Earth 2

Figure 1. Terrians, those mysterious inhabitants of the planet G889 in the 1990s TV series Earth2.

I just started a new job, and yesterday was my first day off since the first of the year. A few days ago I noticed some headlines about the discovery of an “Earth twin” – e.g., Most Earth-Like Alien Planet Possibly Found. Since similar headlines have been pretty frequent recently, I figured the subject matter would be, as usual, some familiar phantom planet, such as GJ 581 g or Tau Ceti e. So with all my work-related chaos, I didn’t pay much attention. I surmised that if this were a truly new and remarkable discovery, there’d be plenty of follow-up forthcoming in higher-impact press.

As it turns out, there’s been very little. Yet this really is novel stuff.

The object in question is a Kepler Object of Interest or KOI, meaning it’s a candidate transiting planet, not a confirmed detection. Its current designation is KOI-172.02. The fullest account I could find through a casual search was the NASA press release of January 7 announcing 461 new planet candidates. The authors, alas, were stingy with their details. They write that “four of the potential new planets are less than twice the size of Earth and orbit in their sun’s habitable zone,” but with a single exception they never mention which four KOIs those might be. The only member of the Goldilocks quartet to be identified by number was 172.02, which is described as “approximately 1.5 times the radius of Earth” and orbiting in a period of 242 days. Not much to go on, and certainly no hint as to how far away this putative system might be.

Statements by astronomers in various news stories offered a tad more detail. Natalie Batalha noted that the host star has spectral type G, and Mario Livio speculated, “Maybe there's no land life, but perhaps very clever dolphins.”

Yesterday I finally had time to dig around. I consulted the nifty NASA Exoplanet Archive, which maintains a list of all KOIs with the most recent data. From that archive I constructed my own short list of potentially Earthlike KOIs. Given the currency of the parameters, it’s more robust than similar lists that have appeared over the past two years. Table 1 presents my back alley distillation of the latest numbers:

Table 1. Current Kepler Objects of Interest (KOI) with potentially Earthlike equilibrium temperatures

Column 1 gives the KOI designation; column 2 the planet period in days; column 3 the semimajor axis in astronomical units (AU); column 4 the planet radius in Earth units (Rea); column 5 the planet equilibrium temperature (Teq); column 6 the stellar effective temperature (Teff), and column 7 the stellar radius in Solar units (Rsol). Temperatures are in Kelvin.

-------------------------------------------------------

I used an arbitrary lower limit of 4000 K for stellar Teff to screen out the squirmiest M dwarfs, and my cut-offs for planet Teq were guided by published values for Venus, Earth, and Mars (not all necessarily reliable). Let me go on the record as a moaner and complainer about the scarcity of consistent Teq data for the Solar planets. I spent a lot of time Googling but turned up no reliable sources other than Wikipedia, which tells me the following: the mean surface temperature of Mars might be 210 K; the temperature at Venus’ cloud tops might be an oddly chilly 230 K (although as everybody agrees, the Venusian surface temperature is about 735 K); the Teq of Earth might be 255 K; and the mean surface temperature of Earth might be 288 K. Those numbers inspired my upper and lower limits of 210 and 300 K for Table 1.

To select planetary radii, I used the rule of thumb that any object larger than 2 Rea most likely has a hydrogen atmosphere, especially objects in the Super Earth mass range (which today I’m calling 2 Mea-9 Mea). Fortunately, Li Zeng and Dimitar Sasselov just circulated a preprint of their new article on the mass-radius relationship for solid planets, which updates the scales published last year by various Kepler scientists (Zeng & Sasselov 2013). According to their new chart, a planet of 1.4 Rea (e.g., KOI-3010.01) is likely to have a mass between 2 and 5 Mea, with the most lightweight possibility involving a layered composition of 25% rock/metal and 75% water, and the most heavyweight involving a collisionally stripped core composed mainly of metal. But since nobody knows if such severe collisional stripping ever happens outside numerical simulations (Marcus et al. 2010), a more likely upper mass limit would be 3.5 Mea, corresponding to the same rock/metal structure as Earth. Thus, 1.4 Rea is a suitable radius for anybody in search of singing cetaceans or calculating lizards.

By the time we get to 1.9 Rea (e.g., KOI-2762.01), outcomes become much fuzzier. That girth might represent an all-water world with a mass of 3 Rea, a 50-50 water plus rock/metal planet with a mass of 4.5 Mea, or a truly terrestrial planet with a rock/metal composition like Earth and a chunky mass of 10-12 Mea. Still worse, for planets on temperate orbits (i.e., all the KOIs in Table 1), it could also include an ice/rock or rock/metal planet with a hydrogen atmosphere and a mass between 3 and 8 Mea. As a result, I refuse to get excited by any candidate bigger than 1.7 Rea.

Now I’m more curious than ever to know which three KOIs apart from 172.02 were nominated by the Kepler folks for the title of “potentially possibly habitable.” Several of the objects in Table 1 have been noted in passing in previous mission publications (Kaltenegger & Sasselov 2011, Batalha et al. 2012), but without any fanfare whatsoever.

The current poster child, KOI-172.02, has the longest orbital period and the widest semimajor axis of any object on my list, and not coincidentally it’s the one that’s least likely to be tidally locked. That seems to be the main reason why it was singled out. 

According to Franck Selsis and colleagues, tidal locking can be predicted for planets on circular orbits within a radius of 0.6 AU around a star of 0.9 Msol; within 0.5 AU around a star of 0.8-0.6 Msol; and within 0.45 AU around a star of 0.5 Msol (Selsis et al. 2007). A moderate orbital eccentricity would be the only protection against locking into a permanent dayside and a permanent nightside, since an elliptical orbit like Mercury’s, with an eccentricity of 0.2, is more likely to result in spin-orbit resonance than in a 1:1 lock of rotation and revolution. Since most of the stars in Table 1 have Teq between 4000 and 5000 K, and since the main sequence mass range corresponding to that temperature range is about 0.6-0.85 Msol, all the associated KOIs will have experienced some form of spin-orbit synchronization. Not that 1:1 tidal locking or a higher-order spin-orbit resonance (e.g., 2:1, like Mercury) necessarily poses any major threat to habitability – it just wouldn’t be Earthlike.

Other data on 172.02 make it less attractive as a candidate Earth twin. The host star's Teff is actually 100 K higher than the temperature of our Sun, while the planet's semimajor axis is only a little larger than that of Venus. The result is a planet substantially hotter than Earth. It would take a very special mix of physical conditions to create a habitable environment out of such an object, even in the best-case scenarios: a water world of 2 Mea or a rocky planet of 5 Mea, either of which is consistent with a radius of 1.54 Rea. My favorites in Table 1 are 701.03 and 3010.01. Both are cooler than 172.02, and as we saw above, the radius of 3010.01 is consistent with a biophilic composition.  Both planets would be tidally synchronized with their host stars, but that's not a deal breaker.

The bottom line is that we shouldn’t break out the good champagne for any of these KOIs. None has been confirmed even as a planetary candidate, and all associated data are subject to revision. Some of these interesting objects may turn out to be false positives; others may have their parameters redefined in ways that would exclude them from consideration of habitability. Regardless, we can still find solace, however small, in the fact that astronomers have finally identified a handful of intriguing and so far viable contenders for the title of Earth 2. Confirmation of one of them, or of some still unpublicized KOI or candidate, might come sometime in 2013.

Figure 2. Another Earth. Has anyone on this Earth actually seen that movie?

And yet – in the current Zeitgeist, is it already too late to celebrate another Earth? Science headlines (as well as some publicity-seeking astronomers) have cried “Earth twin!” for so long now that their audience has gotten jaded. When the NASA news on KOI-172.02 perfused cyberspace this past Thursday, a blogger with the Washington Post, Alexandra Petri, responded tartly with a post entitled No More New Planets, Please. Most of the 61 accumulated comments shared Ms. Petri’s lack of enthusiasm, as the online discussion quickly devolved into jokes and bickering about politics, religion, and the end of the world. One early commenter identified as “felpin” made me laugh out loud: “I turned off the science news on my Google news because [of] the endless parade of planets .... almost as boring as the Kardashian news.”

So what’s the antidote for Kardashian exoplanets?

Less hype and more context in astronomical journalism. More cautious choice of sound bites in press releases and media conferences. Richer presentation of detail in all media. More education of the curious about the mysteries and wonders of our planet-haunted universe.

In short, this blog.

POSTSCRIPT, Jauary 17
The morning after I uploaded this post I saw a brand-new preprint by Eric Gaidos on the very same topic - “Candidate Planets in the Habitable Zones of Kepler Stars.” I was perplexed by the fact that for candidates around 2 Rea or smaller, his list and my list show minimal overlap. Even for overlapping systems, the parameters he reported sometimes differed from the ones shown on the NASA site. I gather that Gaidos used the brand-new data release but conducted his own analyses to recalibrate many values. For the record, the systems that we identified in common are KOI-701.03, KOI-2418.01, and KOI-3010.01, but Gaidos presents a much larger radius for the first of these (2.04 Rea), which would have made me disqualify it. He also reports an age that is only one-eighth of what NASA says, and a stellar effective temperature that is much hotter. Notably, Gaidos doesn't include NASA's poster child, KOI-172.02, and generally his list seems to have few KOIs with only 3 digits preceding the decimal point. Puzzling.

My takeaway from this exercise is that when you're trying to understand potential Earth twins located hundreds of parsecs away, you can be pretty darn sure that most or all of your data are unreliable. Gaidos notes that different analyses of the Teff of Kepler stars may disagree by 200 K, which is more than enough to push planetary Teq out of the habitable range. So, for now, most everybody seems to be engaging in elaborate hand-waving.

References

Batalha N, Rowe JF, Bryson ST, Barclay T, Burke CJ, Caldwell DA, and 66 others. (2012) Planetary candidates observed by Kepler III: Analysis of the first 16 months of data. In press. Abstract: http://adsabs.harvard.edu/abs/2012arXiv1202.5852B
Gaidos E. (2013) Candidate planets in the habitable zones of Kepler stars. Abstract: http://adsabs.harvard.edu/abs/2013arXiv1301.2384G 
Harrington JD, Johnson M. (2013) NASA’s Kepler mission discovers 461 new planet candidates. Release 13-008. Monday, January 7, 2013. http://www.nasa.gov/home/hqnews/2013/jan/HQ_13-008_KEPLER_New_Planets.html
Kaltenegger L, Sasselov D. (2011) Exploring the habitable zone for Kepler planetary candidates. Astrophysical Journal Letters 736, L25
Marcus RA, Sasselov D, Lars Hernquist, and Sarah T. Stewart. (2010) Minimum radii of Super-Earths: Constraints from giant impacts. Astrophysical Journal Letters 712, L73–L76.
Selsis F, J. F.Kasting, B. Levrard, J. Paillet, I. Ribas, and X. Delfosse. (2007) Habitable planets around the star Gliese 581? Astronomy & Astrophysics 476, 1373-1387.
Zeng L, Sasselov D. (2013) A detailed model grid for solid planets from 0.1 through 100 Earth masses. In press. Abstract: http://adsabs.harvard.edu/abs/2013arXiv1301.0818Z