Using the James Webb Space Telescope (JWST), astronomers have confirmed that the planet TRAPPIST-1b probably has no atmosphere. Researchers have been excited to use the new telescope to explore it and its six siblings, which are all roughly the size of Earth and which orbit a star 12 parsecs (39 light years) from Earth. The system is a unique laboratory for studying how environmental conditions arise on planets — and how they might become suitable for life.
Last November and December, JWST searched for an atmosphere on TRAPPIST-1b by looking for heat radiating from it. With the telescope’s ability to study infrared light, “you can actually measure the glow of the planet”, says Thomas Greene, an astronomer at NASA’s Ames Research Center in Mountain View, California. He and his colleagues report their results today in Nature1.
Although the finding might sound disappointing to those hoping for an atmosphere, scientists say that the work showcases JWST’s transformational power and opens the door for more results from the TRAPPIST-1 system.
Previous studies with the Hubble and Spitzer space telescopes, using a different technique, showed that TRAPPIST-1b — the innermost planet in the system — probably doesn’t have a large puffy atmosphere made mostly of hydrogen2. But researchers couldn’t rule out whether it has a dense atmosphere, as Earth might have had billions of years ago.
A planetary laboratory
JWST looked at TRAPPIST-1 in mid-infrared wavelengths of light — 20 times redder than the human eye can see — to see how that radiation changed as TRAPPIST-1b moved behind the star. By measuring the brightness of the star and planet together compared with that of the star alone, astronomers could calculate how much came from the planet.
If TRAPPIST-1b had an atmosphere, it would have re-circulated the energy absorbed from the star and appeared less bright than Greene and his colleagues measured. The observations did not reveal any carbon dioxide on the planet, which JWST might have been able to spot.
It’s not surprising that TRAPPIST-1b has no atmosphere, because it is blasted by four times as much radiation as Earth receives from the Sun. TRAPPIST-1 is also wracked by stellar flares and other activity that sends radiation across its planets, potentially scouring away atmospheres. Still, understanding these conditions is crucial because M dwarf stars — cool, dim stars such as TRAPPIST-1 — often have Earth-sized planets orbiting them.
A key early step in studying the TRAPPIST-1 system is to take advantage of JWST’s power to understand the star itself, says Julien de Wit, an exoplanet researcher at the Massachusetts Institute of Technology in Cambridge: “know thy star” should come before “know thy planets”. Astronomers should work together to study TRAPPIST-1 with JWST using multiple techniques, to understand the star as fully as possible, he says. Otherwise, researchers will struggle to interpret what they are seeing in planetary observations, because the star’s activity could be contaminating those measurements.
More discoveries are sure to come. Other research teams have been using JWST to study TRAPPIST-1b, as well as other planets in the system. That includes TRAPPIST-1b’s neighbor, TRAPPIST-1c, a planet that is close enough to its star for JWST to study its glow. Publications on all of these are expected soon.