TRAPPIST-1b and c, the two innermost planets in the three-planet system TRAPPIST-1, are primarily rocky, unlike gas giants such as Jupiter, according to a group of astronomers led by MIT scientist Julien de Wit.
This artist’s impression shows an imagined view from the surface one of the three exoplanets orbiting the ultracool dwarf star TRAPPIST-1. These alien worlds – named TRAPPIST-1b, c and d – have sizes and temperatures similar to those of Venus and Earth. TRAPPIST-1b and TRAPPIST-1c have orbital periods of 1.51 days and 2.42 days, respectively, and the third planet, TRAPPIST-1d, has a less well determined period in the range 4.5 to 72.8-days. Image credit: M. Kornmesser / ESO.
TRAPPIST-1 is an ultracool dwarf star located only 38.8 light-years away in the constellation of Aquarius.
The star is much cooler and redder than our Sun, with an age constrained to be 500 million years old, and with a luminosity, mass, and radius of 0.05%, 8% and 11.5% those of the Sun, respectively.
On May 2, Dr. de Wit and co-authors announced they had discovered three short-period Earth-sized planets close to the habitable zone of the TRAPPIST-1 system: TRAPPIST-1b, c and d.
Now, in a paper published this week in the journal Nature, the team reports that TRAPPIST-1 b and c – the two innermost planets in the system – are primarily rocky.
The findings strengthen the case that these alien worlds may indeed be habitable.
The astronomers also determined that the atmospheres of both planets are very compact, similar to the atmospheres of Earth, Venus, and Mars.
They came to this conclusion after making a preliminary screening of the planets’ atmospheres, just days after announcing the discovery of the planetary system.
On May 4, they commandeered the NASA/ESA Hubble Space Telescope and pointed it at the system’s star to catch a rare event: a double transit, the moment when two planets almost simultaneously pass in front of their star.
The team realized the planets would transit just two weeks before the event, thanks to refined estimates of the planets’ orbital configuration.
“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,” Dr. de Wit said.
“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.”
Using Hubble, the astronomers recorded a combined transmission spectrum of TRAPPIST-1b and c, meaning that as first one planet then the other crossed in front of the star, they were able to measure the changes in wavelength as the amount of starlight dipped with each transit.
“Now we can say that these planets are rocky,” Dr. de Wit said. “Now the question is what kind of atmosphere do they have?”
“The plausible scenarios include something like Venus, where the atmosphere is dominated by carbon dioxide, or an Earth-like atmosphere with heavy clouds, or even something like Mars with a depleted atmosphere.”
The team’s next step will be to try to disentangle all possible scenarios that exist for these planets.
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Julien de Wit et al. A combined transmission spectrum of the Earth-sized exoplanets TRAPPIST-1 b and c. Nature, published online July 20, 2016; doi: 10.1038/nature18641
