HomeScience & TechNASA's Webb measures the temperature of a rocky exoplanet

NASA’s Webb measures the temperature of a rocky exoplanet

An international team of researchers used NASA’s James Webb Space Telescope to measure the temperature of the rocky exoplanet TRAPPIST-1b. The measurement is based on the planet’s thermal emission: thermal energy given off in the form of infrared light detected by the Webb Mid-Infrared Instrument (MIRI). The result shows that the dayside of the planet has a temperature of about 500 Kelvin (roughly 450 degrees Fahrenheit) and suggests that it has no significant atmosphere.

This is the first detection of any form of light emitted by an exoplanet as small and cold as the rocky planets in our own solar system. The result represents an important step in determining whether planets orbiting small, active stars like TRAPPIST-1 can sustain the atmosphere necessary for life. It also bodes well for Webb’s ability to characterize temperate Earth-sized exoplanets with MIRI.

“These observations really take advantage of Webb’s mid-infrared capability,” 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 telescope has had the sensitivity to measure such faint mid-infrared light.”

Rocky planets orbiting ultracool red dwarfs

In early 2017, astronomers announced the discovery of seven rocky planets orbiting an ultracool red dwarf (or M dwarf) 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.

 They all orbit much closer to their star than any of our planets orbit the Sun they all fit comfortably into Mercury’s orbit they receive comparable amounts of energy from their small star.

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

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

Co-author Elsa Ducrot of the French Commission for Alternative Energies and Atomic Energy (CEA) in France, who was on the team that carried out earlier studies of the TRAPPIST-1 system, added: “It is 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 observing the atmospheres of rocky planets.”

Atmosphere detection (or not)

Previous observations of TRAPPIST-1 b using the Hubble and Spitzer Space Telescopes found no evidence for a puffy atmosphere, but were unable to rule out a dense atmosphere.

One way to reduce uncertainty is to measure the planet’s temperature. “This planet is tidally locked, with one side facing the star all the time and the other in permanent darkness,” said CEA’s Pierre-Olivier Lagage, co-author of the paper. “If it has an atmosphere that circulates and redistributes the heat, the day side will be cooler than if there was 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 behind the star. Although not hot enough to emit visible light of its own, TRAPPIST-1 b has an infrared glow. By subtracting the brightness of the star itself (during the secondary eclipse) from the brightness of the star and planet combined, they were able to successfully calculate how much infrared light the planet emits.

Measuring minute changes in brightness

Webb’s detection of the secondary eclipse is a significant milestone in itself. With a star more than 1000 times brighter than the planet, the change in brightness is less than 0.1%.

“There was also some concern that we would miss the eclipse. All the planets are pulling on each other, so the orbits aren’t perfect,” said Taylor Bell, a 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 minutes.”

The team analyzed data from five separate secondary observations of the eclipse. “We compared the results with computer models showing what the temperature should be in different scenarios,” Ducrot explained. “The results are almost perfectly consistent with a black body made of bare rock and no atmosphere to circulate heat. We also saw no evidence of light absorption by carbon dioxide, which would be apparent in these measurements.”

This research was conducted as part of the Webb Guaranteed Time Observation (GTO) 1177 program, one of eight programs from the first year of Webb science designed to help fully characterize the TRAPPIST-1 system. Additional secondary observations of the TRAPPIST-1 b eclipse 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 will allow them to see how the temperature changes from day to night and confirm whether the planet has an atmosphere or not.

Read Now : <strong>Telescope can’t record atmosphere but JWST has the best view yet of a planet in star system</strong>

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