This graph compares the daytime temperature of TRAPPIST-1 b measured with Webb's Mid-Infrared Instrument (MIRI) with computer models of what the temperature would be under various conditions.

This graph compares the daytime temperature of TRAPPIST-1 b measured with Webb’s Mid-Infrared Instrument (MIRI) with computer models of what the temperature would be under various conditions. – NASA, ESA, CSA, J. OLMSTED (STSCI)


An international team of researchers used the James Webb Space Telescope to measure the temperature of the rocky exoplanet TRAPPIST-1 b, located 40 light years away.

The measurement is based on the planet’s thermal emission, that is, the thermal energy emitted in the form of infrared light detected by Webb’s Mid-Infrared Instrument (MIRI).

The result indicates that the dayside of the planet has a temperature of around 230 degrees Celsius (450 degrees Fahrenheit) and suggests it has no significant atmosphere.

This is the first detection of any form of light emitted by an exoplanet as small and as 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 they can maintain the atmospheres necessary to sustain life. It also bodes well for Webb’s ability to use the MIRI instrument to characterize Earth-sized exoplanets with temperate temperatures.

“These observations take great advantage of Webb’s mid-infrared capabilities,” said it’s a statement Thomas Greene, an astrophysicist at NASA Ames Research Center and lead author of the study published in the scientific journal Nature. “No previous telescope had the sensitivity to measure such faint infrared light.”

In early 2017, astronomers reported the discovery of seven rocky planets orbiting an ultracool red dwarf (or M dwarf) star 40 light-years from Earth. What is remarkable about the planets is their similarity in size and mass to the inner rocky planets of our own solar system. Although they all orbit much closer to their stars than any of our planets orbit the Sun – they could all fit comfortably in Mercury’s orbit – they receive comparable amounts of energy from their little star.

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

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

Co-author Elsa Ducrot, from the French Commission for Atomic Energy and Alternative Energies (CEA) in France, who was part of the team that carried out previous studies of the TRAPPIST-1 system, added: “It is easier to characterize terrestrial planets that move around smaller stars and colder If we want to understand habitability around M-type stars, the TRAPPIST-1 system is a great laboratory. These are the best targets we have for observing the atmospheres of rocky planets.”

Previous observations of TRAPPIST-1 b with the Hubble and Spitzer Space Telescopes found no evidence of a puffy atmosphere, but could not rule out a thick atmosphere.

One way to reduce uncertainty is to measure the planet’s temperature. “This planet is tidally locked, with one side facing the star at all times and the other in permanent darkness,” said Pierre-Olivier Lagage of the CEA, co-author of the paper. “If you have an atmosphere to circulate and redistribute heat, the dayside will be cooler than if there’s no atmosphere.”

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

Webb’s detection of a secondary eclipse is in itself an important milestone. The observed star is more than 1,000 times brighter than the planet, and the change in brightness is less than 0.1%.

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

The team analyzed data from five separate observations of minor eclipses. “We compared the results with computer models that showed what the temperature should be in different scenarios,” explained Ducrot. “The results are almost perfectly consistent with a blackbody made of bare rock and no atmosphere to circulate the heat. We also didn’t see any signs of light being absorbed by the carbon dioxide, which would be evident in these measurements.“.

This research was conducted as part of Webb’s Time Guaranteed Observations (GTO) program number 1177, which is one of eight programs in Webb’s first year of scientific investigations designed to help fully characterize the TRAPPIST-1 system. Additional observations of TRAPPIST-1’s secondary eclipses are currently underway, so now that they know how good the data can be, the team hopes to later capture a full phase curve showing the change in brightness over the entire orbit. This will allow them to see how the temperature changes from the day side to the night side and confirm whether or not the planet has an atmosphere.

“There was a target that I dreamed of having,” said Lagage, who worked on developing the MIRI instrument for more than two decades. “And that was it. It’s the first time we’ve been able to detect emission from a rocky, temperate planet.. This is a really important step in the history of exoplanet discovery.”