An international team of researchers using NASA’s James Webb Space Telescope has measured the temperature of the rocky exoplanet TRAPPIST-1 b. The measurement is based on the planet’s thermal radiation: thermal energy emitted in the form of infrared light is detected by the Webb Mid-Infrared Instrument (MIRI). The result shows that the planet has a daytime temperature of about 500 Kelvin (about 450 degrees Fahrenheit) and does not have a significant atmosphere.

This is the first detection any a form of light emitted by an exoplanet as small and cold as the rocky planets in our solar system. The result marks an important step in determining whether planets orbiting small, active stars like TRAPPIST-1 can support the atmosphere necessary to support life. It also bodes well for Webb’s ability to characterize temperate Earth-sized exoplanets with MIRI.

“These observations really take advantage of Webb’s mid-infrared capabilities,” said Thomas Green, an astrophysicist at NASA Ames Research Center and lead author of the study. Nature. “No previous telescope had the sensitivity to measure such weak mid-infrared light.”

This plot compares the daytime temperature of TRAPPIST-1 b measured by the Webb Mid-Infrared Instrument (MIRI) with computer models of the temperature under different conditions.

Rocky planets orbiting extremely cold red dwarfs

In early 2017, astronomers announced the discovery of seven rocky planets orbiting an ultra-cold red dwarf star (or M-dwarf) 40 light-years from Earth. The remarkable thing about the planets is their similarity in size and mass to the inner rocky planets of our solar system. Although they all orbit much closer to their stars than our planets around the Sun – they can all fit comfortably in Mercury’s orbit – they get comparable amounts of energy from their tiny stars.

The innermost planet, TRAPPIST-1 b, has an orbital distance of about one percent that of Earth and receives about four times as much energy as Earth receives from the Sun. Although not in the system’s habitable zone, its observations can provide important information about both its sister planets and planets of other M-dwarf systems.

“There are ten times more of these stars in the Milky Way than stars like the Sun, and they’re twice as likely to have rocky planets as stars like the Sun,” Green said. “But they’re also very active—they’re very bright when they’re young, and they emit flares and X-rays that can destroy the atmosphere.”

Co-author Elsa Ducrot of the Alternative Energy and Atomic Energy Commission (CEA) in France, who was on the team that conducted previous studies of the TRAPPIST-1 system, added: “It’s easier to characterize terrestrial planets around smaller, cooler stars. “The TRAPPIST-1 system is an excellent laboratory if we want to understand the possibility of habitability around it. These are the best objects we have for studying the atmospheres of rocky planets.”

Atmosphere detection (or not)

Previous observations of TRAPPIST-1 b using the Hubble and Spitzer space telescopes found no evidence of a loose atmosphere, but could not rule out the presence of a dense atmosphere either.

One way to reduce uncertainty is to measure the temperature of the planet. “This planet is tidally locked, with one side always facing the star and the other side in perpetual darkness,” said Pierre-Olivier Lagage, CEA’s co-author of the paper. “If it has an atmosphere to circulate and redistribute heat, it will be colder during the day than it is without an atmosphere.”

The team used a technique called secondary dimming photometry, in which MIRI measures the change in brightness in the system as the planet moves behind the star. Although TRAPPIST-1b is not hot enough to emit its own visible light, it does have an infrared glow. By subtracting the star’s brightness (during the second eclipse) separately from the total brightness of the star and planet, they were able to successfully calculate how much infrared light the planet was emitting.

This light curve shows the change in brightness of the TRAPPIST-1 system as the innermost planet TRAPPIST-1 b moves behind the star. This phenomenon is known as secondary eclipse.

Measuring small changes in brightness

Webb’s discovery of the secondary solar eclipse is an important milestone in itself. Because the star is 1000 times brighter than the planet, the variation in brightness is less than 0.1%.

“There were also some fears that we would miss the eclipse. “All the planets are pulling towards each other, so the orbits aren’t perfect,” said Taylor Bell, a postdoctoral researcher at the Bay Area Institute for Environmental Studies who analyzed the data. “But it was simply amazing: the eclipse time we saw in the data matched the predicted time in minutes.”

The team analyzed data from five separate observations of secondary eclipses. “We compared the results with computer models that show what the temperature should be in different scenarios,” Ducrou said. “The results are almost entirely consistent with a black body made of bare rock and there is no atmosphere to circulate heat. Also, we did not see any evidence of light absorption by carbon dioxide in these measurements.”

This work was conducted as part of the Webb Guaranteed Time Observation (GTO) 1177 program, one of eight programs in the first year of science designed to help fully characterize Webb’s TRAPPIST-1 system. Additional secondary observations of the TRAPPIST-1 b eclipse are currently underway, and now that they know how good the data can be, the team hopes to finally capture a full phase curve showing the change in brightness in orbit. This will allow them to see how the temperature changes from day to night and confirm whether the planet has an atmosphere.

“There was only one goal that I dreamed of,” said Lage, who has worked on the development of the MIRI instrument for over two decades. “And that was it. It’s the first time we’ve been able to detect radiation from a rocky planet with a temperate climate. This is a really important step in the history of exoplanets discovery.”