A new study has found that several exoplanets orbiting the nearby ultra-cool dwarf star TRAPPIST-1, which are all made mostly of rock, could hold more water than Earth. The exoplanets' densities, now known much more precisely than before, suggest that some of them could have up to 5 percent of their mass in the form of water, about 250 times more than Earth's oceans.
The fourth planet out is the most similar to Earth. It seems to be the rockiest planet of the seven, and has the potential to host liquid water. ExoPlanets around the faint red star TRAPPIST-1, just 40 light-years from Earth, were first detected by the TRAPPIST-South telescope at ESO’s La Silla Observatory in 2016.
A team of scientists led by Simon Grimm at the University of Bern in Switzerland have now applied very complex computer modelling methods to all the available data and have determined the planets’ densities with much better precision than was possible before.
"The TRAPPIST-1 planets are so close together that they interfere with each other gravitationally, so the times when they pass in front of the star shift slightly. These shifts depend on the planets' masses, their distances, and other orbital parameters. With a computer model, we simulate the planets' orbits until the calculated transits agree with the observed values, and hence derive the planetary masses." Says Grimm
The measurements of the densities, when combined with models of the planets’ compositions, strongly suggest that the seven TRAPPIST-1 planets are not barren rocky worlds. They seem to contain significant amounts of volatile material, probably water, amounting to up to 5% the planet's mass in some cases. By comparison, the Earth has only about 0.02% water mass.
"Densities, while important clues to the planets' compositions, do not say anything about habitability. However, our study is an important step forward as we continue to explore whether these planets could support life," said Brice-Olivier Demory, a co-author at the University of Bern.
Two of the other planets orbiting the star are likely to have rocky cores and be surrounded by atmospheres much thicker than Earth's whilst others may be layered with Ice.