The seven-planet system orbiting TRAPPIST-1 is one of the more intriguing exoplanetary discoveries of the last few years and the possibility for one or more of these worlds to be habitable is extremely exciting. Three of the seven Earth-sized planets in this system are in a region around the star where they could have liquid water on their surfaces.
But could is not must. And researchers are trying to better understand the more detailed properties of these planets to work out whether they can truly support life. In a new paper published in Astronomy & Astrophysics, an international group of researchers has modeled the potential interior structure of the TRAPPIST-1 planets, which are referred to as letters from b to h in order of distance from the star. The interiors of most of the planets are at least partially molten due to gravitational effects.
“Assuming the planets are composed of water ice, rock, and iron, we determine how much of each might be present, and how thick the different layers would be," lead author Dr Amy Barr said in a statement. "Because the masses and radii of the planets are not very well-constrained, we show the full range of possible interior structures and interior compositions."
“Because the TRAPPIST-1 star is very old and dim, the surfaces of the planets have relatively cool temperatures by planetary standards, ranging from 400 degrees Kelvin (260°F), which is cooler than Venus, to 167 degrees Kelvin (-159°F), which is colder than Earth’s poles,” Barr added. “The planets also orbit very close to the star, with orbital periods of a few days. Because their orbits are eccentric – not quite circular – these planets could experience tidal heating just like the moons of Jupiter and Saturn.”
The models show that based on tidal heating planet e is the most suitable planet for life. Planet d is likely to have similar conditions but is also likely to be covered by a global ocean. Planets b and c could have a molten mantle and planet c at least has a solid surface, suggesting that it might look like the tidally heated moon Io.
Planets f, g, and h couldn’t be constrained exactly due to the probable higher presence of ice on their surfaces. More data and better modeling will hopefully provide answers regarding these other planets, but it’s a shame as it's recently been shown that the further out a planet is, the more likely it is to keep its atmosphere.
TRAPPIST-1 is a red dwarf smaller and dimmer than our Sun and its seven planets orbit very close together and very close to the star. It is located 39.46 light-years from Earth.