Friday, April 19, 2013

Holy Grail, Earthman!

Figure 1. Artist’s view of Kepler-62f, a likely terrestrial planet orbiting in the habitable zone of an amber K2 star located about 368 parsecs (1200 light years) away in the constellation Lyra.
It’s happened. The Holy Grail of exoplanetary science has been glimpsed, if not yet grasped. Yesterday the Kepler Mission reported a transiting planet, most likely of rocky composition, orbiting in the habitable zone of its amber host star. This planet, Kepler-62f, has an estimated diameter of 17,985 km (11,140 miles), corresponding to 1.41 Earth radii (Rea), and an equilibrium temperature (Teq) of 205 K, similar to the Teq of Mars. It also has four inner companions, one of which (Kepler-62e) may be a second candidate for habitability. While neither Kepler-62e nor Kepler-62f can honestly be described as “Earth Twins,” The New York Times declares that they’re still “promising places to live.” (And if the Times doesn’t know real estate, who does?)

No transit timing variations have been observed for any of the planets in this system, nor did a radial velocity search detect any variability consistent with planetary motion. These null results mean that we have no secure way to determine the mass of any of these planets, except by using theoretical models to estimate their likely composition and density.

According to the latest mass-radius relationships published by Lissauer et al. (2013), the radius of Kepler-62f is consistent with a range of masses and compositions, depending on whether it is entirely rocky or whether it has a significant icy component. At its likely maximum mass of 3.5 times Earth (3.5 Mea), 62f would have exactly the same iron/silicate composition as our home planet, and thus qualify as a true Super Earth. Exchanging some proportion of rock and metal for ices would result in increasingly lower masses, down to about 1.5 Mea for a planet that is 75% rock and 25% ice, and less than 1 Mea for a half rock, half ice planet. With its larger radius of 1.61 Rea, Kepler-62e might be an iron/silicate planet of 6.5 Mea, or it might be an Earth analog of 1 Mea with a 1% hydrogen atmosphere. If it has an icy component, its mass could range from about 2.5 Mea for a 75% rocky/25% icy composition, down to 1 Mea for a half rock, half ice planet.

Borucki et al. (2013) favor the rocky or icy options, arguing that both planets have probably “lost their primordial or outgassed hydrogen envelope.”   

Table 1. Parameters of the Kepler-62 system

Rea = radius in Earth units; Period = orbital period in days; a = semimajor axis in astronomical units (AU); e = orbital eccentricity; Teq = equilibrium temperature in Kelvin.


As it turns out, Kepler-62e is just the grown-up name for KOI 701.03, which I discussed three months ago in a posting titled Earth 2. Back then, available data indicated a slightly smaller radius and a slightly lower Teq, making 701.03 one of my favorite candidates for a second Earth. The new data make it less attractive, but they compensate by yielding the unexpected planets 62c  –  a Hot Mars!  and 62f, our likeliest habitable Super Earth.

Table 1 and Figure 2 summarize the system architecture, which is very interesting. We see five low-mass planets orbiting in a region that is equivalent, temperature-wise, to the Solar System inside the orbit of Mars. Four out of five planets must be less massive than 10 Mea (as constrained by their radii and thermal environment), while the largest planet (62d) has an upper mass limit of 14 Mea.

The orbits of the three inner planets are closely packed within a semimajor axis of 0.12 astronomical units (AU). Among low-mass planets, this configuration is already quite familiar. If 62e and 62f had not been observed in transit, the reduced three-planet system would look much like a dozen other confirmed Kepler systems with two or three planets.

Given their small radii and high Teq, the two inner planets must be rocky. Kepler-62b is probably about 2.5 Mea; it is also hot, desolate, and probably airless. Kepler-62c is a new example of the growing class of extrasolar subterrestrials: similar in diameter to Mars, and probably similar in mass also. We can imagine it as an enlarged Mercury, slightly cooler but just as inhospitable.

Kepler-62d is more difficult to assess. With a very modest hydrogen/helium atmosphere, it could be as lightweight as 4 Mea, although this model is disfavored by Borucki et al. With a composition equivalent to Earth, it would reach its upper limit of 14 Mea. More likely is a mass somewhere between these extremes, with a structure that includes a water layer above a core of rock and metal, and possibly some contribution from hydrogen. 

Figure 2. Orbital architecture of the Kepler-62 system. Planets are shown at their relative sizes. 

The host star is a dim K2 dwarf with a mass of 0.69 Msol, a radius of 0.64 Rsol, an effective temperature of 4925 K, and a luminosity only 21% Solar. The star also has a remarkably low metallicity of -0.37 (all values Borucki et al. 2013). Although stellar enrichment in metals is strongly associated with the formation of gas giants like Jupiter and Saturn, recent findings by radial velocity and transit searches indicate that small rocky planets are likely around stars even less metallic than Kepler-62.

Although the orbits of 62e and 62f are similar in period and semimajor axis to those of Mercury and Venus, respectively, the low effective temperature of the host star means that their respective thermal environments are more reminiscent of Venus and Mars. Lisa Kaltenegger, one of the co-authors of the discovery paper, opined that Kepler-62e – the “Super Venus” – probably has temperatures “like Washington in May” (although she didn’t say whether she meant the balmy District of Columbia or the damp Northwestern state). However, if that planet’s atmosphere has significant greenhouse gases, it would be much hotter than any inhabited region of Earth, even DC in August.

For Kepler-62f, greenhouse gases would make the difference between a frigid environment with limited or nonexistent surface water and a truly biophilic ocean planet like Earth. Naturally, Kepler astronomers favor optimistic scenarios for both planets; Kaltenegger predicts “endless oceans.”

According to models published by Franck Selsis et al. (2007), planets orbiting within 0.5 AU of a K2 star like Kepler-62 will be tidally locked. This constraint applies to all of the system’s planets except for Kepler-62f, which is likely to have a rapid rotation like Earth, Mars, or Jupiter. Even if its hotter sibling, Kepler-62e, has a permanent dayside and nightside, life-friendly environments are still possible there under the right conditions.

As we have seen over the past decades, early exoplanetary data are always tentative, so that subsequent observations and analyses can shatter any rosy mind-picture we may have formed of this or that alien world. Nevertheless, enough solid evidence has now accumulated to demonstrate that Earthlike planets must orbit in the habitable zones of star systems across our Galaxy. And it looks increasingly likely that they can be found even in systems like Kepler-62, which bears no resemblance to home. At last, at last we can celebrate: we have a plausible candidate for Earth 2.

Figure 3. The Ace of Cups, signifier of joy, abundance, fertility, and nourishment

Lissauer JJ, Jontof-Hutter D, Rowe JF, Fabrycky DC, Lopez ED, Agol E, et al. (2013) All six planets known to orbit Kepler-11 have low densities. In press:   
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 Express 18 April 2013. 10.1126/science.1234702
Selsis F, Kasting JF, Levrard B, Paillet J, Ribas I, and Delfosse X. (2007) Habitable planets around the star Gliese 581? Astronomy & Astrophysics 476, 1373-1387.

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