Earth Similarity Index Provides a Logical Breakpoint for Naming Planets

T he recent confirmation of Kepler 22b, a 2.4 Earth-radii planet located in the habitable zone of a G-type star, is heralding a new era of charting extrasolar planets. This discovery grants the International Astronomical Union (IAU) an opportunity to make a hard break from current precedent: it is time for some extrasolar planets to be officially named. Why are names needed? Within the astronomy community, the current designations are reasonably informative; but outside astronomy, they are hopelessly disengaging (e.g., Lyra, 2009; Mascaro, 2010). The NBC Nightly News devoted 42 seconds to the discovery of Kepler 22b, during which host Brian Williams quipped, “We can work on the name” (NBC, 2011).

In a recent issue of Astrobiology, Schulze-Makuch et al. (2011) offered a useful tool to target officially sanctioned names to the bodies of greatest interest. The Earth Similarity Index (ESI) quantifies the weighted similarity between a planetary body and Earth, based on planet radius, bulk density, surface temperature, and escape velocity. After normalization on a zero-to-one scale, only one extrasolar planet (prior to the announcement of Kepler 22b) is estimated to have an ESI greater than 0.8 (the contested Gliese 581g). This break (or perhaps a more conservative 0.9) provides a logical threshold above which newly discovered planets should be officially named. The IAU notes that the impending flood of new planets would make any naming schemes obsolete quickly. But high ESI objects—no matter how numerous they ultimately turn out to be—will captivate the interest of astronomers for decades before they can be glimpsed and centuries before they might be visited.

Is this proposal biased? Yes. Anthropocentric? Certainly. No naming scheme will be perfect. But extrasolar planet science is on the leading edge of discovery, and imperfect monikers are a small price to pay to increase public engagement. It is an age not unlike the late 1700s, when the discovery of Uranus by William Herschel ushered in a new era of astronomy. Although Uranus was originally named Georgium Sidus (after King George III), this did not stifle progress or confuse the public. In fact, it started a controversy that only served to draw attention to Herschel's novel techniques and increase funding for cosmological research (Holmes, 2008). Names offer an organic connection to something, and official sanction for a select group of planets that holds fascination for all of humankind will pay real dividends with trivial risks.