We reported a few months ago the discovery of three potentially habitable exoplanets "just" 40 light-years away. Now, two of those planets have been confirmed to be rocky.
After the first discovery was announced on May 2, the team was able to conduct a follow-up observation on May 4, catching two birds with one stone. Two of the three planets were passing in front of the star in a double transit, a perfect opportunity to study them at the same time.
“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,” said lead author Julien de Wit in a statement. “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 scientists used Hubble to look at the changes in light from the star TRAPPIST-1, a dim red dwarf with 8 percent the mass of the Sun. As the planets pass in front of the star, its light dips a small fraction. To assess the planetary atmospheres, they looked at the dips in many different wavelengths.
“The data turned out to be pristine, absolutely perfect, and the observations were the best that we could have expected,” continued de Wit. “The force was certainly with us.”
As reported in Nature, the dips were seen to not vary significantly from wavelength to wavelength, implying that the planets lack a large puffy atmosphere that would block some types of light and not others. The planets must thus be rocky, similar to Earth and Venus.
“Now we can say that these planets are rocky. Now the question is, what kind of atmosphere do they have?” said de Wit. “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 next step is to try to disentangle all these possible scenarios that exist for these terrestrial planets.”
The planets, TRAPPIST-1b and TRAPPIST-1c, have a radius 1.11 and 1.05 times that of Earth and they are within the habitable zone of the star. Depending on what the atmosphere is made of, they could have liquid water on their surface and maybe even life. But more analysis is necessary.
“With more observations using Hubble, and further down the road with [the James Webb Space Telescope], 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.”