Friday, July 5, 2013

Welcome Our Newest Neighbors

Artist’s impression of the view from GJ 667C f, a Super Earth orbiting in the system habitable zone. Looming above the horizon is the host star, GJ 667C , an M dwarf. To the left is an inner planet, GJ 667C c, visible as a crescent. To the right, higher in the sky, is the binary pair GJ 667AB, both K dwarfs. Image credit: European Southern Observatory


[As of September 2014, much of the information in this post has been superseded. For the results of more recent research, see GJ 667C: Just Two Planets.]

Guillem Anglada-Escudé and Mikko Tuomi, rising stars of Bayesian-flavored astronomy, recently returned to the headlines with a rigorous analysis that confirms the presence of six planets around the nearby M dwarf Gliese 667C.

Both separately and together, these two investigators have already produced a remarkable series of re-analyses of existing radial velocity data on stars in the Sun’s back yard, as I’ve discussed here and here. Their latest study, which will appear in Astronomy & Astrophysics, has raised the biggest waves yet, with a nod from Wikipedia’s “In the News” section and immediate acceptance of their findings in the Extrasolar Planets Encyclopaedia. Even better, their results bring closure to a series of conflicting interpretations of the planetary system around GJ 667C.

The headline-grabber in the present investigation is this well-supported finding: not one, not two, but three Super Earths orbit in the host star’s narrow habitable zone, along with at least three other Super Earths on both warmer and cooler orbits.

The host star is the smallest member of a triple star system located just 6.84 parsecs (22 light years) away (EPE). The two larger stars, GJ 667A and GJ 667B, are amber dwarfs of spectral types K3 and K5, sharing a close binary orbit. The third star, GJ 667C, orbits at an estimated separation of 230 astronomical units (AU). It is a dwarf in just about every way. Its mass is 33% Solar (0.33 Msol), its effective temperature is 3350 K (58% Solar), its luminosity is just 1.4% Solar, and its metallicity (proportion of elements heavier than helium) is -0.55, where Solar metallicity = 0. Although the spectral type of star C is usually defined as M1.5, Anglada-Escudé and colleagues suggest that M3 or M4 would be more accurate, given the star’s brightness and metallicity.

The six-planet system they describe represents an architectural type that has become very familiar in recent years: a tightly packed collection of small planets orbiting inside a semimajor axis of 1 AU (equivalent to the distance of the Earth from the Sun). A similar architecture has been observed around HD 40307 (6 planets), Kepler-11 (6 planets), Kepler-20 (5 planets), Kepler-33 (5 planets), and Kepler-62 (5 planets), while 11 more systems, both near and far, represent scaled-down versions with three to four small planets (61 Virginis, 82 Eridani, GJ 163, GJ 581, HD 31527, HD 39194, HD 69830, HD 136352, Kepler-18, Kepler-37, and Kepler-42). Collectively, these compact systems represent a substantial fraction of all known planetary systems containing three or more planets.

Unlike most multiplanet systems, in which the innermost planet is the least massive, the hottest planet of GJ 667C is also the heaviest, with a minimum mass (m sin i) that is 5.6 times Earth (Mea). Nevertheless, all six planets are fairly similar in mass, ranging from 2.7 to 5.6 Mea. Whether they are also similar in composition (all enveloped in hydrogen, all rocky, all icy, or all a rock/ice blend), or whether they represent a variety of interior structures, will remain a mystery for the indefinite future.

Parameters for Six Planets Orbiting GJ 667C

Column 1 gives the current alphabetic designation; column 2 the minimum planet mass in Earth units; column 3 the semimajor axis in astronomical units (AU); column 4 the orbital eccentricity; and column 5 the orbital period in days. All values Anglada-Escudé et al. 2013.

At least from my back alley perspective, this new study is exemplary for its thorough consideration of alternative explanations for the radial velocity data, as well as its careful test of the dynamical plausibility of the proposed orbital architecture (see this 32-second animation). As the authors note, “the dynamics of the system are far from trivial” on account of its tightly packed configuration. To illustrate: at their closest approach, planets c and f are less than 5 million kilometers apart, compared to about 41 million kilometers for the inferior conjunction of Venus and Earth.

Despite such cozy orbits, however, Anglada-Escudé and colleagues find that the proposed system can remain stable over million-year time scales. Although resource limitations prevented them from conducting a truly wide-ranging exploration of the orbital dynamics, with billion-year integration times, their preliminary results indicate that stability is preserved as long as the true masses of the six planets are less than twice as large as their currently defined minimum values. At the maximum permitted values, four out of six planets would still fit the traditional definition of Super Earths (less than 10 Mea), while the other two would approach Uranus in mass.

The new analysis also returned evidence for a seventh planet, h. This one is a potentially Earth-mass object orbiting between planets b and c in a period of about 17 days. The authors consider it “a physically viable planet that might be confirmed with a few more observations,” without pressing the claim.

From 2012, an artist’s impression of GJ 667C c. This is a much wetter and rosier view than the one at the top of this posting. Image credit: European Southern Observatory.

Another major strength of this study is its extended discussion of habitability. Anglada-Escude and colleagues set the boundaries for the habitable zone around GJ 667C at 0.111 AU-0.246 AU, which comfortably accommodates planets c, f, and e. Whether these worlds can truly sustain surface bodies of liquid water depends on their true masses and radii. Their masses may be retrieved by future analyses of the system dynamics, but their radii (and thus their densities and approximate physical compositions) remain out of reach, since these objects cannot be observed in transit.  

We can be sure of one thing, nevertheless: given their short orbital periods and small eccentricities, all three planets in the habitable zone will be tidally locked, each with a permanent day-side and night-side (Selsis et al. 2007). This configuration might have interesting consequences in terms of physical environments and biological evolution, especially if the three planets present a variety of structures and topographies.

Their minimum masses – 2.7 Mea each for planets f and e, 3.8 Mea for planet c – are consistent with hydrogenated atmospheres, like those of the puffy planets around Kepler-11. Especially relevant is the case of Kepler-11f, a Super Earth of just 2 Mea, whose ample radius of 2.48 Rea requires a deep atmosphere containing 4% hydrogen (Lissauer et al. 2013). All three habitable zone planets around GJ 677C are cooler and more massive than Kepler-11f, perhaps increasing their chances of retaining hydrogen envelopes. Whether they actually do may depend on the past behavior of the host star; young M dwarfs produce plentiful radiation in the extreme ultraviolet that can sputter away hydrogen in the atmospheres of close-in planets (Barnes et al. 2012, Pierrehumber & Gaidos 2011). For that reason, planets orbiting G-type stars like Kepler-11 and our Sun may not furnish appropriate analogs for M dwarf planets.

Little research to date has explored the habitability of 3 Mea planets with atmospheres that are 1%-3% hydrogen. Nevertheless, hydrogen is a greenhouse gas, and if it comprised just 1% of the atmosphere of GJ 667C c, f, or e, it would raise their temperatures too high to sustain liquid water. Even if one or more of these worlds retained little hydrogen, or none at all, their masses could still be a problem. All are quite massive in relation to Earth and Venus, and it remains a matter of serious debate whether such heavyweight objects can sustain plate tectonics and magnetic fields (O’Neill & Lenardic 2007, Morard et al. 2011, Lenardic & Crowley 2013, Noack & Breuer 2013). Yet both features are considered indispensable for habitability.

Still – only a biophobe could fail to note that, despite every uncertainty, this new model of the GJ 667 C system is much friendlier to life than previous offerings. The earliest characterization of the system, by Bonfils and colleagues using the HARPS spectrograph (first circulated as a preprint in 2011 but not formally published until 2013), noted only one planet (c) in the habitable zone, defining it as a Super Earth with a minimum mass of 3.9 Mea and a period of about 28 days. A follow-up study by the same research group presented an even larger and less hospitable value of 4.25 Mea for the same object (Delfosse et al. 2013; first circulated in 2012). A reanalysis of the data of Delfosse et al. by Anglada-Escude and colleagues (2012) raised the mass of planet c still higher, to 4.54 Mea. Around the same time, a reanalysis of the data of Bonfils et al. by Philip Gregory (2012) made it higher still, at 4.8 Mea, while suggesting with admirable prescience that the data implied a six-planet system, including two additional Super Earths in the habitable zone.

Gregory’s hypothetical line-up offered two new planets: d, with a period of 31 days and a minimum mass of 3.1 Mea, and e, with 39 days and 2.4 Mea. Unfortunately, such a packed system would likely be unstable. In any event, the latest reanalysis by Anglada-Escude’s group brings us an even more attractive – and reliably stable – six-planet system in which the 28-day planet (c) has 3.8 Mea (similar to the original value by Bonfils and colleagues), the 39-day planet (now f rather than e) has 2.7 Mea, and the new 62-day planet (e) also has 2.7 Mea. Gregory’s object at 31 days now looks like a mirage.

In the biophilic view, it is better to have one small planet in the habitable zone than none at all, and better still to have three. Our own Solar System belongs to that rarefied minority, given its windfall of three rocky planets orbiting between 0.7 and 1.5 AU. GJ 667 C seems to have had similar luck in its much smaller habitable zone. Maybe one of its temperate Super Earths has enjoyed the kind of charmed history that seems essential for complex life.

  Image Credit: Wikimedia Commons

Anglada-Escudé G, Arriagada P, Vogt SS, Rivera EJ, Butler RP, Crane JD, and 11 others. (2012) A planetary system around the nearby M dwarf GJ 667 C with at least one super-Earth in its habitable zone. Astrophysical Journal Letters 751, L16. Abstract:
Anglada-Escudé G, Tuomi M, Gerlach E, Barnes R, Heller R, Jenkins JS, Wende S, Vogt SS, Butler RP, Reiners A, Jones HRA. (2013) A dynamically-packed planetary system around GJ 667C with three super-Earths in its habitable zone. Astronomy & Astrophysics, in press. Abstract:
Barnes R, Meadows VS, Domagal-Goldman SD, Heller R, Jackson B, Lopez-Morales M, Tanner A, Gomez-Perez N, Ruedas T. (2012) Habitability of Planets Orbiting Cool Stars. In The Sixteenth Cambridge Workshop on Cool Stars, Stellar Systems, and the Sun. Edited by Christopher M. Johns-Krull, Matthew K. Browning, and Andrew A. West. ASP Conference Series, Vol. 448. San Francisco: Astronomical Society of the Pacific.
Delfosse X, Bonfils X, Forveille T, Udry S, Mayor M, F. Bouchy F, et al. (2013) The HARPS search for southern extra-solar planets. XXXV. Super-Earths around the M-dwarf neighbors Gl 433 and Gl 667C. Astronomy & Astrophysics 553, A8. Abstract:
Gregory PC. (2012) Additional Keplerian signals in the HARPS data for Gliese 667C from a Bayesian re-analysis. Monthly Notices of the Royal Astronomical Society in press. Abstract:
Lenardic A, Crowley JW. (2013) On the notion of well-defined tectonic regimes for terrestrial planets in this solar system and others. Astrophysical Journal 755, 132.
Morard G, Bouchet J, Valencia D, Mazevet S, Guyot F. (2011) The melting curve of iron at extreme pressures: implications for planetary cores. High Energy Density Physics 7, 141-144.
Noack L, Breuer D. (2013) Plate tectonics on rocky exoplanets: Influence of initial conditions and mantle rheology. Planetary and Space Science, in press.
O’Neill C, Lenardic A. (2007) Geological consequences of super-sized Earths. Geophysical Research Letters 34, L19204.
Pierrehumbert R, Gaidos E. (2011) Hydrogen greenhouse planets beyond the habitable zone. Astrophysical Journal Letters 734, L13. Abstract:
Selsis F, Kasting JF, Levrard B, Paillet J, Ribas I, Delfosse X. (2007) Habitable planets around the star Gliese 581? Astronomy & Astrophysics 476, 1373-1387.

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