Free Astronomy Magazine May-June 2023

MAY-JUNE 2023 C omparison of the dayside tem- perature of TRAPPIST-1 b as measured using Webb’s Mid-In- frared Instrument (MIRI) to com- puter models showing what the temperature would be under vari- ous conditions. The models take into account the known properties of the system, including the temperature of the star and the planet’s orbital distance. The temperature of the dayside of Mercury is also shown for reference. [NASA, ESA, CSA, Joseph Olmsted (STScI). Science: Thomas P. Greene (NASA Ames), Taylor Bell (BAERI), Elsa Ducrot (CEA), Pierre-Olivier Lagage (CEA)] from the brightness of the star and planet combined, they were able to successfully calculate how much in- frared light is being given off by the planet. Webb’s detection of a secondary eclipse is itself a major 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’d miss the eclipse. The planets all tug on each other, so the orbits are not perfect,” said Taylor Bell, the post- doctoral researcher at the Bay Area Environmental Research Institute who analyzed the data. “But it was just amazing: The time of the eclipse that we saw in the data matched the predicted time within a couple of minutes.” The team analyzed data from five separate secondary eclipse observa- tions. “We compared the results to computer models showing what the temperature should be in different scenarios,” explained Ducrot. “The results are almost perfectly consis- tent 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 car- bon dioxide, which would be appar- ent in these measurements.” This research was conducted as part of Webb Guaranteed Time Observa- tion (GTO) program 1177, which is one of eight programs from Webb’s first year of science designed to help fully characterize the TRAPPIST-1 system. Additional secondary eclipse observations of TRAPPIST-1 b are currently in progress, 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 over the entire orbit. This will allow them to see how the temperature changes from the day to the nightside and confirm if the planet has an atmos- phere or not. “There was one target that I dreamed of having,” said Lagage, who worked on the development of the MIRI instrument for more than two decades. “And it was this one. This is the first time we can detect the emission from a rocky, temper- ate planet. It’s a really important step in the story of discovering exo- planets. ” !