The planets of TRAPPIST-1 are not only rocky, but some of them could hold as much as 250 times more water than Earth’s oceans, according to their latest density measurements.


Though on many of the worlds this water would be frozen into ice sheets or in the form of water vapour in the planet’s atmosphere, there is potential that there could be liquid water on some of the planets.

The red dwarf star TRAPPIST-1 has interested scientists since the announcement that it ws surrounded by seven Earth-mass, potentially rocky, planets on 22 February 2017.

Astronomers immediately began to study the planetary system as closely as possible, trying to measure the size and mass of the planets as accurately as possible, in order to calculate their density.

“We now know more about TRAPPIST-1 than any other planetary system apart from our own.

The improved densities in our study dramatically refine our understanding of the nature of these mysterious worlds,” Sean Carey, manager of Spitzer Science Centre at the California Institute of Technology.

The densest planets are not the ones that are the closest to the star
Caroline Dorn, University of Zurich

By interpreting these densities, the planetary scientists were able to determine what these planets might look like:

  • TRAPPIST-1b, the innermost planet, has a rocky core surrounded by an atmosphere thicker than Earth.
  • TRAPPIST-1c is also rocky, but with a thinner atmosphere than b.
  • The third planet out, TRAPPIST-1d, is the lightest of the worlds. It’s thought that it has a layer of water, either as an ocean or an ice sheet, and is surrounded by an envelope of volatile substances.
  • The only planet denser than Earth is TRAPPIST-1e, and so may have an iron core but it doesn’t appear to have an atmosphere, ice layer or ocean.
  • TRAPPIST-1f, g and h are all so far away from the host star that there could be water ice across their surfaces. The could have a think atmosphere, but it would be unlikely to contain heavy gases such as carbon dioxide.

“It is interesting that the densest planets are not the ones that are the closest to the star, and that the colder planets cannot harbour thick atmospheres,” says Caroline Dorn, who took part in the study, from University of Zurich.

More detail about what these atmospheres might be like is still a matter of debate.

Observations with the Hubble Space Telescope have found no traces of a Neptune-like puffy hydrogen atmosphere, around planets d, e and f, but the observatory doesn’t really have the capability to tell us much more about these worlds.

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It is likely astronomers will have to wait until 2019, when the James Webb Space Telescope is due to launch, to find out what these planets are really like.


Ezzy Pearson is the News Editor of BBC Sky at Night Magazine. Her first book about the history of robotic planetary landers is out now from The History Press.