Subterrestrials

 Figure 1. Subterrestrial planets orbiting Sun-like stars, with the Earth included for comparison.

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Over the past few weeks, Kepler Mission scientists have reported two new transiting systems containing members of a tiny but growing extrasolar population: planets smaller than Earth, which I christen “subterrestrials” (Figure 1). Although objects of this size account for more than one-third of the planets and all of the spheroidal moons and dwarf planets in the Solar System, they represent only 2% of all transiting planets, and less than 1% of the vaguely defined census of confirmed exoplanets. (I say “vaguely” because the Extrasolar Planets Encyclopaedia currently lists 861 exoplanets discovered by all methods, but it omits some of the newest and smallest transiting subterrestrials. Qui peut dire pourquoi?)

The two newest systems are especially interesting (Figure 2). Kepler-68, the subject of a forthcoming article by Ronald Gilliland and colleagues, centers on a G-type star almost identical in mass to our Sun, although it is evidently older, hotter, and more bloated in radius. The star’s three companions define an inner system that includes two transiting low-mass planets (b and c) and an outer system that includes a single non-transiting gas giant (d) whose orbital period is about 580 days. Kepler-68 therefore meets my definition of a mixed-mass system. Still better, it joins the exclusive club of extrasolar systems that contain at least one gas giant and at least two low-mass planets (like our Solar System). Since most known mixed-mass systems (e.g., 55 Cancri, Mu Arae) contain only one low-mass planet, that club previously included just three members: GJ 876, HD 10180, and Kepler-30. Even with its enlarged membership, only two systems, HD 10180 and Kepler-68, present low-mass planets in adjacent orbits plus a gas giant on a wider orbit. This arrangement is one of the most distinctive features of the architecture of our Solar System.

Figure 2. Kepler-37 and Kepler 68, two new systems with subterrestrial and Super Earth planets, represented at the same scale. Orbital dimensions are measured in astronomical units (AU). Red lines mark the planets of Kepler-68; blue lines mark those of Kepler-37. Pink fill indicates rock/metal composition; purple indicates an additional water or hydrogen envelope, or both. Only one of these objects has an estimated mass: Kepler-68b, at 8.3 Earth masses (Mea). Kepler-68 also harbors a third planet, a Jupiter-mass giant with a semimajor axis of 1.4 AU, similar to the orbit of Mars.

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Nevertheless, it was the second new subterrestrial system that got all the headlines: Kepler-37, announced in Nature (Barclay et al. 2013), featured in Wikipedia news, and reported in global media outlets ranging from the Los Angeles Times to the Malaysia Chronicle. The system’s primary appears to be a K-type star whose mass is 80% of Solar (0.80 Msol). Three low-mass planets have been detected, all transiting and all confined within an astrocentric radius of 0.21 astronomical units (AU). The international flood of headlines originated in the fact that Kepler-37b is the smallest exoplanet yet detected (der kleinste Exoplanet! il più piccolo esopianeta!) – so small that it barely exceeds the diameter of the Earth’s Moon. Its characterization represents a new frontier in exoplanetary astronomy.

With these additions we now have at least seven subterrestrial exoplanets, orbiting in four different Kepler multiplanet systems (Table 1). Their host stars include a very small M dwarf, Kepler-42, which is similar in mass to GJ 1214 and Barnard’s Star; a K-type star, Kepler-37; a G-type star, Kepler-20, which is similar in metallicity to our Sun but less massive by 9%; and a more massive and metal-rich G-type star, Kepler-68. The range of masses and metallicities represented by this group implies that low-mass rocky planets like those in our Solar System are common in the Milky Way Galaxy, whether in habitable orbits or not. 

Table 1. All confirmed subterrestrial exoplanets. Column 2 lists the planet radius in Earth units (Rea); column 3, the orbital period in days; column 4, the semimajor axis in AU; column 5, the estimated equilibrium temperature in Kelvin; column 6, the distance of the star in parsecs; column 7, the stellar metallicity; column 8, the stellar mass in Solar units (Msol); and column 9, the stellar radius in Solar units (Rsol).

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With their high equilibrium temperatures and small radii, most or all of the objects in Table 1 must be rocky. Planets so lightweight have difficulty retaining volatiles; they cannot sustain hydrogen envelopes. Most, if not all, are also too warm to retain ices. They may be cousins of Mercury, which after all is one of the Earth’s siblings.

Another potential subterrestrial object, KIC 12557548 b, has been interpreted as a solitary companion to a K-type star of 0.70 Msol located about 470 parsecs away (Rappaport et al. 2012). The star exhibits unusual transits at intervals of less than 16 hours: regular in period but irregular in depth. Two studies have explained this behavior as the signature of a low-mass rocky planet undergoing catastrophic disintegration (Rappaport et al. 2012, Perez-Becker & Chiang 2013). In these models, the disintegrating planet is less massive than Mars. Successive transits vary in depth because the planet constantly sheds a cloud of dust that streams after it like a comet’s tail. As the cloud disperses semi-chaotically, it varies in size, and the area of the star occulted during each transit varies along with it. Perez-Becker & Chiang detail a scenario in which the planet was originally similar to Mercury (0.06 Mea) but by now has lost 80% of its mass, so that it is as lightweight as the Moon. At the implied rate of loss, this object will disappear completely within 100 million years. Given its peculiar physical status, I don’t include it in my personal census of subterrestrials.

The emergence of this new subpopulation of small planets adds substantially to our understanding of the likely distribution of Earth-like planets in the Galaxy. We now know that low-mass planets can range continuously from less than the mass of Mercury to twice the mass of Uranus. They occur preferentially alongside similar planets, so that a single system (e.g., Kepler-20 or Kepler-68) can harbor both rocky terrestrial planets (like Venus) and more massive gas dwarfs (like Uranus) in close proximity. Unfortunately, the known terrestrials and subterrestrials are still confined to short-period orbits, with very few detected on orbits longer than 100 days. Further collection and analysis of Kepler data should correct at least some of that bias.

REFERENCES

Barclay T, Rowe JF, Lissauer JJ, Huber D, Fressin F, Howell SB, and 58 others. (2013) A sub-Mercury-sized exoplanet. Nature 494, 452-454.

Gilliland RL, Marcy GW, Rowe JF, Rogers L, Torres G, Fressin F, and 27 others. (2013) Kepler-68: Three planets, one with a density between that of Earth and ice giants. Astrophysical Journal, in press.

Perez-Becker D, Chiang E. (2013) Catastrophic evaporation of rocky planets. Monthly Notices of the Royal Astronomical Society, in press.

Rappaport S, Levine A, Chiang E, El Mellah I, Jenkins J, Kalomeni B, Kotson M, Nelson L, Rousseau-Nepton L, Tran K. (2012) Possible disintegrating short-period Super-Mercury orbiting KIC 12557548. (2012) Astrophysical Journal 752, 1.