Kepler-452b: Latest Hope for Another Earth

Figure 1. Imaginary view of the surface of an Earth-like exoplanet, created to illustrate the formal announcement of Kepler-452b, a Super Earth or gas dwarf orbiting a metal-rich yellow star. Unfortunately, the estimated radius of this object is 63% larger than Earth, This value is inconsistent with an Earth-like composition. Credit: SETI Institute/Danielle Futselaar 

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Earth 2.0! Again! No really! This time the hype machine seems to be on overdrive. After decades of waking to National Public Radio, the first time I ever heard an exoplanet headline on the morning news was Thursday, July 23, and it was all about Kepler-452b. More details emerged over the course of the day (e.g., video, press release), and eventually I found a copy of the official discovery paper by Jon Jenkins and colleagues.

With that data under my skull, I can tell you that the planet in question is no match for Earth. The host star and the orbital elements are everything you could wish for – spectral type G2, local year longer than ours – but the gosh darn radius (1.63 times Earth) is just too big. I noticed skepticism even in mainstream news accounts, which generally referred to “Earth’s cousin” instead of “Earth’s twin,” along with murmurs that an object that is (or was) possibly potentially habitable under certain improbable conditions isn’t really all that sexy. Indeed, Wired suggested that the whole story was “kind of meaningless.”

Table 1 summarizes the confirmed Kepler planets that have been proposed to date as Earth-like worlds orbiting in their host stars’ habitable zones (HZ). (For other postings on this topic in 2015, see Much Ado About Earth 2 and What Kepler Hasn’t Told Us.)

Table 1. Small, Cool Extrasolar Planets

Column 1 presents the host star’s name; column 2 the stellar effective temperature (Teff) in Kelvin; column 3 the stellar mass in Solar units (Msol); column 4 the stellar metallicity; column 5 the stellar age in billions of years (Gyr); column 6 the distance to the system in parsecs; column 7 the planet name; column 8 the KOI number; column 9 the planet radius in Earth units (Rea); column 10 the orbital semimajor axis in astronomical units (Earth’s orbit = 1); column 11 the planet equilibrium temperature (Teq) in Kelvin; and column 12 the orbital period in days. 

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Kepler-452b stands out from the pack in several ways. Most important are the statistics on its host, whose mass and effective temperature are almost identical to our Sun’s. Accordingly, a habitable orbit in this system exceeds 300 days. Thus Earth’s new cousin has one of the longest orbits in the Kepler dataset, as well as the widest semimajor axis of any HZ candidate.

At 430 parsecs (1400 light years), this system is also more distant than any of the others, suggesting that detectable Earth-size planets around G-type stars are notably less abundant than those around M or K stars. Kepler-452 itself is also older than all but one star in Table 1, with a radius 11% larger than the Sun’s (data not shown), indicating that it is expanding and growing hotter with age. These factors result in an estimated equilibrium temperature (Teq) of 265 K for Kepler-452b. This is a little hotter than Earth, which has Teq of 255 K and a mean surface temperature of 288 K. 

Kepler-452 is also the most metallic star in Table 1, raising the chances that it might harbor one or more gas giants in addition to its single confirmed planet. Analogs of Jupiter or Saturn on wider orbits would make a very interesting system architecture; maybe future radial velocity observations can assess their potential presence. 

But the least appealing superlative associated with Kepler-452b concerns its radius, which at 1.63 Rea is by far the largest in Table 1. Including error margins – minus 0.20, plus 0.23 – blurs the picture a bit, but overall makes it uglier. At 1.43 Rea, Kepler-452b could be an iron/silicate planet of about 3 Earth masses (3 Mea) or a 50% rock/50% ice world of 1.5 Mea (Dressing et al. 2015). The iron/silicate option would make it a good “potentially habitable Super Earth,” but the icy option would raise questions about the chemical diversity needed for biogenesis. At the preferred value of 1.63 Rea, however, an Earth-like composition with an iron core and a silicate mantle would raise the mass to 5.5 Mea (Dressing et al. 2015), which is arguably too high for plate tectonics or a carbon cycle. A 50% rock/50% ice composition would correspond to a mass of 2.5 Mea. More likely for a radius around 1.6 Rea, however, is a planet with the same mass as Earth and a puffy atmosphere with 1% hydrogen/helium (Rogers 2015), corresponding to a lifeless Hellworld. 

Finally, at the high value of 1.86 Rea, current theoretical models would readily define Kepler-452b as a low-mass, low-density gas dwarf with a substantial hydrogen/helium envelope (Rogers 2015, Dressing et al. 2015, Wolfgang et al. 2015). Indeed, one recent study (Lammer et al. 2014) suggests that virtually all planets of 2 Mea or more orbiting in the circumstellar HZ will retain hydrogen atmospheres. The authors singled out Kepler-62f – perhaps the most impressive candidate in Table 1 – as a likely gas dwarf with an extensive hydrogen envelope, presenting a detailed argument to invalidate claims by Borucki et al. (2013) that the planet could have an Earth-like composition. Ouch!

And so – while I can’t retroactively cancel my joyous response to the announcement of Kepler-62f – the skepticism I’ve honed over the duration of the Kepler mission stifles any similar rejoicing for Kepler-452b. That doesn’t mean I’ve lost my optimism regarding the possibility of complex life beyond the Earth. Indeed, Kepler data demonstrate that small planets are abundant in the Milky Way, and what we know about the evolution of the Earth argues that some of those planets are bound to sustain life. But it’s equally plain that on orbits of 100 days or more, Earth-size planets are no more common (and probably less common) than small gas dwarfs of 2-3 Rea. Even telescopes as sensitive as Kepler have great difficulty detecting Earth-size planets on such long orbits. 

present ideals & future likelihoods 

Ideal radii for potentially habitable terrestrial planets are about 1.3 Rea and smaller, implying iron/silicate planets of 2.5 Mea or less. At 1.2 Rea, the corresponding mass is about 2 Mea, while at 1.1 Rea, it’s 1.5 Mea. That relatively narrow range of masses and radii appears to define the population of bona fide Super Earths. Larger radii require either large quantities of rocky mass that might jeopardize geologic activity; fractions of ices that might be too high to enable mixing of heavy elements with water; or hydrogen/helium envelopes that would rule out water in its liquid state altogether. Indeed, a version of Earth scaled up to 10 Mea (a mass so far not attested for purely rocky objects) would have a radius of only 1.9 Mea. 

As we’ve seen, Kepler isn’t very sensitive to planets of 1.3 Rea or less. A recent iteration of the Extrasolar Planets Encyclopaedia listed 160 transiting planets in this range, representing about 15% of the confirmed Kepler sample. The vast majority have orbits shorter than 25 days. Only six occur in systems with a single transiting planet (a description that applies to Kepler-452). This bias toward multiple companions has everything to do with the protocols used to validate Kepler candidates. Members of multi-planet systems are much easier to confirm than singletons, which might require data on transit timing or radial velocity variations to ensure their reality. Yet future confirmations of Kepler planets, including those in the desired range of radii and masses, are likely to involve increasing numbers of singleton systems. The low-hanging fruits of planetary multiplicity have already been mostly plucked.  

In the discovery paper, Jenkins and colleagues include an interesting paragraph on potential system architectures for Kepler-452. They calculate that if the system included a Venus analog with the same inclination to planet b’s orbital plane as Venus to Earth, the likelihood of its detection (along with Kepler-452b) would be only 10%. An analog of Mars would be still more difficult to detect, while all three planets together would have just a 2% chance of discovery.

Could the likeliest Earth-like planets in the Kepler dataset turn out to be in systems with only one transiting object?

REFERENCES
Borucki WJ, Agol E, Fressin F, Kaltenegger L, Rowe J, Isaacson H, et al. (2013) Kepler-62: A five-planet system with planets of 1.4 and 1.6 Earth radii in the habitable zone. Science 340, 587-590. DOI:10.1126/science.1234702 Abstract: http://adsabs.harvard.edu/abs/2013Sci...340..587B. 

Dressing CD, Charbonneau D, Dumusque X, Gettel S, Pepe F, Cameron AC, et al. (2015) The mass of Kepler-93b and the composition of terrestrial planets. Astrophysical Journal 800, 135. Abstract: http://adsabs.harvard.edu/abs/2015ApJ...800..135D 
Jenkins J, Twicken JD, Batalha NM, Caldwell DA, Cochran WD, Endl M, et al. (2015) Discovery and validation of Kepler-452b: A 1.6 Rea Super Earth exoplanet in the habitable zone of a G2 star. Astronomical Journal 150, 56. 
Lammer H, Stokl A, Erkaev NV, Dorfi EA, Odert P, Gudel M, Kulikov YN, Kislyakova KG, Leitzinger M. (2014) Origin and loss of nebula-captured hydrogen envelopes from ‘sub’- to ‘super-Earths’ in the habitable zone of Sun-like stars. Monthly Notices of the Royal Astronomical Society 439, 3225-3238. Abstract: http://adsabs.harvard.edu/abs/2014MNRAS.439.3225L 
Rogers L. (2015) Most 1.6 Earth-radius planets are not rocky. Astrophysical Journal 801, 41. Abstract: 2014arXiv1407.4457R 
Wolfgang A, Rogers LA, Ford EB. (2015) Probabilistic mass-radius relationship for sub-Neptune-sized planets. Astrophysical Journal, in press.