Something in the Air
The James Webb Space Telescope is looking for atmospheres— and signs of life—on planets circling the star TRAPPIST-1
Scientists believe that data from the James Webb Space Telescope (JWST) will enable them to figure out whether a group of nearby exoplanets are habitable or not, a NASA expert says.
JWST should be able to determine whether any of the planets orbiting the star TRAPPIST-1 have atmospheres, according to NASA astrobiologist Shawn Domagal-goldman. And If the exoplanets have the right kind of atmospheres, they may also have life.
Despite decades of scanning the skies for radio signals and an ever-growing list of exoplanets—the name given to planets orbiting other stars—scientists have never found evidence that life exists anywhere but Earth.
The TRAPPIST-1 System
The TRAPPIST-1 star system is a collection of at least seven exoplanets orbiting a dwarf star roughly 40 light-years away from Earth that is considered to be the leading candidate for potentially habitable worlds. While preliminary investigations have revealed that some of the planets are rocky and possibly temperate, the key question of whether or not they have atmospheres has been impossible to settle so far.
That was before JWST, NASA’S most powerful space telescope ever, was launched in December. By using a huge array of mirrors to help it capture as much light as possible, JWST has been peering into the cosmos and has already taken the deepest infrared image of the universe ever. For several weeks, it’s also had its sights trained on TRAPPIST-1.
“The thing that I think is universally interesting to just about everybody is whether or not these planets have atmospheres,” Domagal-goldman says. “It’s critical for habitability. The moon gets just as much energy from the sun as the Earth does, but it doesn’t hold on to its atmosphere and so it doesn’t have any oceans and it doesn’t have any global biosphere.”
By contrast, the TRAPPIST worlds are large enough to hold onto their atmospheres since their greater mass means more gravity. However, the unknown factor is whether their star TRAPPIST-1 has
been violent enough to blast these atmospheres away over time with outbursts of high-energy radiation. “So it’s a first-order question to answer for the habitability of these worlds: whether or not they do have atmospheres. It turns out that’s also the easiest thing that Webb can observe,” Domagal-goldman said.
The Stuff of Life
Once an atmosphere is detected on any of the TRAPPIST worlds, the next question will be: what are they made of? Scientists’ will stop wondering if these planets might have life, and will shift to finding out whether or not they actually do.
This gets more complicated, involving in-depth measurements of the planets via a technique known as transit spectroscopy. Essentially, scientists will use JWST to capture light from TRAPPIST-1 whenever it passes through the atmospheres of the exoplanets. If this light encounters any gasses, like oxygen, carbon dioxide, water vapor, or methane, then this will leave a telltale trace in the starlight that JWST can detect.
“If you want to look for the kinds of biosignatures that we have on Earth, which is to say oxygen and ozone from plants and algae, that’s a great biosignature because if you see it, it’s less likely to be produced by-non biological processes,” Domagal-goldman says.
One potential problem, though, is if a planet’s atmosphere is high in oxygen and ozone (ozone is atmospheric oxygen but with three oxygen atoms instead of two) the risk is that both of these will be mostly trapped below the planets’ clouds making them harder to observe.
“If we’re talking about a habitable planet making lots of oxygen, we’re talking about a global ocean, and you’re gonna have a hydrological cycle and cloud decks. If you have cloud decks, it’s going to basically block the lower atmosphere where all that oxygen is sitting,” Domagal-goldman says.
Fortunately, there’s another gas that can also be a telltale sign of life: methane, a gas that is known to be produced by living organisms on this planet. So much so, that scientists have been working to try to stop cows belching out so much of it, since methane is a strong greenhouse gas. The presence of methane in another world’s atmosphere would be especially exciting if it were mixed together with other gasses that cause it to break down.
“If you see the methane in the context of these other gasses that are destroying the methane, and you know those other gasses are also being replenished, that’s an indicator that the methane is not just there, but it’s being reproduced super rapidly,” says Domagal-goldman. “If you know that the methane is being destroyed rapidly, it has to be replenished rapidly and that rapid replenishment is the biosignature.”
It’s uncertain when a breakthrough discovery about the TRAPPIST-1 planets can be expected, but with an estimated 20 years ahead of it, Webb has plenty of time to look.
“If you asked me before the TRAPPIST-1 worlds were discovered eight years ago if JWST was going to be doing any science on this class of worlds I would have said it’s possible, but I would have tempered my enthusiasm because it would have required a darn near perfect telescope and darn near perfect targets,” Domagal-goldman says. “We now know that Webb is now acting darn near perfectly. And when we found the TRAPPIST worlds those were the darn near perfect targets.”
“For exploration of potentially habitable worlds, the question of whether the TRAPPIST planets in the habitable zone have atmospheres is the most important scientific question or observation that anyone’s going to be doing, with JWST or any other facility, probably for the next five or 10 years,” he says.