Free Astronomy Magazine May-June 2023

49 MAY-JUNE 2023 ASTRO PUBLISHING are 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 they’re young, and they give off flares and X-rays that can wipe out an atmosphere.” Co-author Elsa Ducrot from the French Alternative Energies and Atomic Energy Commission (CEA) in France, who was on the team that conducted earlier studies of the TRAPPIST-1 system, added, “It’s easier to characterize terrestrial planets around smaller, cooler stars. If we want to understand 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.” Previous observations of TRAPPIST-1 b with the Hub- ble and Spitzer space telescopes found no evidence for a puffy atmosphere, but were not able to rule out a dense one. One way to reduce the 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 from CEA, a co-author on the paper. “If it has an atmosphere to circulate and redistrib- ute the heat, the dayside will be cooler than if there is no atmosphere.” The team used a technique called secondary eclipse photometry, in which MIRI measured the change in brightness from the system as the planet moved be- hind the star. Although TRAPPIST-1 b is not hot enough to give off its own visible light, it does have an infrared glow. By subtracting the brightness of the star on its own (during the secondary eclipse) T his illustration shows what the hot rocky exoplanet TRAPPIST-1 b could look like based on this work. TRAPPIST-1 b, the innermost of seven known planets in the TRAPPIST-1 system, orbits its star at a distance of 0.011 AU, completing one circuit in just 1.51 Earth- days. TRAPPIST-1 b is slightly larger than Earth, but has around the same density, which indicates that it must have a rocky composition. Webb’s measurement of mid-infrared light given off by TRAPPIST-1 b suggests that the planet does not have any substantial atmos- phere. The star, TRAPPIST-1, is an ultracool red dwarf (M dwarf) with a temperature of only 2,566 kelvins and a mass just 0.09 times the mass of the Sun. This il- lustration is based on new data gathered by Webb’s Mid-Infrared Instrument (MIRI) as well as previous ob- servations from other ground- and space-based tele- scopes. Webb has not captured any images of the planet. [NASA, ESA, CSA, Joseph Olmsted (STScI). Sci- ence: Thomas P. Greene (NASA Ames), Taylor Bell (BAERI), Elsa Ducrot (CEA), Pierre-Olivier Lagage (CEA)]