I readily admit that one of my pet peeves going back almost 20 years to the discovery of the first extrasolar planets has been overblown claims about the potential habitability of some of these discoveries. This has been one of the motivations for my ongoing series of Habitable Planet Reality Check posts in recent months on this web site (for the full background story, see Habitable Planet Reality Check: Kepler 186f).
This past week, a team led by Robert Wittenmyer (UNSW Australia/University of Southern Queensland) announced the discovery of a potential super-Earth orbiting on the inside edge of the habitable zone of the nearby red dwarf star, GJ 832. And in a refreshingly honest change of pace, instead of claiming that this new discovery might be a habitable planet (as far too many other teams have done with similar discoveries), this team makes the statement right up front that they do not believe that the planet they found, GJ 832c, is likely to be habitable and is more likely to be a uninhabitable “super-Venus”.
This intellectual honesty about the potential nature of this newly-discovered planet probably explains why there had been little attention paid to it during most of the last week in the various astronomical news outlets. That changed on June 25 with the wildly optimistic press release by Abel Mendez Torres of the Planetary Habitability Laboratory at the University of Puerto Rico at Arecibo which, despite the actual claims made by Wittenmyer and his team in their discovery paper, rates GJ 832c at 0.81 on their Earth Similarity Index (ESI). The press release goes on to state that GJ 832c is one of the best Earth-like candidates found so far and certainly the closest (which once again makes me question the value of PHL’s rating system but that is a topic for another post). But while the discovery of GJ 832c did not initially attract the attention of many people, it did get mine and deserves a closer look as a counterexample to some of the more dubious claims made about the potential habitability of extrasolar planet discoveries.
GJ 832 (or Gliese 832) is a spectral type M1V red dwarf star with a surface temperature of 3472 K and an estimated mass of 0.45 times that of the Sun. Although there seems to be range of luminosity values quoted for GJ 832 in astronomical literature, the most recent value by Bonfils et al. (given as one of two values in Table 5 in the first updated version of the preprint of the paper by Wittenmyer et al.) places it at 0.026 times that of the Sun. Located in the southern constellation of Grus (The Crane) at a distance of 16.1 light years, GJ 832 is among the closest stars to our Solar System.
In 2008, Jeremy Bailey (Macquarie University in Australia) and his team, which included famed extrasolar planet hunters, Paul Butler and Geoff Marcy, announced the discovery of the first planet found to be orbiting GJ 832 using precision Doppler velocity measurements made as part of the Anglo-Australian Planet Search (AAPS). With an orbital period currently estimated to be 10 years, an orbital radius of 3.6 AU and a minimum mass (or mpsini) of about 0.7 times that of Jupiter, this was the first example of a Jupiter-analog discovered orbiting a red dwarf star.
The newly announced discovery by Wittenmyer and his team also using the Doppler velocity technique adds a second planet to this nearby planetary system. This team combined data gathered from three different planet search programs (including the AAPS) acquired over the course of 15 years to uncover the Doppler signature of a significantly smaller planet with a minimum mass of 5.4 times that of the Earth in a much tighter orbit about GJ 832. With the addition of GJ 832c, this planetary system begins to resemble a scaled-down version of our Solar System with a smaller planet closer in and a Jupiter-like planet in a more distant orbit.
The characteristics of this planetary system, based on information from the preprint of the discovery paper by Wittenmyer et al., are summarized below in Table I. The mean effective stellar flux, Seff, or insolation (with Earth’s insolation defined as 1) of these two planets is calculated using the data given in the discovery paper, the luminosity of GJ 832 from Bonfils et al. and takes into account the scaling of Seff from the effects of the moderate eccentricity of the new planet’s orbit as prescribed by Dressing et al..
Table I: Properties of Planets in GJ 832 System
|Mass (Earth = 1)||
|Orbit Radius (AU)||
|Mean Seff (Earth = 1)||
As I have mentioned earlier, I have to give credit to Wittenmyer and his team for resisting the temptation of claiming that GJ 832c is a potentially habitable super-Earth. Wittenmyer et al. state openly in their paper that they find it likely that GJ 832c is massive enough to hold onto a dense atmosphere that would enhance the greenhouse effect making their discovery more likely to be a super-Venus than a habitable super-Earth.
Based on the latest models of planetary habitability by Kopparapu et al., the inner edge of the habitable zone of GJ 832 for a planet with a mass of 5 times that of the Earth (as conservatively defined by the onset of a runaway greenhouse effect) has an Seff of 1.0 or about the same value as Earth. While my calculation of the position of the inner edge of the habitable zone relative to GJ 832 differs from that which is given in the paper by Wittenmyer et al., that value of Seff corresponds to a distance of 0.163 AU (compared to the value of 0.130 AU given in the preprint of the discovery paper which seems to correspond to the highly optimistic “Recent Venus” definition for the inner boundary of the habitable zone). The distance of 0.163 AU places GJ 832c right at the innermost edge of this system’s habitable zone for a planet of this presumed size. Since GJ 832c orbits so close to its sun, it is almost certainly a synchronous rotator or, given the moderate eccentricity of its orbit, it could possibly be a slow rotator locked into some other spin-orbit resonance (e.g. the 3:2 resonance of Mercury in our own Solar System). While Wittenmyer et al. specifically cites work recently published by Jun Yang et al. as well as earlier work by others which suggests that the inner edge of the habitable zone might actually be significantly closer for slowly rotating planets, Wittenmyer and his team continue to resist the temptation to label their find as potentially habitable owing mainly to its large mass and probable dense atmosphere.
Even though Wittenmyer et al. do not explicitly state it in their discovery paper, there are other issues that further complicate the potential habitability of GJ 832c. Like all planets found using precision Doppler velocity measurements, only the minimum mass of the planet, or mpsini, can be determined since the inclination of the planet’s orbit to our line of sight, i, can not be determined except with information from other sources. Given a completely random orientation of the orbit of GJ 832c, there is about a 63% chance that the actual mass of GJ 832c exceeds six times that of the Earth and is most likely a mini-Neptune or larger (see “Habitable Planet Reality Check: Terrestrial Planet Size Limit“). And even if the mass of GJ 832c turns out to be less than six times that of Earth, it could still be one of the newly recognized class of low-mass low density planets known as a gas dwarf. In any of these cases, these non-terrestrial types of planets are unlikely to be habitable no matter what their insolation values are.
While Wittenmyer et al. purposely avoid any claims about the potential habitability of GJ 832c, they do openly consider the possibility that any moon that their find might posses could be potentially habitable if it were large enough. Given that the mass ratio for the Pluto-Charon system in our Solar System is about 9:1, they argue it is possible that GJ 832c could have a moon with a mass maybe half that of the Earth. Given the proximity of GJ 832c to its sun and assuming a mass of 5.4 times that of the Earth, any moons it might have in prograde orbits with orbital radii less than ~138,000 kilometers (corresponding to an orbital period of ~2.5 days) would be stable over astronomically long timescales. Further, Wittenmyer et al. argue that any potentially habitable moon of GJ 832c would need to have an orbital radius greater than roughly ~100,000 kilometers (corresponding to an orbital period of ~1.6 days) in order to avoid excessive tidal heating. Given the narrow range of orbital radii available to any potentially habitable moon, Wittenmyer et al. state in their paper “although the idea of a habitable exomoon companion to GJ 832c is certainly interesting, the odds seem stacked against the existence of such an object”.
While I agree with their assessment, I also have to point out that even if a moon of sufficient size did exist in this narrow orbital range, it would still be unlikely to be habitable. While my calculations show that GJ 836c is at the inner most edge of the habitable zone for a body with five times the mass of the Earth, the inner edge of the habitable zone for smaller bodies lies farther out still. Based on the work of the influence of planetary mass on the position of the inner edge of the habitable zone by Kopparapu et al., the effective stellar flux, Seff, for the inner edge of the habitable zone (as conservatively defined by the onset of a runaway greenhouse effect) for bodies with 0.1 to 1 times the mass of the Earth is in the 0.83 to 0.93 range. These values correspond to distances of 0.176 to 0.166 AU, respectively, from GJ 832 or just beyond the orbit of GJ 832c. And since any such moon of GJ 832c would be a synchronous rotator with its period of rotation equal to its orbital period of about two days, there is little likelihood that the position of the inner edge of the habitable as a result of slow rotation would differ enough to matter. Even given the uncertainties about the characteristics of this system, I find it unlikely that any potential large moon of GJ 836c might be habitable.
My hat is off to Robert Wittenmyer and his team for resisting the temptation of labeling their new planetary discovery as being potentially habitable despite the fact it lies near the inner edge of the habitable zone even as it is most conservatively defined. Although their laudable honesty has probably cost them some media attention, in their discovery paper they rightfully characterize their planet as most likely possessing a dense atmosphere that would enhance the greenhouse effect making GJ 832c more like a hellish super-Venus than a habitable super-Earth.
Since GJ 832c was discovered using the Doppler velocity technique which only provides a measure of the minimum mass for this new find, there are better than even odds that its mass exceeds ten times that of the Earth making is likely that GJ 832c a mini-Neptune or larger. Even if the actual mass of GJ 832c is less than ten Earth-masses, it could still be a member of the newly-discovered class of planets known as gas dwarfs. Taken together, the odds are not in favor of GJ 832c even being a terrestrial planet at all never mind a potentially habitable one. In the face of these facts, the claim by the Planetary Habitability Laboratory in their June 25 press release that GJ 832c is “one of the top three most Earth-like planets according to the ESI” is premature at best and borders on the absurd at worst. Either way, this inflated claim calls into question the scientific value of the Earth Similarity Index and how it is calculated.
While Wittenmyer et al. openly speculate in their discovery paper that GJ 832c could posses a moon that might be habitable if it is of sufficient size and occupies a rather narrow range of orbital radii, they find the odds are against it. I would further argue that any such moon is also likely to be just beyond the inner edge of the habitable zone for bodies in the 0.1 to 1 Earth-mass range decreasing the odds of a there being a habitable moon further still.
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“Abundance of Earth Analogs”, Drew Ex Machina. June 25, 2014 [Post]
“Habitable Planet Reality Check: Kapteyn b”, Drew Ex Machina, June 6, 2014 [Post]
“Habitable Planet Reality Check: 55 Cancri f”, Drew Ex Machina, May 7, 2014 [Post]
“Habitable Planet Reality Check: Kepler 186f”, Drew Ex Machina, April 20, 2014 [Post]
“Habitable Planet Reality Check: Terrestrial Planet Size Limit”, Drew Ex Machina, July 24, 2014 [Post]
“The Transition from Super Earth to Mini Neptune”, Drew Ex Machina, March 29, 2014 [Post]
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