Wednesday, September 3, 2014

Gliese 667C: Just Two Planets

The constellation Scorpius, with the position of the triple star system GJ 667C marked by a green square. The bright red star in this image is Antares (Alpha Scorpii). The Pipe Nebula is visible above GJ 667C.
The astronomers who exposed those allegedly habitable Super Earths around GJ 581 as mere phantoms have set their sights on the next leading candidate for Most Earthlike Extrasolar Planet, namely GJ 667C f

And poof! There it goes, just another figment of red noise

A preprint by Paul Robertson and Suvrath Mahadevan appeared earlier this week, presenting their analysis of correlated noise in the radial velocity data on Gliese 667C (abbreviated GJ 667C). Although various studies have reported evidence for three, four, six, or even seven low-mass planets around this nearby M dwarf (Gregory 2012, Anglada-Escude et al. 2012, 2013), Robertson & Mahadevan find only two planetary signals in the data, consistent with the two planets originally reported by Delfosse and colleagues (2013). As they note, both their approach and their results were anticipated by an earlier duo, Feroz & Hobson (2014)

Parameters for the Two-Planet System Around 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 Robertson & Mahadevan 2014.
With just 33% of our Sun’s mass (0.33 Msol), GJ 667C can sustain conditions suitable for surface water only in a narrow annulus between 0.111 and 0.246 astronomical units (AU). These boundaries define the system’s habitable zone. In the Solar System, this annulus would fall well inside the orbit of Mercury, while our own habitable zone (obviously) centers on the orbit of Earth at 1 AU. 

An earlier analysis by Guillem Anglada-Escude and colleagues (2013), which I reported last year with great enthusiasm, indicated as many as three small planets (c, f, and e) in the habitable zone around GJ 667C. Candidates f and e seemed the most promising, since each had an m sin (i) or minimum mass estimated at 2.7 times Earth (2.7 Mea). Planet c, however, seemed iffy, since various groups placed its minimum mass between 3.9 and 4.8 Mea. Data from the Kepler Mission suggest that planets of this mass are typically enveloped in hydrogen-rich atmospheres, as long as they are far enough from the host star (e.g., as far away as the habitable zone) to avoid ablation of lightweight gases. Unfortunately, hydrogen creates a powerful greenhouse effect, so any planet much heavier than 2 or 3 Mea that is cool enough to avoid atmospheric stripping is also cool enough to retain an envelope that effectively prohibits biogenesis. It’s an extrasolar Catch-22.

This latest work by Robertson and Mahadevan confirms the reality of planet c, while demonstrating that candidates d, e, and f are just artifacts of correlated noise.  Thus, it’s still safe to claim that GJ 667 has a Super Earth in its habitable zone. Unfortunately, the minimum mass of this object is just as high as it ever was, currently estimated at 4.1 Mea. Its actual mass is inevitably higher, maybe much higher. So even if GJ 667C c orbits at the right distance from its parent star, it’s nothing like Earth.

Given this development, we can no longer claim that the Sun’s back yard contains any candidates for extraterrestrial life. In fact, the inventory of compact, low-mass multiplanet systems in the Solar neighborhood just shrank by 8%. Until this week we could point to 12 different stars within 40 parsecs (130 light years) that supported at least three low-mass planets, with no bumbling gas giants in sight. Now the number is down to 11, and only one of those 11 has more than three planetary candidates. Still worse, none of the planets around any of these 11 stars is both small enough and cool enough to qualify as possibly potentially habitable.

Where will the axe fall next? My bet’s on HD 40307. Ain’t science grand!

All Systems with Three or More Low-Mass Planets Detected by Radial Velocity

Semimajor axes are measured in astronomical units (AU), where 1 AU is the Earth-Sun separation; planet masses are indicated in Earth units (ME); star masses appear in Solar units at right.

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 556, A126. Abstract:
Delfosse X, Bonfils X, Forveille T, Udry S, Mayor M, F. Bouchy F, et al. (2013) The HARPS search for southern extra-solar planets. XXXIII. Super-Earths around the M-dwarf neighbors Gl 433 and Gl 667C. Astronomy & Astrophysics 553, A8. Abstract:
Feroz F, Hobson MP. (2014) Bayesian analysis of radial velocity data of GJ667C with correlated noise: Evidence for only two planets. Monthly Notices of the Royal Astronomical Society 437, 3540-3549. Abstract:
Gregory PC. (2012) Additional Keplerian signals in the HARPS data for Gliese 667C from a Bayesian re-analysis. (unpublished). Abstract:
Robertson P, Mahadevan S, Endl M, Roy A. (2014) Stellar activity masquerading as planets in the habitable zone of the M dwarf Gliese 581. Science Express (3 July 2014), doi:10.1126/science.1253253
Robertson P, Mahadevan S. (2014) Disentangling planets and stellar activity for Gliese 667C. Astrophysical Journal Letters, in press. Abstract:


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