The Search for Extraterrestrial Intelligence in Earth's Solar Transit Zone

Abstract

Over the past few years, astronomers have detected thousands of planets and candidate planets by observing their periodic transits in front of their host stars. A related method, called transit spectroscopy, might soon allow studies of the chemical imprints of life in extrasolar planetary atmospheres. Here, we address the reciprocal question, namely, from where is Earth detectable by extrasolar observers using similar methods. We explore Earth's transit zone (ETZ), the projection of a band around Earth's ecliptic onto the celestial plane, where observers can detect Earth transits across the Sun. ETZ is between 0.520° and 0.537° wide due to the noncircular Earth orbit. The restricted Earth transit zone (rETZ), where Earth transits the Sun less than 0.5 solar radii from its center, is about 0.262° wide. We first compile a target list of 45 K and 37 G dwarf stars inside the rETZ and within 1 kpc (about 3260 light-years) using the Hipparcos catalogue. We then greatly enlarge the number of potential targets by constructing an analytic galactic disk model and find that about 10⁵ K and G dwarf stars should reside within the rETZ. The ongoing Gaia space mission can potentially discover all G dwarfs among them (several 10⁴) within the next 5 years. Many more potentially habitable planets orbit dim, unknown M stars in ETZ and other stars that traversed ETZ thousands of years ago. If any of these planets host intelligent observers, they could have identified Earth as a habitable, or even as a living, world long ago, and we could be receiving their broadcasts today. The K2 mission, the Allen Telescope Array, the upcoming Square Kilometer Array, or the Green Bank Telescope might detect such deliberate extraterrestrial messages. Ultimately, ETZ would be an ideal region to be monitored by the Breakthrough Listen Initiatives, an upcoming survey that will constitute the most comprehensive search for extraterrestrial intelligence so far. Key Words: Astrobiology—Extraterrestrial life—Intelligence—Life detection—SETI. Astrobiology 16, 259–270.

1. Introduction

Now that thousands of extrasolar planets have been discovered in a small part of the sky by the Kepler space telescope (Batalha et al., 2013; Lissauer et al., 2014; Rowe et al., 2014), and Earth-sized habitable zone (HZ) planets turn out to orbit about one out of 10 stars (Kaltenegger and Sasselov, 2011; Dressing and Charbonneau, 2013; Petigura et al., 2013), the search for life outside the Solar System has experienced a substantial impetus. Recent studies suggest that deciphering the biorelevant constituents from Earth-like planetary atmospheres—such as oxygen, water, carbon dioxide, and methane—is feasible by taking high-accuracy transit transmission spectra of these world's atmospheres (Ehrenreich et al., 2006; Kaltenegger and Traub, 2009; Belu et al., 2011; Rauer et al., 2011; von Paris et al., 2011; Benneke and Seager, 2012). In addition to other techniques such as spatially resolved spectroscopy from space (Leger et al., 2015), transit observations are thus one key technology with which to characterize extrasolar, inhabited worlds.

In the spirit of Carl Sagan's view of Earth as “a pale blue dot” from the perspective of an observer on the receding Voyager 1 spacecraft (Sagan, 1994), we propose a similar, but more probing, question. Supposing that extraterrestrial observers are using similar transit techniques and are also surveying the Galaxy for habitable worlds with atmospheres that are obviously altered by life: Which of them could discover Earth as a planet that supports life?

The planetary systems, from which it is possible to detect Earth transiting in front of the Sun, are all located in a thin strip around the ecliptic as projected onto the Galaxy, which we refer to as Earth's transit zone (ETZ). Possible extraterrestrials that observe Earth from within ETZ will have direct evidence to classify Earth as a living planet perhaps worth the effort of deliberate broadcasts. Hence, stellar systems within ETZ are promising targets for us to search for extraterrestrial intelligence (SETI).

More than 100 SETI surveys have been performed in the past century (Tarter, 2001). Searches have focused either on nearby systems (Turnbull and Tarter, 2003a), mostly because an extrasolar signal emitted with a given power is stronger and more likely to be detected at closer distances, or on stellar clusters (Turnbull and Tarter, 2003b) where potential messages from many objects can be collected in a single observational run. The first targeted SETI was project Ozma in 1960, which used the 26 m radio telescope of the National Radio Astronomy Observatory. One of the most popular surveys is SETI@home (Korpela et al., 2001), which digests data of the 300 m Arecibo radio telescope. Project Phoenix (Backus and the Project Phoenix Team, 2001), which was conducted by the SETI Institute at different sites between 1995 and 2004, is one of the most ambitious searches ever undertaken, covering more than 800 stars as far as 240 light-years (ly), or about 74 parsecs (pc), from Earth. More recent searches include the first very long baseline interferometric SETI (Rampadarath et al., 2012), placing an upper limit of 7 MW Hz⁻¹ on the power output of any isotropic emitter in the planetary system around the M dwarf Gl 581. The Allen Telescope Array (ATA), jointly operated by the SETI Institute and the Radio Astronomy Laboratory (University of California), has been used for targeted surveys around the galactic center (Tarter et al., 2002; Williams et al., 2009). Later, Tellis and Marcy (2015) searched for optical laser emission from more than one thousand Kepler objects of interest with the Keck telescope. And most recently, Yuri Milner and Stephen Hawking announced the “Breakthrough Listen Initiatives,”¹ a new effort involving a US $100 million investment over the next 10 years to search for extraterrestrial broadcasts using both radio and optical wavelengths (Merali, 2015).

Major obstacles for SETI include the questions of where to point, in which frequency range to search, and which sensitivity be required. If messages from other observers were transmitted to us intentionally (so-called “beacons”), then their power could be relatively strong. However, if their broadcasts reach us unintentionally (by “leakage”), they might be very weak and short-lived (Forgan and Nichol, 2011). On the other hand, these leakage messages could be much more abundant in the Universe, because beacons would require the emitter to have (1) an interest in interstellar communication as well as (2) the capabilities to do so, and (3) they would need to actually identify Earth as a worthwhile target. Ultimately, while leakage transmissions should preferably reach us from regions with higher stellar (more precisely: planetary) densities, simply because we should expect more emitters where there are more potentially habitable worlds, a preferred celestial region for intended messages has not been agreed upon. Here, we suggest that this preferred region is ETZ and present a list of targets to listen for intended transmissions.

The idea to search for intended messages within ETZ is not new (Filippova and Strelnitskij, 1988), but it has never been treated in the refereed literature. Filippova (1990) was one of the first to present a list of nearby stars (29 most KGF dwarfs) near the ecliptic to be monitored, and Castellano et al. (2004) presented a list of 17 G dwarfs from the Hipparcos catalogue (ESA, 1997). The latter report was reconsidered by Shostak and Villard (2004), who proposed to survey the Sun's anti-direction for broadcasts and utilize Earth's solar transit as a means of synchronization between potential senders and us. Conn Henry et al. (2008) proposed a near-ecliptic SETI survey using the ATA. A broader approach was taken by Nussinov (2009), who proposed that solar transits of any of the Solar System planets might draw the interest of extraterrestrials. With the repurposed K2 mission now monitoring thousands of stars near the ecliptic, with the Square Kilometer Array (SKA) starting observations within a few years from now, and with the Breakthrough Listen Initiatives about to be launched, a detailed and updated treatment of ETZ is now timely.

Here, we present a more rigorous geometric description of ETZ (Section 2) than given in the abovementioned studies. We almost triple the list of target stars for SETI in ETZ (Section 3.1), and we compare the number of stars on that target list with the number of stars actually expected in ETZ and within 1 kiloparsec (kpc)² to the Sun, based on stellar density measurements (Section 3.2).

2. Earth's Transit Zone

2.1. Geometrical construction

We start our geometrical construction of ETZ by assuming that Earth's solar orbit is circular. For extrasolar observers to see Earth completely in transit, as opposed to a grazing transit, they must see the whole planetary disk passing in front of the Sun. Such a full transit can be observed from inside a zone with an angular size φ^ETZ = 2(α - γ), where α = arctan(R _⊙/a), a = 1 AU, and γ = arcsin(R _⊕/t) (Fig. 1). Using Pythagoras' theorem for t as the hypotenuse of the bold triangle in Fig. 1, we obtain t = (a ² + R _⊙ ²)^1/2 and \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} \begin{align*}\varphi^ { { \rm ETZ } } = 2 \left( { \rm arctan } \left( \frac { R_ { \odot } } { a } \right) - { \rm arcsin } \left( \frac { R_ { \oplus } } { \sqrt { a^2 + R_ { \odot } ^2 } } \right) \right) \tag { 1 } \end{align*} \end{document}

FIG. 1.

Geometrical construction of ETZ. The yellow circle represents the Sun, the blue circle Earth (sizes not to scale). Within φ^ETZ, the whole disk of Earth performs a transit while at φ^graze the Earth scrapes through a grazing transit. The projection of the stripe defined by φ^ETZ on the sky defines ETZ around the Sun. (Color graphics available at www.liebertonline.com/ast)

With R _⊕ ≪ R _⊙ and arctan(x) ≈ x for x → 1, Eq. 1 becomes φ^ETZ ≈ R _⊙/a, as used by Castellano et al. (2004). While these authors compute φ^ETZ ≈ 2 × 0.267° = 0.534°, our Eq. 1 yields φ^ETZ ≈ 0.528°. This result is also somewhat more precise than the approximation of 2 × 0.28° = 0.56° by Nussinov (2009). As an aside, the somewhat wider celestial band spanned by φ^graze = 2(α + β), with β = arctan(R _⊕/t), defines the outermost zone from which a grazing transit can be seen, and it has a width of 0.538°.

To estimate how the area within ETZ (A _ETZ) compares to the total area of the celestial plane (A _tot), note that A _tot = 4π, while \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} \begin{align*}A_{ \rm ETZ} = \int_0^{2 \pi} {\rm d} \phi \int_{ \pi / 2 - \varphi^{ \rm ETZ} / 2}^{ \pi / 2 + \varphi^{ \rm ETZ} / 2} { \rm d} \theta \,\sin \theta \approx 4.608 \times 10^{ - 3}A_{ \rm tot} \tag{2}\end{align*} \end{document}

Constraining SETI to ETZ thus implies a substantial reduction of the celestial area to be surveyed.

The transit impact parameter b, which describes the minimum separation of the crossing planet from the stellar center in units of stellar radii, is crucial for an observer. For grazing transits (b → 1), the transit duration D can be very short, making characterization of the planetary atmosphere extremely challenging, because even an extraterrestrial observer would be confronted with the solar photometric variability. It is thus plausible to constrain the search window to stars from which they would watch Earth's solar transit with b ≤ 0.5. Replacing R _⊙ in Eq. 1 with R _⊙/2 yields a restricted Earth transit zone (rETZ) that is 0.262° wide.

With Earth's orbital mean motion (ω) of about 360°/365.25 days ≈0.0411°/h, the duration D of the Earth transit at b = 0 (over the solar diameter) is \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} \begin{align*}D\approx \frac { \varphi^ { \rm ETZ } } { \omega } \approx 12 { \rm h } :51 \min \tag { 3 } \end{align*} \end{document}

If the transit were observed with b ≤ 0.5, then its duration would always be longer than 3^1/2 D/2 ≈ 11 h : 08 min—a reasonable duration that might allow extraterrestrials to detect Earth's atmospheric biomarkers.

2.2. Variation of Earth's transit zone

Various physical effects complicate the picture shown in Fig. 1, such as

• Earth's variable orbital eccentricity (e)

• Earth's apsidal precession

• perturbations of Earth's orbit from the Moon

• solar motions due to planetary perturbations (mostly from Jupiter)

• the precession of the ecliptic

all of which we discuss in the following.

Different from what has been assumed in Section 2.1, Earth's solar orbit is not circular but has an eccentricity (e) of about 0.016. Hence, the distance between Earth and the Sun (a) is a function of the geocentric ecliptic longitude λ. Earth's closest solar approach, or perihelion, occurs at the longitude of the periapsis, currently almost exactly λ = 283° (around January 4 in this decade). At that instant, the distance between the Sun and Earth is r _min = a(1 - e) ≈ 0.9833 AU. With the Sun at λ = 283°, potential observers of Earth's transit would be located in the anti-direction at λ = 103°. Hence, at λ = 103° in ETZ, \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document}$$\varphi_{ \rm max}^{ \rm ETZ}$$ \end{document} = 0.537°. In turn, at aphelion (currently at λ = 103° around July 4), observers at λ = 283° see Earth's full solar transit within an angle \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document}$$\varphi_{ \rm max}^{ \rm ETZ}$$ \end{document} = 0.520°.

In the long run, gravitational perturbations on Earth's orbit from the other planets (most of all Jupiter and Venus) cause Earth's eccentricity to vary between almost 0 and about 0.06 (Laskar et al., 1993), with the most relevant beat periods occurring on timescales of hundreds of thousands of years (Laskar et al., 2011a). Hence, some parts of ETZ were as narrow as 0.498° or as wide as 0.562° during these phases. Orbital perturbations from Ceres and Vesta induce chaos on a similar timescale (Laskar et al., 2011b), preventing an exact reconstruction of an ancient ETZ. As Earth's argument of perihelion also evolves, causing the orbital ellipse to revolve once in 112 kyr, those variations in the width of ETZ should have been smeared out over the ecliptic longitude.

As for the location of ETZ, variations of the Sun's position relative to the Solar System barycenter, mostly due to Jupiter's perturbations, have the most relevant effect on b of up to 2.5%. Earth's movement around the Earth-Moon barycenter and the 5.14° misalignment of the lunar orbital plane to the ecliptic induce additional variations of Earth's solar transit impact parameter. But the maximum vertical displacement of Earth due to the Moon is just 423 km, or 0.06% of the solar radius. Hence, this effect is irrelevant for our considerations. Planetary precession, that is, variations of the ecliptic with respect to the invariable plane due to planetary perturbations, are also negligible, since they are about 100 times weaker than lunisolar effects (Williams, 1994).

3. Host Stars of Potential Observers in Earth's Transit Zone

3.1. SETI targets

Figure 2 shows the path of ETZ on the celestial plane. The all-sky view in the left panel is a blend of optical data (Mellinger, 2009), a JHK infrared composite of the Two Micron All Sky Survey (2MASS, Skrutskie et al., 2006), and radio emission between 30 and 70 GHz detected by the Planck satellite (The Planck Collaboration, 2006). The data were retrieved with the Aladin Sky Atlas³ v7.5 (Bonnarel et al., 2000). The arrows in the center of the left panel indicate the origin of the galactic reference frame. Ecliptic coordinates are overplotted to illustrate the orientation of ETZ with respect to the galactic plane.

FIG. 2.

Projection of ETZ onto the celestial plane. Left panel (above): All-sky view in a galactic reference frame with ecliptic coordinates, based on optical, infrared (2MASS), and radio (Planck) data. ETZ is indicated as a gray band curved along the ecliptic. The rectangle around (0°,0°) indicates the zoom shown in right panel. Two red arrows label the origin of the galactic coordinate frame. Right panel (facing page): Zoom into the region around (0°,0°), now in an ecliptic reference frame. The wide, light shaded region has a width of 0.528°, following Eq. 1, while the dark shaded region highlights the rETZ with a width of 0.262°, where Earth transits the Sun with an impact parameter b ≤ 0.5. The position of one ETZ G dwarf is indicated with a black arrow. (Color graphics available at www.liebertonline.com/ast)

At the Sun's location within the Milky Way, the galactic disk has a width of about 600 pc (Binney and Tremaine, 2008). As the Solar System is tilted against the galactic plane by about 63°, ETZ's pathlength through the disk is roughly 1 kpc long. This length is similar to the width of the galactic habitable zone (Lineweaver et al., 2004), which extends from about 7.5 to 9 kpc from the galactic center, the Sun being at about 8.5 kpc (for counter-arguments see Prantzos, 2008). Stars beyond 1 kpc from the Sun may thus not host inhabited planets in the first place. Consequently, we use the SIMBAD Astronomical Database⁴ to retrieve all stars within 1 kpc to the Sun that reside in the rETZ, where Earth's b ≤ 0.5. Our query code reads

region(ZONE ecl,0:0 +0:0,360:0d 0:262d) & plx >=1

where the “plx” (for parallax) condition is measured in milliarcseconds and the string “ecl” sets the ecliptic coordinate system. This search yields 234 stars, including 15 M, 67 K, 46 G, 58 F, 29 A, and 13 B stars. There is no O star, and five of the remaining six objects are not classified. Finally, the sample contains a very nearby white dwarf (“van Maanen's star,” van Maanen, 1917) at a distance of only 4 pc. Obviously, these counts dramatically underestimate the number of M dwarfs, which are the dominant type of star in the solar neighborhood.

We filter 113 K and G stars from this sample. A more sophisticated approach would make use of the stellar ages (if known) for the remaining K and G stars, as was done by Turnbull and Tarter (2003a), since some of these targets might still be very young with little time for the emergence of an intelligent species. Nevertheless, most of these stars should be of similar age as the Sun since they are in the solar neighborhood of the Milky Way. The rejection of giants and subgiants finally leaves us with 45 K and 37 G dwarfs. Table 1 lists their coordinates, proper motions, parallaxes, distances, V magnitudes, and spectral types. Parallaxes are obtained from the Hipparcos catalogue (van Leeuwen, 2007), and distances in units of parsecs are calculated as (parallax)⁻¹.

Table 1.

K Dwarf Stars (45 by Numbers) and G Dwarf Stars (37) within 1 kpc from the Sun and inside the Restricted Earth Transit Zone

Identifier	ICRS coordinates J2000/2000	Ecliptic coordinates J2000/2000	Proper Motion mas/yr	Parallax mas	Distance pc	V mag.	Spectral Type
HR 222	00 48 22.97665 + 05 16 50.2129	013.1821587 + 00.0825339	757.11 −1141.33	134.14 (±0.51)	\documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document}$$7.45^{+ 0.03}_{- 0.03}$$ \end{document}	5.74	K2.5V
GJ 757	19 24 34.23191 −22 03 43.8557	289.5287641 −00.0448508	−236.43 −454.54	34.11 (±2.11)	\documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document}$$29.32^{+ 1.93}_{- 1.71}$$ \end{document}	10.9	K8Vk:
HD 53532	07 06 16.80335 + 22 41 00.5537	105.2557439 + 00.1174720	−89.98 −78.56	23.60 (±0.82)	\documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document}$$42.37^{+ 1.53}_{- 1.42}$$ \end{document}	8.27	G0V
HD 115153	13 15 30.76740 −08 03 18.5701	200.4670388 −00.0647171	42.18 58.71	23.76 (±0.67)	\documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document}$$42.09^{+ 1.22}_{- 1.15}$$ \end{document}	8.06	G5
LTT 11954	06 57 46.33755 + 22 53 33.1761	103.2837246 + 00.1168203	−142.85 −142.91	23.12 (±0.66)	\documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document}$$43.25^{+ 1.27}_{- 1.20}$$ \end{document}	7.63	G0
HD 108754	12 29 42.73517 −03 19 58.6869	188.1369160 −00.1147546	−327.65 −562.26	20.56 (±0.99)	\documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document}$$48.64^{+ 2.46}_{- 2.23}$$ \end{document}	9.03	G7V
G 3-38	02 03 33.03157 + 12 35 04.9890	033.1168429 + 00.0348568	381.09 −51.86	20.30 (±3.71)	\documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document}$$49.26^{+ 11.02}_{- 7.61}$$ \end{document}	11.54	K7
HD 20477	03 18 19.98264 + 18 10 17.8452	051.9747902 −00.0924300	−84.56 −103.48	20.16 (±0.64)	\documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document}$$49.60^{+ 1.63}_{- 1.53}$$ \end{document}	7.54	G0
HD 78451	09 08 49.27841 + 16 25 03.5462	134.7386517 + 00.0053713	−40.57 −71.38	18.21 (±2.91)	\documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document}$$54.91^{+ 10.44}_{- 7.57}$$ \end{document}	9.47	G5
StKM 2-1619	22 31 18.99284 −09 25 34.7527	336.0020753 −00.1250324	−141.48 −195.84	17.90 (±2.60)	\documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document}$$55.87^{+ 9.49}_{- 7.09}$$ \end{document}	11.15	K5V
LTT 6827	17 05 59.62335 −22 51 24.3597	257.5762891 + 00.0021104	34.76 −325.61	17.55 (±2.03)	\documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document}$$56.98^{+ 7.45}_{- 5.91}$$ \end{document}	10.0	G
NLTT 20353	08 50 36.87345 + 17 41 21.5587	130.2052944 + 00.0036610	−149.74 −59.67	17.07 (±2.27)	\documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document}$$58.58^{+ 8.99}_{- 6.88}$$ \end{document}	9.323	K0
G 156-19	22 33 21.52793 −09 03 48.8063	336.6043076 + 00.0261599	298.54 −62.00	16.17 (±0.96)	\documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document}$$61.84^{+ 3.90}_{- 3.47}$$ \end{document}	8.71	G0
HD 123453	14 08 04.09936 −12 55 42.1319	214.2696634 + 00.0156760	116.89 −110.14	15.90 (±1.39)	\documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document}$$62.89^{+ 6.03}_{- 5.06}$$ \end{document}	7.95	G9V:+..
BD +18 487	03 27 55.30483 + 18 52 56.4159	054.3533816 + 00.0238173	13.83 −59.44	14.41 (±1.43)	\documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document}$$69.40^{+ 7.65}_{- 6.27}$$ \end{document}	8.48	G0
HD 224448	23 58 04.536 −00 07 41.49	359.507608 + 00.073754	−45.28 −19.25	14.13 (±1.25)	\documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document}$$70.77^{+ 6.87}_{- 5.76}$$ \end{document}	9.01	G0
LTT 8911	22 12 50.82036 −10 55 31.3525	331.2230435 + 00.1223774	178.99 −183.40	13.85 (±2.09)	\documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document}$$72.20^{+ 12.83}_{- 9.47}$$ \end{document}	10.8	K3
LTT 7183	18 05 46.73402 −23 31 03.8191	271.3247005 −00.0850796	25.24 −212.03	13.13 (±1.18)	\documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document}$$76.16^{+ 7.52}_{- 6.28}$$ \end{document}	9.05	G7V
LTT 7494	18 54 12.798 −22 54 25.32	282.466510 −00.052375	−161.7 −368.0	13.00 (±7.00)	\documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document}$$76.92^{+ 89.74}_{- 26.92}$$ \end{document}	8.48	G5
HD 81563	09 26 49.40417 + 14 55 40.7481	139.3192061 −00.1051346	21.47 −91.74	12.22 (±0.92)	\documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document}$$81.83^{+ 6.66}_{- 5.73}$$ \end{document}	8.27	G0
HD 121865	13 58 26.84077 −12 03 33.4252	211.7657893 + 00.0303109	133.80 −150.88	11.91 (±0.88)	\documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document}$$83.96^{+ 6.70}_{- 5.78}$$ \end{document}	7.05	G5
HD 204433	21 28 50.03197 −14 49 34.9337	319.8123553 + 00.0494591	−21.87 −100.07	9.94 (±1.37)	\documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document}$$100.60^{+ 16.08}_{- 12.19}$$ \end{document}	9.1	G2V
HD 284145	04 04 56.52494 + 20 51 23.3074	063.2768930 + 00.0460690	10.54 −53.70	9.00 (±1.38)	\documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document}$$111.11^{+ 20.12}_{- 14.77}$$ \end{document}	8.98	G0
HD 109376	12 34 13.27236 −03 43 16.1748	189.3239649 −00.0283740	14.16 −57.19	8.58 (±0.86)	\documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document}$$116.55^{+ 12.98}_{- 10.62}$$ \end{document}	8.57	G5
HD 244354	05 31 04.38928 + 23 12 34.7855	083.3563104 −00.0630026	10.45 −38.04	7.95 (±1.29)	\documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document}$$125.79^{+ 24.34}_{- 17.56}$$ \end{document}	9.11	G0
HD 152956	16 57 30.23696 −22 38 37.1391	255.6074904 + 00.0183989	−0.12 −29.91	7.53 (±1.42)	\documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document}$$132.80^{+ 30.86}_{- 21.07}$$ \end{document}	9.78	G0V
HD 117004	13 27 29.65820 −09 11 33.7450	203.6390818 −00.0160648	−55.67 −12.36	7.13 (±1.42)	\documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document}$$140.25^{+ 34.88}_{- 23.29}$$ \end{document}	9.48	G0
HD 207921	21 52 54.70756 −12 58 41.6150	325.9382627 −00.1118968	−35.35 −79.37	7.02 (±0.92)	\documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document}$$142.45^{+ 21.48}_{- 16.51}$$ \end{document}	8.77	G3V
HD 15002	02 25 27.73995 + 14 32 35.7739	038.7897351 + 00.1198081	20.82 −49.85	6.74 (±1.13)	\documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document}$$148.37^{+ 29.89}_{- 21.30}$$ \end{document}	9.19	G0
HD 31363	04 56 06.63500 + 22 34 35.6076	075.2805471 −00.0502722	38.65 −53.50	6.64 (±0.86)	\documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document}$$150.60^{+ 22.41}_{- 17.27}$$ \end{document}	7.17	K0
HD 218553	23 09 01.53846 −05 20 30.5894	346.1996508 + 00.1116472	16.63 2.73	6.51 (±1.10)	\documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document}$$153.61^{+ 31.23}_{- 22.20}$$ \end{document}	8.61	K0
HD 210241	22 09 18.13872 −11 21 58.3422	330.2538687 + 00.0175564	4.42 −42.40	6.43 (±0.73)	\documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document}$$155.52^{+ 19.92}_{- 15.86}$$ \end{document}	7.86	K0
HD 110918	12 45 32.02766 −04 48 39.3989	192.3457381 + 00.0737266	−18.62 7.16	6.39 (±0.56)	\documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document}$$156.50^{+ 15.03}_{- 12.61}$$ \end{document}	7.16	K2
HD 106352	12 14 10.58599 −01 38 31.7169	183.9043926 −00.0981432	92.45 −98.42	5.94 (±1.42)	\documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document}$$168.35^{+ 52.89}_{- 32.48}$$ \end{document}	9.71	G5
LTT 8795	22 00 13.00098 −12 13 29.6524	327.8705876 −00.0123927	169.20 −93.07	5.75 (±1.35)	\documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document}$$173.91^{+ 53.36}_{- 33.07}$$ \end{document}	9.23	G5V
BD-22 4248	16 56 41.05634 −22 40 27.2304	255.4227874 −00.0323920	−11.85 −2.97	5.61 (±2.70)	\documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document}$$178.25^{+ 165.39}_{- 57.92}$$ \end{document}	9.84	G5
HD 14243	02 18 34.05265 + 13 57 00.9562	037.0170514 + 00.0997387	6.36 −14.36	5.54 (±1.14)	\documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document}$$180.51^{+ 46.77}_{- 30.81}$$ \end{document}	8.44	K0
HD 71907	08 30 31.01625 + 18 56 34.7115	125.2737250 −00.0074509	−13.48 −11.24	5.50 (±1.13)	\documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document}$$181.82^{+ 47.02}_{- 30.99}$$ \end{document}	8.63	K2
HD 3819	00 40 49.92047 + 04 28 41.9361	011.1373198 + 00.0778609	−11.92 −20.75	5.16 (±0.72)	\documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document}$$193.80^{+ 31.43}_{- 23.73}$$ \end{document}	7.97	G5
HD 66287	08 03 34.13418 + 20 20 18.6938	118.7779183 −00.0675791	13.93 −27.38	5.16 (±0.66)	\documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document}$$193.80^{+ 28.42}_{- 21.98}$$ \end{document}	7.03	G5
HD 61660	07 41 12.69882 + 21 26 48.8100	113.4412891 + 00.0431439	11.69 −42.56	4.91 (±0.91)	\documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document}$$203.67^{+ 46.33}_{- 31.85}$$ \end{document}	7.87	K2
G 14−32	13 08 25.78882 −07 18 30.4548	198.5601373 −00.0373871	−219.83 82.11	4.71 (±1.03)	\documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document}$$212.31^{+ 59.43}_{- 38.01}$$ \end{document}	8.35	G5
HD 218399	23 07 50.70029 −05 41 51.5236	345.7908778 −00.1024129	20.51 −3.66	4.55 (±1.34)	\documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document}$$219.78^{+ 91.75}_{- 50.00}$$ \end{document}	8.54	K0
44 Cnc	08 43 08.35527 + 18 09 01.9705	128.3684830 −00.0222521	−1.25 −3.04	4.49 (±0.82)	\documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document}$$222.72^{+ 49.76}_{- 34.39}$$ \end{document}	8.03	K0
72 Aqr	22 50 46.30141 −07 18 42.9276	341.2553953 + 00.0346433	−25.71 −14.01	4.48 (±0.65)	\documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document}$$223.21^{+ 37.89}_{- 28.29}$$ \end{document}	7.0	K0
HD 16178	02 36 11.96153 + 15 08 55.3242	041.4430635 −00.1219804	−1.98 −40.02	4.38 (±0.74)	\documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document}$$228.31^{+ 46.42}_{- 33.00}$$ \end{document}	8.4	K0
HD 72115	08 31 41.29613 + 18 59 15.8392	125.5314085 + 00.1034379	−26.31 −20.99	4.12 (±0.51)	\documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document}$$242.72^{+ 34.29}_{- 26.74}$$ \end{document}	6.45	K0
HD 218846	23 11 17.17017 −05 05 55.5145	346.8126666 + 00.1170492	15.77 4.11	4.10 (±1.04)	\documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document}$$243.90^{+ 82.90}_{- 49.36}$$ \end{document}	8.85	K0
HD 2690	00 30 31.24132 + 03 16 00.7985	008.2962799 −00.0255395	16.51 −0.46	4.01 (±0.98)	\documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document}$$249.38^{+ 80.66}_{- 48.98}$$ \end{document}	8.44	K2
HD 940	00 13 47.57526 + 01 23 00.9161	003.7137699 −00.1011061	−25.67 −14.22	3.97 (±0.61)	\documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document}$$251.89^{+ 45.73}_{- 33.55}$$ \end{document}	6.77	K0
HD 17795	02 51 52.36942 + 16 29 51.8022	045.4438836 + 00.0334572	−3.84 −15.93	3.97 (±0.93)	\documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document}$$251.89^{+ 77.06}_{- 47.81}$$ \end{document}	8.13	K5
HD 5214	00 53 58.22129 + 05 48 32.9485	014.6686315 + 00.0303990	27.17 3.95	3.86 (±0.57)	\documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document}$$259.07^{+ 44.88}_{- 33.33}$$ \end{document}	7.92	G5
HD 50318	06 53 36.58256 + 22 46 34.2676	102.3401752 −00.0908021	−0.68 −8.08	3.86 (±0.86)	\documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document}$$259.07^{+ 74.27}_{- 47.20}$$ \end{document}	8.22	K0
HD 29097	04 35 36.74097 + 22 01 14.4366	070.5072283 −00.0023000	16.31 −24.59	3.85 (±1.46)	\documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document}$$259.07^{+ 158.67}_{- 71.42}$$ \end{document}	8.21	K0
BD +20 1979	08 01 42.33483 + 20 26 49.9871	118.3283130 −00.0497126	−7.39 −10.44	3.50 (±1.08)	\documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document}$$285.71^{+ 127.51}_{- 67.37}$$ \end{document}	9.05	G5
HD 54427	07 09 44.41414 + 22 21 52.5414	106.0864687 −00.1063125	17.69 −12.19	3.48 (±0.68)	\documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document}$$287.36^{+ 69.79}_{- 46.97}$$ \end{document}	7.56	K2
HD 118866	13 39 43.95528 −10 14 51.5276	206.8316729 + 00.1025172	−22.75 9.18	3.46 (±0.93)	\documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document}$$289.02^{+ 106.24}_{- 61.23}$$ \end{document}	8.44	K2
HD 39184	05 51 50.65689 + 23 22 58.0589	088.1285815 −00.0432527	−8.36 −20.44	3.27 (±0.72)	\documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document}$$305.81^{+ 86.35}_{- 55.19}$$ \end{document}	6.98	K5
BD +18 472	03 23 39.55279 + 18 33 20.7050	053.2946619 −00.0423735	−3.38 −11.71	3.12 (±1.66)	\documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document}$$320.51^{+ 364.42}_{- 111.31}$$ \end{document}	8.93	K0
HD 7990	01 19 28.31292 + 08 23 46.9468	021.5030840 + 00.0134072	−14.37 34.86	3.01 (±0.65)	\documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document}$$332.23^{+ 91.50}_{- 59.00}$$ \end{document}	7.91	K0
HD 118777	13 39 07.04823 −10 23 24.7498	206.7420336 −00.0850406	9.77 −22.94	2.94 (±1.43)	\documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document}$$340.14^{+ 322.12}_{- 111.30}$$ \end{document}	8.47	K0
BD-13 6044	21 53 49.08240 −12 54 16.5428	326.1709588 −00.1173764	−11.92 −35.21	2.88 (±1.83)	\documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document}$$347.22^{+ 605.16}_{- 134.91}$$ \end{document}	10.13	G0
BD +15 2068	09 32 57.78706 + 14 24 26.9151	140.8924859 −00.1306424	35.74 −46.11	2.73 (±1.27)	\documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document}$$366.30^{+ 318.63}_{- 116.30}$$ \end{document}	9.25	K0
HD 70842	08 24 49.68026 + 19 10 13.0292	123.9140629 −00.1079141	1.97 −3.67	2.66 (±1.19)	\documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document}$$375.94^{+ 304.33}_{- 116.20}$$ \end{document}	8.91	K0
HD 90762	10 28 55.89407 + 09 33 54.3328	155.4041598 + 00.0379253	14.48 −30.82	2.57 (±0.85)	\documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document}$$389.11^{+ 192.29}_{- 96.71}$$ \end{document}	8.44	G5
HD 88680	10 13 50.38192 + 10 50 31.3131	151.4804623 −00.1136457	5.36 4.87	2.54 (±0.71)	\documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document}$$393.70^{+ 152.75}_{- 86.01}$$ \end{document}	8.02	G5
BD +04 122	00 48 02.45463 + 05 06 12.7318	013.0348238 −00.0474695	0.11 −0.90	2.50 (±1.02)	\documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document}$$400.00^{+ 275.68}_{- 115.91}$$ \end{document}	8.99	K2
HD 121185	13 54 07.92284 −11 41 43.3350	210.6494013 + 00.0044963	0.12 18.81	2.50 (±0.69)	\documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document}$$400.00^{+ 152.49}_{- 86.52}$$ \end{document}	7.37	K2
HD 63105	07 47 55.89599 + 21 02 05.5168	115.0551127 −00.0880687	−12.51 −8.90	2.20 (±0.67)	\documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document}$$454.55^{+ 199.05}_{- 106.11}$$ \end{document}	7.78	K2
HD 81541	09 26 40.48560 + 15 06 57.9390	139.2263973 + 00.0623751	−13.49 −2.81	2.09 (±1.26)	\documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document}$$478.47^{+ 726.35}_{- 170.96}$$ \end{document}	9.19	G
HD 3082	00 34 07.51580 + 03 48 32.0706	009.3364777 + 00.1184635	4.52 2.76	2.04 (±1.23)	\documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document}$$490.20^{+ 744.37}_{- 184.39}$$ \end{document}	8.86	K2
HD 46279	06 33 27.26922 + 23 10 43.3360	097.6842660 −00.0377662	0.08 −4.62	1.80 (±1.18)	\documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document}$$555.56^{+ 1057.35}_{- 219.99}$$ \end{document}	8.37	K0
BD +15 391	02 49 04.16610 + 16 07 22.0335	044.6909552 −00.1281619	−16.32 −1.78	1.78 (±1.23)	\documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document}$$561.80^{+ 1256.38}_{- 229.57}$$ \end{document}	9.21	K0
HD 222294	23 39 23.37144 −02 09 16.4705	354.4165961 + 00.0690674	5.02 −1.40	1.76 (±1.22)	\documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document}$$568.18^{+ 1283.67}_{- 232.61}$$ \end{document}	8.64	K5
HD 116210	13 22 18.76319 −08 41 58.5069	202.2678209 −00.0325619	−2.02 6.12	1.72 (±0.94)	\documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document}$$581.40^{+ 700.66}_{- 205.46}$$ \end{document}	8.57	G5
HD 10917	01 47 21.58819 + 10 57 13.2703	028.8344633 −00.1139321	6.56 −2.02	1.69 (±0.97)	\documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document}$$591.72^{+ 797.17}_{- 215.78}$$ \end{document}	8.77	K2
HD 102105	11 45 05.53604 + 01 34 43.3932	175.9530492 −00.0326170	−7.50 9.64	1.49 (±1.01)	\documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document}$$671.14^{+ 1412.19}_{- 271.14}$$ \end{document}	8.59	K5
HD 107136	12 19 07.46767 −02 02 24.8982	185.1972389 + 00.0269111	−27.53 5.57	1.44 (±1.27)	\documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document}$$694.44^{+ 5187.91}_{- 325.44}$$ \end{document}	9.24	K2
HD 106351	12 14 09.77273 −01 35 26.8989	183.8809015 −00.0523691	−38.18 10.20	1.42 (±1.07)	\documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document}$$704.23^{+ 2152.92}_{- 302.62}$$ \end{document}	9.05	G5
HD 90225	10 25 02.00411 + 09 58 56.3018	154.3577234 + 00.0752950	−16.81 −20.19	1.37 (±0.84)	\documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document}$$729.93^{+ 1156.87}_{- 277.44}$$ \end{document}	8.25	K0
HD 64768	07 56 15.07077 + 20 49 03.0689	117.0041988 + 00.0614611	3.88 −14.67	1.27 (±0.95)	\documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document}$$787.40^{+ 2337.60}_{- 336.95}$$ \end{document}	8.32	K0
HD 216905	22 57 04.57976 −06 48 56.5140	342.8906179 −00.1031304	17.41 −7.73	1.07 (±1.46)	\documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document}$$934.58 ^{+ {\rm nan}}_{- 539.32}$$ \end{document}	9.35	K0

Objects are sorted by increasing distance to the Sun.

Castellano et al. (2004) presented a list of 17 G dwarfs from the Hipparcos catalogue that lie within 0.534° around the ecliptic, roughly corresponding to the unrestricted Earth transit zone. Seven of these G dwarfs are also listed in Table 1,⁵ which contains only stars in the rETZ. Observers on planets orbiting the remaining 10 G dwarfs in Castellano et al. (2004) would possibly have a harder time identifying Earth as supporting life via transit observations; hence, their host stars deserve lower priority for SETI.

3.2. Galactic stellar model for targets

Beyond those 82 immediate SETI targets listed in Table 1, how many stars may nowadays⁶ host observers of Earth transits? To address this question, we estimate the number of stars that actually reside in the rETZ.

Although M dwarfs are by far the most numerous type of star in the solar neighborhood, numerous issues related to M dwarf habitability have been discussed in the literature, such as tidal locking (Dole, 1964) and the accompanying peril of atmospheric collapse (Joshi et al., 1997), reduced formation of water-rich terrestrial planets (Raymond et al., 2007), weak quiescent ultraviolet radiation from the star (Guo et al., 2010), planetary atmospheric erosion due to weak internal magnetic fields and enhanced stellar winds (Zendejas et al., 2010), forced tidal heating in otherwise habitable moons (Heller, 2012), tidal heating in eccentric terrestrial planets (Barnes et al., 2013), and the possibility of desiccation of terrestrial planets during the pre-main-sequence phase of M dwarfs (Ramirez and Kaltenegger, 2014; Luger and Barnes, 2015). On the other hand, cross-polar circulation may provide a mechanism that prevents atmospheric freeze-out (Haqq-Misra and Kopparapu, 2015), and a stabilizing cloud feedback on terrestrial planets around M dwarfs might even extend the inner edge of the HZ much closer to the star than estimated by 1-D longitudinally averaged atmosphere models (Yang et al., 2013). Further, modest and stable tidal heating might actually render terrestrial planets around M dwarfs habitable for trillions of years (Barnes, 2014), and planets around M dwarfs can also stay in the HZ for much longer times than planets around Sun-like stars (Rushby et al., 2013). For a reappraisal of M dwarf habitability see Tarter et al. (2007).

A major issue concerning the existence of observers on planets around FABO stars is related to their relatively short lifetimes compared to the 4.5 billion years required for an intelligent species to arise on Earth. Given the highly stochastic nature of the origin and evolution of life on our home planet, however, it may still be reasonable to assume that a star-gazing life-form could evolve on a different planet within, say, a billion years. Hence, F dwarf stars could potentially harbor planets who themselves host observers of Earth transits.

We thus consider a “minimal” model that takes into account KG dwarfs⁷ and an “extended” model that invokes MKGF dwarfs. For both models, we assume a vertical stellar density profile \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} \begin{align*} \rho ( z ) = \rho_0{ \rm e}^{ - \mid z \mid / z_0} \tag{4}\end{align*} \end{document}

of the Milky Way, where z is the height above the disk midplane, z ₀ = 300 pc is the local thickness of the thin disk, and ρ ₀ ∈ {0.0092, 0.247} M _⊙ pc⁻³ is the mass density of KG dwarfs (minimal model) and MKGF dwarfs (extended model), respectively, in the midplane (Flynn et al., 2006). At a distance r from the Sun, the stellar density ρ(λ, r) can be described by two contributions, one in the vertical direction and one within the plane. Moving through the ecliptic from geocentric ecliptic longitude λ = 0 to 2π (Fig. 2), ρ(λ, r) oscillates because the ecliptic is tilted against the galactic plane by i = 63°. Hence, \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} \begin{align*}{\rho} ( \lambda{ \prime} , r ) = { \rho_0 \over 2} \left( [ 1- \cos ( 2 \lambda{ \prime} ) ] + [ 1 + \cos ( 2 \lambda{ \prime} ) ] { \rm e}^{ - r \sin ( i ) / z} \right) \tag{5}\end{align*} \end{document}

where λ′ is chosen to equal zero in the midplane, and we assume that z ₀ is constant throughout our region of interest.

To obtain the number of KG dwarfs and MKGF inside the rETZ and within a volume of radius r′, we integrate ρ(λ′, r), weigh it with a factor W = 2.286 × 10⁻³ as per Eq. 2, and divide it by the average stellar mass (M _av ∈ {0.65, 0.5} M _⊙). \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} \begin{align*} N_ { \rm rETZ } ^ { \rm KG } ( r { \prime } ) = \frac { W } { M_{ \rm av } } \int_0^ { r { \prime } } { \rm d } r \ r^2 \int_0^ { 2 \pi } { \rm d } \lambda { \prime } \int_0^ { \pi } { \rm d } \beta \sin ( \beta ) \rho ( \lambda { \prime } , r ) \\ \ = \frac{ 2 \pi \rho_0 W } { M_ { \rm av } } \left( \frac { r { \prime { ^3 } } } { 3 } + \frac { { \rm e } ^ { - \zeta r { \prime } } }{ \zeta^3 } \left[ - \zeta r { \prime } ( \zeta r { \prime } + 2 )- 2 \right] + \frac { 2 } { \zeta^3 } \right) \tag { 6 } \end{align*} \end{document}

with ζ = sin(i)/z ₀.

Figure 3 visualizes \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document}$$N_{ \rm rETZ}^{ \rm KG} ( r{ \prime} )$$ \end{document} with a dashed line (minimal model) and \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document}$$N_{ \rm rETZ}^{ \rm MKGF} ( r{ \prime} )$$ \end{document} with a dotted line (extended model). The black solid line is a cumulative KG dwarf star count based on our SIMBAD search (Table 1) and, since Hipparcos measurements are magnitude-limited, almost complete only out to about 80 pc. At larger distances, stars simply become too faint to be detected or well characterized. The relative abundance of stars with MKGF spectral types in the solar neighborhood (Ledrew, 2001) is shown in the legend of Fig. 3. Our disk model predicts 80,000 KG stars within the rETZ and within 1 kpc, whereas a model assuming a constant KG stellar density yields almost twice that amount (not shown). This suggests that the decrease of the stellar density becomes significant toward 1 kpc. With almost 10⁵ KG dwarfs residing as deep as 1 kpc in ETZ, our minimal model thus predicts about 3 orders of magnitude more KG dwarfs than are currently known due to observational incompleteness. Our extended model, which takes into account MKGF dwarfs, suggests that almost 3 × 10⁵ stars could potentially host observers in the rETZ.

FIG. 3.

Number of main-sequence stars inside the rETZ as a function of distance. The upper dotted line illustrates our disk model (Eq. 6) taking into account MKGF dwarfs, whereas the dashed line considers K and G dwarfs only. According to the latter, almost 10⁵ K and G dwarfs potentially host civilizations who see Earth transiting the Sun. The bottom solid line is based on Hipparcos K and G dwarf counts (Table 1).

Based on recent findings that about 10% of Sun-like stars host a terrestrial planet in the HZ (Kaltenegger and Sasselov, 2011; Dressing and Charbonneau, 2013; Petigura et al., 2013), our collection of 82 K and G dwarfs in the rETZ implies a total of about eight terrestrial planets around them. But more intriguingly, our total estimate of about 10⁵ KG (or 3 × 10⁵ MKGF) dwarfs in the rETZ and within 1 kpc suggests that there might be about 10⁴ (or 3 × 10⁴) terrestrial planets in the HZs around K and G dwarfs and within 1 kpc in the rETZ.

4. Discussion

Beyond planets, their yet unseen moons could host life, too (Williams et al., 1997; Heller and Barnes, 2013). Moreover, intelligent civilizations might also travel between stars (Sagan, 1980) and colonize or terraform worlds (planets or moons) that were originally not habitable. Hence, our estimates might be considered lower limits. Ultimately, while there is a certain stellar population inside ETZ today, stellar proper motions and the variation of the ecliptic cause these targets to float in and out of ETZ over thousands of years. A star with a proper motion of 100 mas/yr could transition ETZ within 20 kyr if its motion were perpendicular to the ecliptic. Hence, a substantial population of civilizations may have seen Earth transiting the Sun long ago and identified Earth as supporting life. Although they might not observe our transits today, they might still be trying to contact us. Stars within about a degree to ETZ thus also serve as potential SETI targets.

Planets orbiting stellar binaries have now been observed (Doyle et al., 2011; Orosz et al., 2012a, 2012b; Welsh et al., 2012; Kostov et al., 2014), and formation models suggest that stellar binarity does not impede planet formation (Gong et al., 2013; Martin et al., 2013). Thus, even if some of the stars listed in Table 1 have low-mass companions, these systems could still host habitable worlds (Forgan, 2014) from a planet formation point of view.

Our study raises immediate science cases for ongoing SETI with the K2 mission (in the optical, Wright et al., 2016), the ATA (radio), the upcoming SKA (radio), and the Breakthrough Listen Initiatives (radio and optical). With integration times of a few minutes, the ATA can detect terrestrial-like radio leakage out to about 80 pc (Blair and the ATA Team, 2009). Consequently, it should be capable of tracing intended broadcasts, supposedly emitted with much higher power, out to a few hundred parsecs. On the Southern Hemisphere, with two operational sites in South Africa and Australia planned to start observations in 2024, the SKA will be the largest radio telescope ever. Its extreme sensitivity, combined with a relatively large field of view of about one square degree (Ekers, 2003), will enable scientists to screen a few dozen of our ETZ targets for extraterrestrial beacons. The ATA on the Northern and the SKA on the Southern Hemisphere have the potential to cover most of the 82 ETZ targets listed in Table 1 in a concerted effort. Ultimately, with their relatively large fields of view, both the ATA and SKA can be used to progressively monitor Earth's whole transit zone for intended broadcasts, including ≈10⁵ nearby K and G dwarfs not listed in Table 1, on a timescale of days only.

Starting in 2016, the Breakthrough Listen Initiatives will combine a radio survey using the 100 m Robert C. Byrd Green Bank Telescope (GBT for short, USA) and the 64 m Parkes Telescope (Australia) with an optical (laser) SETI using the Automated Planet Finder Telescope at Lick Observatory (USA). Four kinds of targets have been announced: (1) 10⁶ nearby stars, (2) the galactic center, (3) the entire plane of the Milky Way, and (4) 100 nearby galaxies. We propose that Earth's solar transit zone offers a compelling alternative or additional target region for the Breakthrough Listen Initiatives. The GBT's half-power beamwidth of about 1 arcmin = 1/60 deg at 13 GHz⁸ implies a sky-projected area of ≈1/3600 deg² that can be scanned at one time (and one frequency range). With the rETZ covering about 2 × 10⁻³ parts of the entire sky, or roughly 80 deg² (Eq. 2), this means about 3 × 10⁵ pointings would be required to fully scan the rETZ with a GBT-like radio telescope. This number is more than 3 orders of magnitude larger than the number of pointings required to scan the 82 K and G dwarfs listed in Table 1. But it is comparable to the number of pointings implied by a targeted survey of the 10⁵ K and G dwarfs (Fig. 3) that we actually suspect to reside inside the rETZ.

Even if intended messages are absent, the aforementioned radio telescopes could discover leakage transmissions. Twenty-two systems in Table 1, and about 100 following our model, are closer than 100 pc. At this proximity, the SKA would be capable of detecting even weak leakage signals with a power less than that of some Earth-based Ballistic Missile Early Warning Systems (Loeb and Zaldarriaga, 2007) or with a power comparable to an airport radar as far as 30 pc from Earth (Schilizzi et al., 2007). In particular, at 7.45 (±0.028) pc or 24.31 (±0.09) ly, HR 222 is by far the closest K dwarf in ETZ.

The ongoing Gaia space mission is supposed to be complete to stars as dim as V = 20 mag (Perryman et al., 2001). This limit coincides almost precisely with the apparent visual magnitude of a Sun-like star at 1 kpc. Hence, the early- to mid-G dwarf population in Table 1 and Fig. 3 should grow dramatically from now 37 to about 10,000 objects and then be complete over the whole range of distances considered. Late K dwarfs with visual magnitudes as low as V = 7 mag might be complete to only ≈100 pc, adding another few hundred targets.

5. Conclusions

The region of the Milky Way from which extraterrestrials might observe non-grazing transits of Earth in front of the Sun appears as a band to both sides of the ecliptic with a total width of about 0.528°, assuming a circular Earth orbit. Taking into account Earth's elliptical orbit, we find that the width of ETZ varies between 0.537° (at geocentric ecliptic longitude λ = 103°) and 0.520° (at λ = 283°). Grazing transits can be observed in a band that is about 0.01° wider. Assuming that observers need to see Earth transiting with an impact parameter b ≤ 0.5 in order to characterize its atmosphere, the width of this rETZ is about 0.262°.

We retrieve 45 K and 37 G dwarf stars within 1 kpc and located inside the rETZ, which implies about eight terrestrial HZ planets based on recent Kepler planet counts. As our sample is severely magnitude limited, we construct a galactic disk model and estimate the true number of K and G dwarfs within 1 kpc and inside ETZ. This number turns out to be about 10⁵, that is, about 10³ times the number of known targets. Hence, our estimate for the number of HZ terrestrial planets in the rETZ goes up to as much as 10⁴. In an extended model that takes into account M and F dwarfs in the rETZ, counts increase by another factor of three.

We have literally adopted Sagan's vision of Earth as “a mote of dust suspended in a sunbeam” (Sagan, 1994): our planet may be seen by distant observers against our Sun's light, allowing them to detect us. As an ultimate consequence, even if our species chose to remain radio-quiet to eschew interstellar contact, we cannot hide from observers located in Earth's solar transit zone, if they exist. Table 1 lists 82 host stars of these potential observers and may serve as an initial priority target list for SETI observations in ETZ. A wider survey of the hundreds of thousands of targets predicted by our galactic model constitutes an effective strategy for the Breakthrough Listen Initiatives and other SETI observations to search for extraterrestrial calling partners.

Footnotes

Acknowledgments

We thank two anonymous referees and Jason Wright for their constructive reports on the initial version of this manuscript. R. Heller has been supported by the Origins Institute at McMaster University, by the Canadian Astrobiology Program (a Collaborative Research and Training Experience Program funded by the Natural Sciences and Engineering Research Council of Canada, NSERC), by the Institute for Astrophysics Göttingen, and by a Fellowship of the German Academic Exchange Service (DAAD). R. E. Pudritz is supported by a Discovery grant from NSERC. He also thanks the Max Planck Institute for Astronomy, Heidelberg, for its hospitality and support of his sabbatical leave during which the manuscript was finalized. This work has made use of NASA's Astrophysics Data System Bibliographic Services and of the SIMBAD database, operated at CDS, Strasbourg, France. Discussions with Dimitris Mislis in the frame of the DFG-funded Graduiertenkolleg 1351 “Extrasolar Planets and Their Host Stars” initiated this study many years ago.

Author Disclosure Statement

No competing financial interests exist.

Abbreviations Used

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