Webb measures the temperature of a rocky world 40 light years away

March 27 () –

An international team of researchers has used the James Webb Space Telescope to measure the temperature of the rocky exoplanet TRAPPIST-1 b, located 40 light years away.

The measurement is based on the planet’s thermal emission, that is, the thermal energy emitted 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 230 degrees Celsius (450 degrees Fahrenheit) and suggests it has no significant atmosphere.

This is the first detection of any form of light emitted by an exoplanet as small and as cold 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 they can maintain atmospheres necessary to support life. It also bodes well for Webb’s ability to use the MIRI instrument to characterize Earth-sized exoplanets with temperate temperatures.

“These observations take very good advantage of Webb’s mid-infrared capability,” said it’s a statement Thomas Greene, an astrophysicist at NASA Ames Research Center and lead author of the study published in the scientific journal Nature. “No previous telescope has had the sensitivity to measure such dim mid-infrared light.”

In early 2017, astronomers reported the discovery of seven rocky planets orbiting an ultracool red dwarf (or M-dwarf) star 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. Although they all orbit much closer to their star than any of our planets orbit the Sun – they could all fit comfortably within the orbit of Mercury – they receive comparable amounts of energy from their small star.

TRAPPIST-1 b, the innermost planet, has an orbital distance about one-hundredth that of Earth and receives approximately four times the amount of energy that Earth does from the Sun. Although it is not within the habitable zone of the system, observations of the planet can provide important information about its sister planets, as well as 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 have rocky planets as stars like the Sun,” Greene 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 destroy an atmosphere.”

Co-author Elsa Ducrot, from the French Commission for Atomic Energy and Alternative Energies (CEA) in France, who was part of the team that carried out previous studies of the TRAPPIST-1 system, added: “It is easier to characterize terrestrial planets that are moving orbiting smaller, cooler stars If we want to understand habitability around M-type 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 Hubble and Spitzer Space Telescopes found no evidence of a puffed-up atmosphere, but they could not rule out a thick atmosphere.

One way to reduce 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 CEA’s Pierre-Olivier Lagage, who is a co-author on the paper. “If you have an atmosphere to circulate and redistribute heat, the dayside will be cooler than if there were no atmosphere.”

The team used a technique called secondary eclipse photometry, in which MIRI measured the change in the brightness of the system 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 star’s own brightness (during the secondary eclipse) from the combined brightness of the star and planet, they were able to successfully calculate how much infrared light the planet emits.

Webb’s detection of a secondary eclipse is itself a major milestone. The observed star is more than 1,000 times brighter than the planet, and the change in brightness is less than 0.1%.

“There was also some fear that we would miss the eclipse. All the planets pull on each other, so the orbits are not perfect.” said Taylor Bell, the postdoctoral researcher at the Bay Area Institute for Environmental Research who analyzed the data. “But it was just amazing: the eclipse time we saw in the data matched the predicted time within a couple of minutes.”

The team analyzed data from five separate observations of secondary eclipses. “We compared the results with computer models that showed what the temperature should be under different scenarios,” Ducrot explained. “The results are almost perfectly consistent with a black body made of bare rock and no atmosphere to circulate the heat. We also did not see any sign of light being absorbed by carbon dioxide, which would be evident in these measurements.“.

This research was conducted as part of Webb’s Guaranteed Time Observations (GTO) program number 1177, which is one of eight programs in Webb’s first year of scientific investigations designed to help fully characterize the TRAPPIST-1 system. Additional observations of secondary eclipses of TRAPPIST-1 by are currently underway, now that they know how good the data can be, the team hopes to later capture a full phase curve showing the change in brightness over the entire duration. orbit. This will allow them to see how the temperature changes from the day side to the night side and confirm whether or not the planet has an atmosphere.

“There was a 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. It is the first time that we can detect the emission of a rocky and temperate planet. This is a really important step in the history of exoplanet discovery.”