Saturday, January 17, 2015

Much Ado About Earth 2

Earth 2 + N: Icons of small transiting exoplanets at their relative sizes, with Earth at the same scale. All these objects are believed to occupy their systems’ habitable zones. To suggest the potential range of morphologies, the extrasolar icons represent a warm ocean planet, an icy ocean planet, a temperate planet with continents and seas, and a Big Mars. Assignment of surface structure and atmosphere is arbitrary, since any of these objects might be an ocean planet or a Big Mars. The three smallest Kepler planets are likely to be tidally locked, so all three appear as Eyeball Earths with standing cyclones around the substellar point. Only Kepler-62f is likely to rotate like Earth and Mars.
Eleven days ago, just in time for Twelfth Night, a research team led by Guillermo Torres reported the “Validation of Twelve Small Kepler Transiting Planets in the Habitable Zone.” The cybermedia loved the story, broadcasting many thrilling variations on the theme of Most Earthlike Planet Yet! Now that the piping pipers and dancing ladies have come and gone, I’ve found time to look behind the barrage of news items and unwrap these Gifts of the Magi.

In fact, Torres and colleagues have brought us only two new Earth-like objects (though some popular accounts suggested other numbers, ranging from one to eight to a thousand). Their official names are Kepler-438b and Kepler-442b. Torres’ collaborators, most of whom are associated with the Kepler Mission, started their search for another Earth 2 about a year ago. This was some time after the announcement of Kepler-62f, the first plausibly terrestrial planet orbiting in an extrasolar habitable zone. They began by selecting all Kepler candidates then believed to be in the habitable zone and smaller than 2.5 Earth radii (2.5 Rea). Then they conducted extensive follow-up observations and analyses to derive the most robust parameters possible for each object of interest.

In the end Torres and colleagues were able to validate a dozen of these smallish planets, though the status of the twelfth is less secure than the others. Meanwhile, a subgroup of his collaborators reported one of the most Earth-like candidates (Kepler-186f) separately from the rest. Then another of those candidates (Kepler-296f) was found to orbit a member of a close binary system, making the planet’s characterization more difficult. Of the remainder, three are clearly larger than 2 Rea, ruling out a terrestrial composition, and only two are smaller than 1.5 Rea, widely regarded as the approximate upper boundary for Earth-like composition. Those two objects were at the center of last week’s hoopla. 

Along with Kepler-62f and Kepler-186f, we now have four robust candidates for terrestrial composition and surface water. And yes, the new ones, especially Kepler-438b, compare very favorably with the earlier candidates, as shown in Table 1:

Four Potentially Earth-like Extrasolar Planets

Column 1 represents the star name; column 2 the stellar effective temperature in Kelvin; column 3 the stellar mass in Solar units; column 4 the stellar metallicity; column 5 the stellar age in billions of years; 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; column 10 the orbital semimajor axis in astronomical units (Earth’s orbit = 1); column 11 the planet equilibrium temperature in Kelvin; and column 12 the orbital period in days. Data on equilibrium temperatures, and all data on Kepler-62, derive from the Kepler Discoveries Table. Other data follow Torres et al. 2015.
If each of these objects had the same iron/silicate composition as Earth, their respective masses would be about 1.5, 1.6, 2.5, and 3.5 Mea (Zeng & Sasselov 2013, Lissauer et al. 2013). Although there is probably an upper mass limit for the habitability of a purely rocky planet, based on the reduced likelihood of plate tectonics at high mass, no consensus has emerged on what the limit might be. My conservative guess is about 3 Mea, disfavoring Kepler-62f.

A somewhat wetter composition – approximately 10% ice, 90% iron/silicate – would reduce the two smaller objects to about 1 Mea and the two larger to about 2 Mea. Such a large watery component, however, would most likely render all these planets uninhabitable, because surface water would be isolated from core metals by an ice layer, and thus would lack the chemical diversity believed necessary for the emergence of life (Alibert 2014).

Regarding planetary climates, the table above presents the equilibrium temperatures (Teq) estimated by Kepler Mission scientists. All planets except Kepler-186f have Teq within the traditional limits (i.e., 185-303 K; Kopparapu et al. 2013). These limits are based on the fact that Earth, with its Teq of 255 K and mean surface temperature of 288 K, is securely parked in a habitable space. However, many studies over the past few years have debated the definition of the habitable zone (e.g., Zsom et al. 2013) as well as the use of Teq in this definition (e.g., Kastings et al. 2014). Some astronomers have argued for extending the habitable zone’s inner limits (Seager 2013), while others have argued against it (Kastings et al. 2014). Amid these debates, all four planets remain promising, even Kepler-186f. Its mass predicts a substantial atmosphere whose greenhouse effect could raise surface temperatures appropriately.

Another key factor in understanding surface conditions on an extrasolar planet is rotation. Three out of four planets in Table 1 orbit within their host star’s tidal locking radius (Selsis et al. 2007). Therefore, their rotation is likely to be “synchronous” with their orbits – in other words, they always turn the same hemisphere toward their suns. A possible outcome of synchronous rotation is loss of atmosphere through freeze-out of volatiles on the permanent night side, rendering the planet uninhabitable. Fortunately, studies old and new (Joshi 2003, Yang et al. 2014) provide scenarios that avoid this outcome.

As Yang and colleagues recently argued, the major variables involved are the synchronous planet’s complement of surface water, its geothermal flux (i.e., volcanism and related processes), and the percentage of its night side covered by land. On an ocean world where geothermal flux is strong and scattered archipelagos are the only land, some sea ice would accumulate on the night side, but liquid water would be abundant everywhere. On a world with oceans, continents, and geothermal flux in the range of Earth values, ice sheets would accumulate on night-side continents, but total freeze-out would not occur and clement conditions would prevail. Only a world with low geothermal flux, limited surface water, and a night side covered by continents would build massive ice sheets and suffer complete loss of liquid water. Thus the odds of a tidally locked planet with livable surface conditions seem favorable.

system architecture and habitability
So far, so good. At least three out of four planets remain plausible candidates for the evolution and survival of life, and the two newest arrivals appear very similar to the old ones. However, Kepler-438b and -442b differ from Kepler-62f and -186f in one important way: each of the new candidates is the only detected planet orbiting its host star. Indeed, their loneliness made it more difficult for Torres’ team to validate them, since validation gets progressively easier as the number of transiting candidates per star increases.

I found this detail surprising and a little disappointing. Each of the older candidates is the outermost planet of five, and in both systems, all orbits are mutually well-aligned and all planets are smaller than 2 Rea. These data tell us that the systems had relatively placid dynamic histories, and that their planets are likely to be rich in refractory elements. They are also consistent with our expectation that planets in the range of 1 to 6 Rea occur in compact systems with neighboring planets of similar size.

So what’s going on with Kepler-438 and Kepler-442? Does each star host only one planet within 0.5 AU? Or does each actually host numerous small planets, except that the Earth-like candidates happen to be significantly misaligned with the rest?

As far as I understand, it’s possible for a planetary system to be “flat” (i.e., co-planar with minimal misalignment), yet our viewing angle can be such that only the inner planets are visible in transit, while the outer planets remain just outside of range. Regardless of viewing angle, however, if an outer planet transits, then mutually aligned inner planets should also transit. At face value, then, Kepler-438b and Kepler-442b do not seem to be members of the compact multiplanet systems we’ve come to know and love. I look forward to future investigations on this point.

superhabitable planets?
Debates aside, all four of these candidates still look promising. Their charms were enhanced by a cover story in Scientific American that appeared on the newsstands when Torres et al. announced their findings: “The Hunt for Planets Better Than Earth.” Written by Rene Heller, this article presents a plain-language summary of Heller & Armstrong’s 2014 study of “Superhabitable Planets” in Astrobiology.

Heller starts by noting that Earth is hardly the “best of all possible worlds,” because its host star will eventually evolve off the main sequence and evaporate all our water. (Only 1.75 billion years to go!) If we want a longer-lived biosphere, we need to find planets that orbit M and K dwarfs, since these types will continue burning on the main sequence for many tens of billions of years. Recognizing that M dwarfs are subject to “powerful stellar flares and other dangerous effects,” Heller singles out K dwarfs in particular as occupants of the “sweet spot of stellar superhabitability.” In addition, he argues that planets more massive than Earth – ideally about 2 Mea – are friendlier to life than planets in the mass range of Venus and Earth. This is because more massive planets will probably have higher levels of geothermal flux, which sustains the carbon cycle and maintains the planetary magnetic field, thereby averting both a CO2 greenhouse and atmospheric erosion by cosmic rays over multi-billion year time scales.

All the Kepler planets summarized on this page are approximately consistent with Heller’s criteria, although none provide a perfect fit: The only candidate orbiting a K dwarf is Kepler-62f, which must be about 3.5 Mea if it is purely rocky.
the problem with M dwarfs
The three best Goldilocks candidates, including the two newest, orbit M dwarfs. These relatively dim stars have masses between about 10% and 60%  Solar (0.1-0.6 Msol). They represent the commonest spectral type in the Galaxy, accounting for 75% of the overall stellar population. They also seem to be rich in small planets, and their habitable zones have much smaller radii than the ones around more massive stars. These criteria mean that habitable planets around M dwarfs have much shorter orbital periods than those around Sun-like stars. Shorter periods, in turn, mean that habitable M dwarf planets are more likely to transit than habitable G dwarf planets, and are easier to detect when they do. 

All these circumstances help to explain the fact that, even though Kepler was specifically designed to study Sun-like stars (spectral types G, early K, and late F), only one of the four candidates discussed here (Kepler-62f) has a reasonably Sun-like host.

Yet M dwarfs still have problems. A brand-new study by Luger & Barnes (2015) provides a convenient rundown:

  • M dwarfs typically emit much of their luminosity in X-rays and extreme ultraviolet wavelengths, which can drive atmospheric escape and harm organisms.
  • They are subject to brief flaring events in which they emit a much higher energy flux, which can destroy volatiles and erode atmospheres, especially in planets orbiting in their close-in habitable zones.
  • They spend a long stretch of their formative years at luminosities one to two orders of magnitude higher than they will be after they settle down on the main sequence. Since planetary systems must form during this epoch, gestating M dwarf planets might develop a runaway greenhouse early on and lose all their volatiles.
  • This luminosity evolution also means that the planets we now observe in M dwarf habitable zones probably formed when habitable temperatures were available only at much wider separations from the central star. Such planets would be born bone-dry.
All these concerns cast doubt on the potential of planets orbiting M dwarfs to support Earth-like environments. Instead of Super Earths, M dwarfs might preferentially harbor Super Mercuries and Super Venuses. Three out of four planets in Table 1 would fit the bill.

EPIC 201367065
We can expect to hear about many more small planets orbiting M dwarfs over the next few years. In fact, the successor mission to Kepler, known as K2, has just reported a system of three planets orbiting an M0 star located only about 45 parsecs (147 light years) away, much closer than any of the planets in Table 1 (Crossfield et al. 2015). The host star was evidently missed by the major catalogs of nearby stars (Henry Draper, Gliese, and Hipparcos). As a result, instead of a familiar HD, GJ, or HR designation, it is known by one of the most unmemorable character strings I’ve ever seen: EPIC 201367065. The planets, which received the usual unglamorous designations b, c, and d, have respective radii of 2.14, 1.72, and 1.52 Rea, and respective orbital periods of 10, 25, and 45 days.

EPIC 201367065 System Architecture

Orbits and planets are represented at their relative sizes, according to Crossfield et al. 2015. According to a widely accepted definition of the habitable zone (Kasting et al. 2014), planet d is too hot for liquid water. However, Zsom et al. 2013 provide certain scenarios (which they concede to be rare) in which this planet might be cool enough for habitability.

Depending on one’s definition of the habitable zone, planet d is located either just inside the inner edge (Zsom et al. 2013), indicating potential surface water if certain finely tuned conditions are met, or significantly starward of the inner edge (Kasting et al. 2014), implying temperatures far too high for liquid water. In addition, within error margins, this planet’s radius might be as small as 1.32 Rea, putting it in the same ballpark as Kepler-442b. As the authors note, “this planet [is] a very interesting potential super-Venus or super-Earth.” Cheers and applause all around!

I’d been worrying that K2 would find only boring old Hot Jupiters, so this early return is a very pleasant surprise . . . even if it compounds the problem of too many interesting planets orbiting the wrong kind of star.

Nonetheless, I'm still a bit troubled that after more than three years of data collection by Kepler, and more than a year of additional analyses, we have only one oversized candidate for the status of terrestrial planet in the habitable zone of a K or G star. Are they intrinsically rare, or just hard to find?

Alibert Y. (2014) On the radius of habitable planets. Astronomy & Astrophysics 561, A41.
Crossfield I, Petigura E, Schlieder J, Howard AW, Fulton BJ, Aller KM, Ciardi DR, Lepine S, Barclay T, et al. (2015) A nearby M star with three transiting Super-Earths discovered by K2. In press.
Heller R. (2015) Better than Earth. Scientific American 312, 32-39.
Heller R, Armstrong J. (2014) Superhabitable Worlds. Astrobiology 14, 50-66.
Joshi MM. (2003) Climate model studies of synchronously rotating planets. Astrobiology 3, 415-427. Abstract:
Kasting JF, Kopparapu R, Ramirez RM, Harman CE. (2014) Remote life-detection criteria, habitable zone boundaries, and the frequency of Earth-like planets around M and late K stars. Proceedings of the National Academy of Sciences 111, 12641-12646. Abstract:
Kopparapu R, Ramirez RM, Kasting JF, Eymet V, Robinson TD, Mahadevan S, Terrien RC, Domagal-Goldman S, Meadows V, Deshpande R. (2013) Habitable zones around main-sequence stars: New estimates. Astrophysical Journal 65, 131. Abstract:
Luger R, Barnes R. (2015) Extreme water loss and abiotic O2 buildup on planets throughout the habitable zones of M dwarfs. In press.
Torres G, Kipping DM, Fressin F, Caldwell DA, Twicken JD, Ballard S, Batalha NM, Bryson ST, Ciardi DR, Henze CE, Howell SB, Isaacson HT, Jenkins JJ, Muirhead PM, Newton ER, Petigura EA, Barclay T, Borucki WJ, Crepp JR, Everett ME, Horch EP, Howard AW, Kolbl R, Marcy GW, McCauliff S, Quintana EV. (2015) Validation of twelve small Kepler transiting planets in the habitable zone. Astrophysical Journal, in press. Abstract:
Yang J, Liu Y, Hu Y, Abbott DS. (2014) Water trapping on tidally locked terrestrial planets requires special conditions. Astrophysical Journal Letters 796, L2. doi:10.1088/2041-8205/796/2/L22
Zsom A, Seager S, de Wit J, Stamenkovic V. (2013) Toward the minimum inner edge distance of the habitable zone. Astrophysical Journal 778, 109. Abstract:
Seager S. (2013) Exoplanet habitability. Science 340, 577-581. Abstract:


1 comment:

  1. Good post.

    I thought the flaring problem tended to be more of an issue with smaller red dwarf stars, not the bigger ones.

    I'm also pretty skeptical of Kepler-438b - the figure they gave for its equivalent in solar insolation is 1.4 times that of Earth. It's probably a super-Venus.