TRAPPIST‑1 gets temperature maps

Astronomers have turned James Webb Space Telescope heat data into the first climate maps of two rocky exoplanets, TRAPPIST-1b and TRAPPIST-1c. (nature.com) A phase curve is a full-orbit movie made from infrared light: as a planet circles its star, Webb records how the system brightens and dims, letting researchers infer where the hot and cool regions sit. The new study used Webb’s Mid-Infrared Instrument at 15 microns to track both planets through their orbits. (nature.com) The team reported that TRAPPIST-1b has a dayside brightness temperature of 490 ± 17 kelvin, no significant nightside emission, and no measurable hotspot shifted away from the point directly under the star. TRAPPIST-1c came in cooler on its dayside at 369 ± 23 kelvin, with nightside flux statistically indistinguishable from TRAPPIST-1b’s. (nature.com) Those patterns match worlds that absorb starlight on one face and reradiate it locally, instead of moving much heat to the far side with thick air. The authors said atmospheres with surface pressures of 1 bar or more and strong greenhouse warming are strongly disfavored for both planets. (nature.com) (arxiv.org) TRAPPIST-1 is a red dwarf about 41 light-years away with seven known Earth-size planets, a system first expanded to seven worlds in February 2017 after Spitzer Space Telescope follow-up. The inner planets are especially useful because they orbit fast enough for Webb to watch large chunks of an orbit in a practical observing campaign. (science.nasa.gov) (spitzer.caltech.edu) (science.nasa.gov) TRAPPIST-1b circles its star every 1.51 days at 0.01154 astronomical units, and the archive lists TRAPPIST-1c at 2.42 days. At those distances, both planets are expected to be tidally locked, with one permanent dayside and one permanent nightside. (science.nasa.gov) (exoplanetarchive.ipac.caltech.edu) This result narrows a question that has hung over red-dwarf planets for years: whether rocky worlds so close to small stars can hang onto dense secondary atmospheres after long exposure to stellar radiation. The Nature Astronomy paper says the new phase curves are consistent with bare rocky surfaces rather than thick atmospheres on either planet. (nature.com) Researchers at the University of Geneva and the University of Bern said the campaign took about 60 hours of continuous observations and allowed temperatures to be constrained on both the day and night sides. That turns a single average temperature into a map of how heat is distributed across each world. (eurekalert.org) The earlier Webb result on TRAPPIST-1b, based on eclipse measurements at 15 microns, had already pointed to little or no heat redistribution and no clear sign of a thick carbon-dioxide atmosphere. The new full-orbit mapping extends that test to both b and c with a more direct read on circulation. (nasa.gov) (nature.com) For now, the two innermost TRAPPIST-1 planets look less like mini-Venuses and more like bare rock under permanent noon and permanent midnight. Webb’s next exoplanet climate tests will be watched just as closely on the cooler worlds farther out in the same system. (nature.com)