The announcement of the discovery of an Earth-sized planet orbiting inside the habitable zone of the distant red dwarf star designated Kepler 186 has resulted in a flurry of stories on the internet and in the traditional press [1, 2]. You will have to forgive me but, I am always skeptical when I hear about claims that some newly discovered planet (or if it is too large, any potential moons it might have) is in the habitable zone. And with the flood of discoveries from NASA’s Kepler mission, this seems to be an ever more frequent occurrence. All too often, after I dig deeper into a claim, I find that the scientists involved have taken liberties with the definition of “habitable” to imply that their just-announced planet discovery might itself be habitable. The press, especially the non-science press, then frequently inflates the purposely guarded but misleading statements of the scientists to claim that a habitable planet has been discovered. This is understandable given that scientists desire to be remembered as finding the first habitable planet and reporters desire to be remembered as being the first to report it.
This is nothing new and extends back almost two decades to the discovery of the first extrasolar planets. Back in January 1996 I was participating in an optical SETI conference in San Jose, California. During the session where I was to present my paper, famed extrasolar planet hunter Geoff Marcy made his presentation of his team’s confirmation of the discovery of 51 Peg b (the first extrasolar planet orbiting a Sun-like star discovered less than four months earlier) and the discovery of their first two planets, 47 UMa b and 70 Vir b . In his presentation he repeated his often made claim of the time that while 70 Vir b was a gas giant and could not be habitable, any moon it might have would have a blackbody temperature of 85° C making it possible that liquid water could exist on its surface. While both statements are technically true, the latter was very misleading and the press was more than happy to repeat the claim that water could exist on 70 Vir b implying life could exist there.
Personally, I found this claim to be intellectually dishonest and a needless attempt to inflate the importance of an already very important discovery. While it is true that a body orbiting 70 Vir b would have a blackbody temperature of 85° C, it would only have that temperature if it were an airless, perfectly radiating blackbody. Fold in the effects of an atmosphere, which any planet or moon would need to possess in order to be habitable, and the greenhouse effect brought about by CO2, water vapor or other gases in its atmosphere would turn any moon of 70 Vir b into an inferno to rival Venus in our own solar system. Just to illustrate the absurdity of the claim, Venus has an effective stellar flux or insolation (i.e. the amount of light and heat a planet receives from its sun, Seff, where Earth is defined as Seff=1) of 1.9 while 70 Vir b had an Seff of 13!!! There is simply no realistic prospect that any potential moon of 70 Vir b was going to have liquid water anywhere on its surface.
Of course later in that same session (with Geoff Marcy in the audience) I made my presentation  which included a discussion of the potential habitability of the first extrasolar planets that had been discovered including those recently found by Geoff Marcy’s team. In my presentation, I stated emphatically that based on the latest models of planetary habitability, any moon of 70 Vir b would experience a runaway greenhouse effect and would have a surface temperature hotter than Venus as well as be just as dry. Geoff Marcy’s claim was just plain misleading and no serious researcher in the field today makes the claim that any potential moon of 70 Vir b might be habitable. Ironically, the other planet Marcy and his team had discovered, 47 UMa b which Marcy calculated had a blackbody temperature of -90° C, is at the outermost edge of what the best models of the time considered to be the habitable zone and it was this fact that spurred my research into habitable moons.
In recent years I have seen the same thing happen repeatedly. Scientists will too often stretch the limits of the definition of the habitable zone far beyond what the best models can support (especially pushing the definition of the inner edge of the habitable zone to almost include Venus) to make the claim that their discovery is in the habitable zone. And there are situations where a super-Earth or other uncomfortably large planet has been claimed to be potentially habitable because it falls inside the habitable zone. While it makes for good press, I simply do not buy it. And the claim seems to be made so frequently lately that I almost ignore such announcements now… until the announcement about Kepler 186f.
Potential Habitability of Kepler 186f
So, how does the claim of Kepler 186f hold up? According to the peer-reviewed paper in Science announcing the discovery , Kepler 186f is the fifth planet found orbiting Kepler 186. This star is a spectral type M1V with a radius that is 47% that of the Sun and a luminosity 4.1% of the Sun’s. It is located 493 ±59 light years away in the constellation of Cygnus. The discovery of the first four, roughly Earth-sized planets orbiting this star was announced earlier this year based on an analysis of the first two years of Kepler data [5, 6] with an additional year’s worth of data required to detect this latest find in its more distant orbit. Based on the Kepler transit data, Kepler 186f has a radius of 1.11 ±0.14 times that of the Earth and an orbital period of 129.9 days. The properties of Kepler 186f and its sister planets pulled from a variety of sources are summarized in the table below.
Table I: Properties of Planets in Kepler 186 System
|Orbit Radius (AU)||0.0378||0.0574||0.0861||0.1216||0.3926|
The reflex motion of these planets orbiting Kepler 186 is too small to be detected using currently available instruments so the masses of these planets (and in turn, a hint at their bulk properties) can not be determined at this time. Kepler 186f could have a mass as small as 0.32 times that of Earth if were composed of water or as great as 3.77 times that of Earth if it were composed of iron. Most likely it lies somewhere between these two extremes. For example, Kepler 186f would have a mass of 1.44 times that of Earth if it had an Earth-like composition. Based on the current analysis of the properties of planets discovered by Kepler which found that planets seem to transition from terrestrial types to gas giants starting at a radius of about 1.5 times that of Earth, it seems probable that Kepler 186f and indeed all of its sister planets are not gas giants. So, it appears that at least in terms of size, Kepler 186f is almost certainly Earth-like and it is not a miniature version of Neptune.
The four known inner planets of the Kepler 186 system have insolation values, Seff, higher than that for Venus – too large by any reasonable measure to be considered habitable. But based on the best conservative models for the limits of the habitable zone, Kepler 186f appears to be near but just inside the outer limit. In the discovery paper, the authors estimate that the insolation of Kepler 186f is 0.32 with an uncertainty of +0.06 and -0.04 . The outer edge of the habitable zone as defined by the maximum greenhouse limit for a star like Kepler 186 with an effective temperature of 3788° K is ~0.24 . If Kepler 186f were a terrestrial planet with a composition and chemistry comparable to the inner planets of our solar system, it would have a dense atmosphere with several bars of CO2 (where Earth’s surface pressure is about one bar) in order to maintain habitable conditions. The exact amount depends on the mix of gases in its atmosphere and the role water clouds and CO2 clouds (the upper atmosphere of this planet is likely cold enough to form CO2 clouds) play in its energy balance. Even if sufficient O2 were present in the atmosphere of this planet, the atmosphere would be deadly to terrestrial animal life including humans. But for life forms that evolved under these conditions, this would not be a problem and Kepler 186f would meet the scientific definition of “habitable”. So, Kepler 186f seems to be Earth-like in size, not a mini Neptune and comfortably inside the outer edge of the habitable zone.
Aside from the unknown bulk properties of Kepler 186f, the one major problem with Kepler 186f is that it orbits relatively close to a dim M-dwarf star which presents its own list of potential problems. But many of these problems are mitigated by the fact that Kepler 186f orbits 0.4 AU from its sun. One potential issue addressed is its spin state. Planets that closely orbit their suns are subject to tidal effects that slow the rotation rate to the point that they are synchronous rotators with the one side perpetually in light and the other in darkness. However, models developed almost two decades ago have shown that even a modestly dense CO2 atmosphere is sufficient to equalize the temperatures on the day and night side of a synchronous rotator [8, 9]. The presence of an ocean, which was not considered in these earliest models, would aid in decreasing temperature extremes further. The uncertainty in the properties of Kepler 186f make it impossible to determine its spin state. It might be a synchronous rotator or not. At very least it is a slow rotator but the presence of a thick, CO2 atmosphere (especially if it is coupled with a circulating ocean) greatly increases the likelihood that it is habitable.
I readily admit that I started my analysis of Kepler 186f with the intent of debunking another questionable claim of the discovery of a potentially habitable planet. Many key properties of Kepler 186f are still unknown but it is definitely an Earth-sized planet that is likely not a gas giant which lies within the habitable zone of an M-dwarf just as the authors of the discovery paper claim . While there are still potential issues associated with the fact that Kepler 186f orbits an M-dwarf star that will need to be addressed by future research, it is potentially the most Earth-like planet found to date. Continued analysis of the Kepler data set should reveal more planets like Kepler 186f and, hopefully, the presence of an Earth-sized planet orbiting within the habitable zone a more Sun-like star – the best possible class of candidates based on our current understanding of habitability and the original purpose of the Kepler mission at its inception.
A French translation of this article by Alexandre Lomaev is also available: “Planètes habitables: le cas de Kepler-186f”, Extrasolar.fr – Encylopédie des Mondes Extérieurs, October 12, 2015 (in French) [Post]
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“The Transition from Super Earth to Mini Neptune”, Drew Ex Machina, March 29, 2014 [Post]
“The Extremes of Habitability”, SETIQuest, Volume 4, Number 2, pp. 1-8, Second Quarter 1998 [Article]
“Detecting Habitable Planets: The Next Decade”, SETIQuest, Volume 4, Number 1, pp. 1-6, First Quarter 1998 [Article]
“Habitable Moons: A New Frontier for Exobiology”, SETIQuest, Volume 3, Number 1, pp. 8-16, First Quarter 1997 [Article]
“Habitable Moons”, Sky & Telescope, Volume 96, Number 6, pp. 50-56, December 1998 [On line version]
(1) “First Potentially Habitable Earth-Sized Planet Confirmed by Keck and Gemini Observatories”, W.M. Keck Observatory Press Release, April 17, 2014 [Press Release]
(2) Elisa V. Quintana et al., “An Earth-Sized Planet in the Habitable Zone of a Cool Star”, Science, pp. 277-280, Vol. 344, No. 6181, 18 April 2014 [Abstract & Paper Access]
(3) Geoffrey W. Marcy and R. Paul Butler, “The First Three Planets”, The Search for Extraterrestrial Intelligence (SETI) in the Optical Spectrum II (San Jose, CA; January 29 – February 1, 1996), Stuart A. Kingsley and Guillermo A. Lemarchand (Editors), Proc. SPIE 2704, pp. 46-49, 1996
(4) Andrew J. LePage, “Rating System for SETI Targets”, The Search for Extraterrestrial Intelligence (SETI) in the Optical Spectrum II (San Jose, CA; January 29 – February 1, 1996), Stuart A. Kingsley and Guillermo A. Lemarchand (Editors), Proc. SPIE 2704, pp. 35-45, 1996 [Abstract]
Note: My paper does not mention the first extrasolar planets because they were discovered after I had already submitted it but I did discuss these discoveries during my actual conference presentation.
(5) J. F. Rowe et al., “Validation of Kepler’s multiple planet candidates. III: Light curve analysis & announcement of hundreds of new multi-planet systems”, Astrophysical Journal, Vol. 784, No. 1, Article ID. 45, March 20, 2014
(6) J. J. Lissauer et al., “Validation of Kepler’s multiple planet candidates. II: Refined statistical framework and systems of special interest”, Astrophysical Journal, Vol. 784, No. 1, Article ID. 44, March 20, 2014
(7) R. K. Kopparapu et al., “Habitable zones around main-sequence stars: new estimates”, Astrophysical Journal, Vol. 765, No. 2, Article ID. 131, March 10, 2013
(8) Robert M. Habrele, Christopher P. McKay, Daniel Tyler and Ray T. Reynolds, “Can Synchronously Rotating Planets Support an Atmosphere?”, in Circumstellar Habitable Zones: Proceedings of the First International Conference (ed. Laurence R. Doyle), Travis House Publications, pp. 29-33, 1996
(9) M.M. Joshi, R.M. Habrele and R.T Reynolds, “Simulations of the Atmosphere of Synchronously Rotating Terrestrial Planets Orbiting M Dwarfs: Conditions for Atmospheric Collapse and the Implications for Habitability”, Icarus, Vol. 129, No. 2, pp. 450-465, October 1997