Earth-sized exoplanets in nearby system ‘rocky’

Exoplanets discovered only 40 light years from Earth have solid bodies suitable for sustaining life.

20th July 2016
Artist's view of planets transiting a red dwarf star in the TRAPPIST-1 system © NASA, ESA, and STScI

Artist's view of planets transiting a red dwarf star in the TRAPPIST-1 system © NASA, ESA, and STScI

So far we’ve discovered more than 3,000 exoplanets circling around the star systems in our galaxy - but finding them is one thing, working out what they are made of is another. Now, scientists from MIT and the University of Liège have found that two exoplanets discovered earlier this year have rocky surfaces, and their size and temperature make them suitable for life. Best of all - they’re only 40 light-years away.

It’s a TRAP(PIST)!

Published in the journal Nature, the team used the Hubble Space Telescope to observe the planets in the TRAPPIST-1 planetary system as they passed simultaneously in front of their host star in a rare ‘double transit’. As the planets passed, they were able to measure changes in the infrared wavelengths emitted as the brightness of the star dipped. These wavelengths did not vary over a narrow range suggesting the exoplanets had a rocky make up, unlike gas giants such as Jupiter, whose wavelengths would have fluctuated significantly due to their large and puffy atmospheres.

It wasn’t all plain sailing for the planet hunters though - they only realised that the double transit would take place just two weeks before it actually happened.

"We thought, maybe we could see if people at Hubble would give us time to do this observation, so we wrote the proposal in less than 24 hours, sent it out, and it was reviewed immediately," says first author Julien de Wit, a postdoc in MIT's Department of Earth, Atmospheric and Planetary Sciences. "Now for the first time we have spectroscopic observations of a double transit, which allows us to get insight on the atmosphere of both planets at the same time.

"The data turned out to be pristine, absolutely perfect, and the observations were the best that we could have expected," de Wit says. "The force was certainly with us."

Something in the air

So the exoplanets are rocky, but what what kind of atmosphere do they have? “The plausible scenarios include something like Venus, where the atmosphere is dominated by carbon dioxide,” de Wit explains, “or an Earth-like atmosphere with heavy clouds, or even something like Mars with a depleted atmosphere.”

It seems amazing that we can discover these exoplanets at all, let alone what their atmosphere is like, but the team plan to use a new, relatively cheap, ground-based infrared telescope called TRAPPIST (TRAnsiting Planets and PlanetesImals Small Telescope), as a prescreening tool to study ultracool dwarf stars, which are less bright making it easier to see planets when objects pass in front, before pointing more powerful telescopes at them. 

"With more observations using Hubble, and further down the road with James Webb [Telescope scheduled to launch in October 2018], we can know not only what kind of atmosphere planets like TRAPPIST-1 have, but also what is within these atmospheres," de Wit says. "And that's very exciting."


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