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
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[As of September 2014, much of the information in this post has been superseded.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.
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
REFERENCES
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: http://adsabs.harvard.edu/abs/2012ApJ...751L..16A
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: http://adsabs.harvard.edu/abs/2013arXiv1306.6074A
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: http://adsabs.harvard.edu/abs/2013A%26A...553A...8D
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: http://adsabs.harvard.edu/abs/2012arXiv1212.4058G
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: http://adsabs.harvard.edu/abs/2011ApJ...734L..13P
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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