One of those programs run by the European Southern Observatory called TRAPPIST, recently found three Earth-size extrasolar planets orbiting a nearby ultracool dwarf star now called TRAPPIST-1.
Stars of this sort are ideal for ground-based planetary transit searches given their small sizes which result in larger decreases in brightness for the transit of a planet of a given size and the tighter orbits of their planetary systems.
If one planet is responsible for these two transit events, 11 different orbital periods ranging from 4.5 days to 72.8 days could satisfy the observations with 18.2 days being the most likely solution by far.
A thorough assessment of the habitability of any extrasolar planet would require a lot of detailed data on the properties of that planet, its atmosphere, its spin state and so on.
Combined with theoretical extrapolations of the factors that keep the Earth habitable, the best we can hope to do at this time is to compare the known properties of extrasolar planets to our current understanding of planetary habitability to determine if an extrasolar planet is “Potentially habitable.”
By “Habitable“, it’s meant to be habitable in an Earth-like sense where the surface conditions allow for the existence of liquid water on the planet’s surface.
The first attribute that provides a clue of the potential habitability of a planet is its radius.
There are no measurements of the masses of these worlds currently available which could help to constrain their bulk properties and determine if they are rocky planets like the Earth or volatile-rich mini-Neptunes with no prospect of being habitable in the conventional sense.
The reflex motion of the three planets orbiting TRAPPIST-1 would be expected to produce variations in the radial velocity of the star of 0.5 to a few meter per second, depending on the planets‘ compositions.
On the limits of the habitable zone based on comprehensive climate and geophysical modeling, the outer boundary of the HZ is conservatively defined as corresponding to the maximum greenhouse limit of a CO2-rich atmosphere where the addition of any more CO2 would not increase a planet’s surface temperature any further.
For an Earth-size planet orbiting TRAPPIST-1, this corresponds to a Seff of 0.92 or a distance of 0.023 AU. However, Gillon et al.
Which run from 0.033 to 0.093 AU, half appear to be beyond the outer edge of the HZ including the most likely solution corresponding to 0.058 AU.
Looking at the relative probabilities of the various orbital solutions, it appears that there is only about a 20% chance that TRAPPIST-1d orbits inside the conservatively defined HZ for a synchronously rotating, Earth-size planet.
Obviously, more data will help to resolve the situation with this planet, but there appears to be only a relatively slim possibility that this extrasolar planet is potentially habitable given what we know today.
Contrary to the claims being made in the media, it appears unlikely that any of the three planets discovered orbiting TRAPPIST-1 are potentially habitable in the Earth-like sense.
The situation with the outer planet is ambiguous and will require more data to define its orbit better but at this time, it seems probable that it orbits beyond the outer edge of the HZ.
