This artist's concept shows what the TRAPPIST-1 planetary system may look like, based on available data about the planets' diameters, masses and distances from the host star, as of February 2018.
In the year since NASA announced the seven Earth-sized planets of the TRAPPIST-1 system, scientists have been working hard to better understand these enticing worlds just 40 light-years away. Thanks to data from a combination of space- and ground-based telescopes, we know more about TRAPPIST-1 than any other planetary system besides our solar system.
A new study in the journal Astronomy and Astrophysics, using data from NASA's Spitzer and Kepler space telescopes, offers the best-yet picture of what these planets are made of. They used the telescope observations to calculate the densities more precisely than ever, then used those numbers in complex simulations. Researchers determined that all of the planets are mostly made of rock. Additionally, some have up to 5 percent of their mass in water, which is 250 times more than the oceans on Earth.
The form that water takes on TRAPPIST-1 planets would depend on how much heat they receive from their ultra-cool dwarf star, which is only about 9 percent as massive as our Sun. Planets closest to the star are more likely to host water in the form of atmospheric vapor, while those farther away may have water frozen on their surfaces as ice. TRAPPIST-1e is the rockiest planet of them all, but is still believed to have the potential to host some liquid water.
The question of the planets' atmospheres is also important for understanding whether liquid water could be present on these surfaces -- an essential ingredient for habitability. NASA's Hubble Space Telescope has now surveyed six of the seven TRAPPIST-1 planets, and new results on four of them are published in Nature Astronomy. In the new study, Hubble reveals that at least three of the TRAPPIST-1 planets -- d, e, and f -- do not seem to contain puffy, hydrogen-rich atmospheres like the gas giants of our own solar system. Hydrogen is a greenhouse gas, and would make these close-in planets hot and inhospitable to life.
In 2016, Hubble observations also did not find evidence for hydrogen atmospheres in c and d. These results and the new ones, instead, favor more compact atmospheres like those of Earth, Venus and Mars. Additional observations are needed to determine the hydrogen content of planet g's atmosphere.
Both studies help pave the way for NASA’s James Webb Space Telescope, scheduled to launch in 2019. Webb will probe deeper into the planetary atmospheres, searching for heavier gases such as carbon dioxide, methane, water and oxygen. The presence of such elements could offer hints of whether life could be present, or if the planets are habitable.
TRAPPIST-1 is named for the Transiting Planets and Planetesimals Small Telescope (TRAPPIST) in Chile, which discovered two of the seven TRAPPIST planets we know of today -- announced in February 2016. NASA's Spitzer Space Telescope, in collaboration with ground-based telescopes, confirmed these planets and uncovered the other five in the system.
Some of TRAPPIST-1’s planets could have life-friendly atmospheres
t’s still too early to pack your bags for TRAPPIST-1. But two new studies probe the likely compositions of the seven Earth-sized worlds orbiting the cool, dim star, and some are looking better and better as places to live (SN: 3/18/17, p. 6).
New mass measurements suggest that the septet probably have rocky surfaces and possibly thin atmospheres, researchers report February 5 in Astronomy & Astrophysics. For at least three of the planets, those atmospheres don’t appear to be too hot for life, many of these same researchers conclude February 5 in Nature Astronomy.
TRAPPIST-1 is about 40 light-years from Earth, and four of its planets lie within or near the habitable zone, the range where temperatures can sustain liquid water. That makes these worlds tempting targets in the search for extraterrestrial life (SN: 12/23/17, p. 25)
One clue to potential habitability is a planet’s mass — something not precisely nailed down in previous measurements of the TRAPPIST-1 worlds. Mass helps determine a planet’s density, which in turn provides clues to its makeup. High density could indicate that a planet doesn’t have an atmosphere. Low density could indicate that a planet is shrouded in a puffy, hydrogen-rich atmosphere that would cause a runaway greenhouse effect.
Using a new computer technique that accounts for the planets’ gravitational tugs on each other, astronomer Simon Grimm of the University of Bern in Switzerland and his colleagues calculated the seven planets’ masses with five to eight times better precision than before. Those measurements suggest that the innermost planet probably has a thick, viscous atmosphere like Venus, Grimm says. The other six, which may be covered in ice or oceans, may have more life-friendly atmospheres. The fourth planet from the star has the same density as Earth and receives the same amount of radiation from its star as Earth, Grimm’s team reports in Astronomy & Astrophysics.
“This is really the cool thing: We have one planet which is very, very similar to the Earth,” Grimm says. “That’s really nice.”
Having an atmosphere could suggest habitability, but not if it’s too hot. So using the Hubble Space Telescope, MIT astronomer Julien de Wit and his colleagues, including some members from Grimm’s team, observed the four middle planets as they passed in front of the star. The team was looking for a signature in near-infrared wavelengths of light filtering through planets’ atmospheres. That would have indicated that the atmospheres were full of heat-trapping hydrogen.
In four different observations, Hubble saw no sign of hydrogen-rich atmospheres around three of the worlds, de Wit and colleagues report in Nature Astronomy. “We ruled out one of the scenarios in which it would have been uninhabitable,” de Wit says.
The new observations don’t necessarily mean the planets have atmospheres, much less ones that are good for life, says planetary scientist Stephen Kane of the University of California, Riverside. It’s still possible that the star’s radiation blew the planets’ atmospheres away earlier in their histories. “That’s something which is still on the table,” he says. “This is a really important piece of that puzzle, but there are many, many pieces.”
Finishing the puzzle may have to wait for the James Webb Space Telescope, scheduled to launch in 2019, which will be powerful enough to figure out all the components of the planets’ atmospheres — if they exist.