A Major milestone was announced Thursday when NASA unveiled Kepler-186 f, a new habitable planet candidate that is, without a doubt, the most Earth-like extrasolar planet known. This planet could legitimately be habitable. Kepler-186 f is one of five known planets orbiting an M1V star with half the radius and mass of Sol. The planet itself is only 1.11 ± 0.14 RE, which suggests that the planet is probably not a mini-Neptune, however we don’t know its mass. If it is composed entirely of volatiles, it has a mass of 0.32 ME, or on the other extreme, if the planet has a pure iron composition, then it has a mass of 3.77 ME. An Earth-like composition places the mass of the planet at 1.44 ME. The planet is probably on the denser end of this, as a low-density planet of this size would probably not have survived the high-XUV stage of the M dwarf’s youth. The other four planets in the system are all less than 1.4 RE, and are likely terrestrial themselves.
The planet gets ~32% of the insolation from its star that Earth gets from ours, which seems a bit on the low end, but there are a couple factors to keep in mind. Because M dwarfs are redder, and because atmospheres scatter blue light, an Earth-like atmosphere for Kepler-186 f would scatter a lower fraction of starlight than Earth’s atmosphere. The insolation from the M dwarf, by virtue of its redder spectrum, gives more heating to a planet than for the same insolation from a G type star. Furthermore, for an Earth-like composition, the planet has a slightly higher mass and could therefore attract a thicker atmosphere, providing more greenhouse heating.
The Planetary Habitability Laboratory lists Kepler-186 f at a rather dismal 17th out of 21. It’s curious they would rank it so pitifully. All of the habitable planet candidates listed as being more Earth-like, with perhaps the exception of Kepler-64 f, are likely low-density mini-Neptunes. Clearly the PHL habitability ranking algorithm needs to be revised. Furthermore, some of the planets listed aren’t even known to exist. Gliese 581 g itself has been effectively disproven.
It’s still possible to imagine a feasibly attainable next step: an Earth-sized planet in the habitable zone of a sun-like star — a true Earth analogue. But this is definitely a remarkable discovery and one that probably won’t seem to be outside the realm of habitability when those future discoveries do come about.
At the risk of sounding pessimistic, this may be the only known terrestrial habitable exoplanet candidate we know of. Kepler-64 f could still be a mini-Neptune. Gone are the days where a 5 Earth-mass planet receiving similar insolation as Venus can stir the imagination with the prospects of luscious fields of green, with kittens playfully swatting at butterflies. As the time approaches where we begin to focus our attention on characterising the atmospheres of habitable planet candidates and searching their spectra for biospheres, we will have to prioritise the planets we look at. The overwhelming majority of the planets currently making people’s “habitable planet candidate” list simply aren’t going to be on the receiving end of that kind of attention. The discovery of planets like Kepler-186 f in the solar neighbourhood is the major next step for searching for an extrasolar biosphere.