An international team of researchers has used the NASA/ESA/CSA James Webb Space Telescope to measure the temperature of the rocky exoplanet TRAPPIST-1 b. The measurement is based on the planet’s thermal emission: heat energy given off in the form of infrared light detected by Webb’s Mid-Infrared Instrument (MIRI). The result indicates that the day side of the planet has a temperature of about 500 Kelvin (roughly 230°C), and suggests that there is no significant atmosphere. This is the first detection of any kind of light emitted from an exoplanet as small and cool as the rocky planets in our own solar system. The result marks an important step in determining whether planets orbiting small active stars like TRAPPIST-1 can maintain the atmospheres needed to support life. It also bodes well for Webb’s ability to characterize Earth-sized temperate exoplanets using MIRI.
“These observations really take advantage of Webb’s mid-infrared capabilities,” said Thomas Greene, an astrophysicist at NASA’s Ames Research Center and lead author of the study, published today in the journal Nature. “No previous telescopes were so sensitive to measure such faint mid-infrared light.”
Rocky planets orbit ultracool red dwarfs
In early 2017, astronomers reported the discovery of seven rocky planets orbiting an ultracool 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 own solar system. While they all orbit much closer to their star than our planets orbit the sun — they could all easily fit into Mercury’s orbit — they receive similar amounts of energy from their tiny star.
TRAPPIST-1 b, the innermost planet, has an orbital distance about one-hundredth that of Earth and receives about four times as much energy as Earth gets from the sun. Although it is not within the system’s habitable zone, observations of the planet could yield important information about its sister planets, as well as those of other M dwarf systems.
“There are ten times as many of these stars in the Milky Way as stars like the sun, and they are twice as likely to be rocky planets as stars like the sun,” Green explained. “But they’re also very active — they’re very bright when they’re young and they give off flares and X-rays that can wipe out an atmosphere.”
Co-author Elsa Ducrot of CEA in France, who was part of the team conducting the initial studies of the TRAPPIST-1 system, added: “It is easier to characterize terrestrial planets around smaller, cooler stars. If we want to understand the habitability around M stars, the TRAPPIST-1 system is a great laboratory. These are the best targets we have for looking at the atmospheres of rocky planets.”
Detect atmosphere (or not)
Previous observations of TRAPPIST-1 b with the NASA/ESA Hubble Space Telescope, as well as NASA’s Spitzer Space Telescope, found no evidence of a swollen atmosphere, but could not rule out a dense atmosphere.
One way to reduce the uncertainty is to measure the temperature of the planet. “This planet is tidally locked, with one side always facing the star and the other in permanent darkness,” said CEA’s Pierre-Olivier Lagage, a co-author of the paper. “If it has an atmosphere to circulate and redistribute the heat, the day side will be cooler than if there is no atmosphere.”
The team used a technique called secondary eclipse photometry, in which MIRI measures the system’s change in 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 alone (during the secondary eclipse) from the brightness of the star and the planet together, they were able to successfully calculate how much infrared light is being emitted by the planet.
Measuring tiny changes in brightness
Webb’s detection of a secondary eclipse is itself an important milestone. With the star more than 1,000 times brighter than the planet, the change in brightness is less than 0.1%.
“There was also some fear that we might miss the eclipse. The planets are all pulling together, so the orbits aren’t perfect. said Taylor Bell, the postdoctoral researcher at the Bay Area Environmental Research Institute who analyzed the data. “But it was just amazing. The eclipse time we saw in the data matched the predicted time within a few minutes.”
Analysis of data from five separate secondary eclipse observations indicates that TRAPPIST-1 b has a daytime temperature of about 500 Kelvin, or about 230°C. The team thinks the most likely interpretation is that the planet has no atmosphere.
“We compared the results with computer models that show what the temperature should be in different scenarios,” Ducrot explained. “The results match almost perfectly with a black body made of bare rock and with no atmosphere to circulate the heat. We also saw no signs of light being absorbed by carbon dioxide, which would be evident in these measurements.”
This study was conducted as part of Guaranteed Time Observation (GTO) Program 1177, one of eight approved GTO and General Observer (GO) programs designed to fully characterize the TRAPPIST-1 system. Additional secondary eclipse observations of TRAPPIST-1 b are currently underway, and now that they know how good the data can be, the team hopes to eventually capture a full phase curve showing the change in brightness across the entire orbit. This allows them to see how the temperature changes from day to night and confirm whether the planet has an atmosphere or not.
“There was one target I dreamed I had”, said Lagage, who spent more than two decades developing the MIRI instrument. “And it was this one. This is the first time we can detect the emission from a rocky temperate planet. It is a very important step in the story of exoplanet discovery.”