TRAPPIST-1 planets orbits suggest they’re soaked in substantial amounts of water
In 2016, astronomers located a batch of presumably rocky exoplanets orbiting TRAPPIST-1, a red dwarf star 40 light years from Earth. While red dwarf stars are generally cooler than our own Sun, some of these planets have tight enough orbits to put them in habitable temperature ranges for life, assuming that life was happy living under a dim red sky. Continuing observations of the TRAPPIST-1 system has revealed more details about these worlds, including the likelihood of vast amounts of liquid water on their rocky surfaces.
Calculating exoplanet composition
At 40 light years away, astronomers aren’t observing signs of water or ice directly. Instead, they’re taking measurements of planets TRAPPIST-1b through TRAPPIST-1h and comparing how they interact with each other’s orbits. Like all matter, each planet creates its own gravitational field that pulls on its neighbors. While they’re not overpowering the pull of their star, researchers have been able to detect small wobbles in the tight orbits of each planet when they move near each other, allowing them to come up with estimates for how massive each planet might be. That information is then compared to estimates for the planets’ volumes, a figure based on how much light each planet blocks when passing in front of the TRAPPIST-1 star. To verify these estimates, researchers then plugged them into simulations of the system, making adjustments until their virtual orbits matched observed data.
Once each planet’s mass and volume were known, researchers calculated their density. The resulting picture of each planet’s composition isn’t inert pieces of rock, but worlds with large amounts of “volatile,” or more dynamic, materials on their surface. Based on the planets’ close proximity to their star, there’s a good chance that this volatile material is some form of water thanks to that molecule’s abundance in the materials that eventually go on to form planets. What’s more, there’s a good possibly that this water exists as a liquid under a thick, steamy atmosphere. Before you picture a second Earth under a reddish sky, planets like TRAPPIST-1b and TRAPPIST-1c may actually make our own planet look dry, as they’ve been estimated to be made of up to five percent water, versus Earth’s 0.02 percent.
Home to water, but without being wet
Not every TRAPPIST-1 planet is expected to be wet, spherical sauna though. TRAPPIST-1d is the smallest of the group, and may have a layer of ice on its surface. TRAPPIST-1e is a little denser than Earth, probably thanks to a more substantial iron core. It likely lacks a considerable atmosphere, with rockier composition overall. Further out, TRAPPIST-1f, g and h are probably too far from their star to maintain a large amount of liquid water, much less vapor. Any water they have would be frozen, and there’s no sign of a substantial atmosphere at this point.
Is there water around every red dwarf?
While there’s more to be learned about the TRAPPIST-1 system, researchers would also like to start using these simulations and analytical techniques on other red dwarf solar systems. It seems that not every star needs to be yellow-hot to be home to some very attractive-looking planets, but it would be great to know just how common these sorts of water-soaked spheres really are.
Source: TRAPPIST-1 planets probably rich in water by ESO, Science Daily