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James Webb’s search for potential life shines light on Earth-like exoplanet with no atmosphere

Artist's impression of what the hot rocky exoplanet TRAPPIST-1 b could look like
Artist's impression of what the hot rocky exoplanet TRAPPIST-1 b could look like Copyright NASA, ESA, CSA, J. Olmsted (STScI), T. P. Greene (NASA Ames), T. Bell (BAERI), E. Ducrot (CEA), P. Lagage (CEA)/NASA, ESA, CSA, J. Olmsted (STScI), T. P. Greene (NASA Ames), T. Bell (BAERI), E. Ducrot (CEA), P. Lagage (CEA)
Copyright NASA, ESA, CSA, J. Olmsted (STScI), T. P. Greene (NASA Ames), T. Bell (BAERI), E. Ducrot (CEA), P. Lagage (CEA)/NASA, ESA, CSA, J. Olmsted (STScI), T. P. Greene (NASA Ames), T. Bell (BAERI), E. Ducrot (CEA), P. Lagage (CEA)
By Luke Hurst
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Researchers used the JWST to measure the temperature of a rocky exoplanet in the promising TRAPPIST-1 star system.

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NASA’s biggest and most powerful space telescope, the James Webb (JWST), has taken the first ever reading of light emitted by a small rocky exoplanet, in the latest display of its capabilities.

The JWST has been showing off the cosmos like it’s never been seen before, capturing awe-inspiring images of galaxies and nebulas light-years in diameter.

But it is also capable of honing in on much smaller targets, such as exoplanets - planets outside of our solar system.

An international team of researchers used the telescope to measure the temperature of a rocky exoplanet in the TRAPPIST-1 star system, home to the largest known batch of Earth-sized rocky planets in the habitable zone.

The habitable zone refers to a planet’s location being not too close and not too far from its host star, meaning temperatures could be just right for liquid water to form, and for an atmosphere to be maintained.

There is therefore considerable interest in the seven known planets within the TRAPPIST-1 system, as potential candidates for hosts for alien life.

The findings from the JWST, however, suggest that the planet TRAPPIST-1 b has no significant atmosphere - and has a daytime temperature of around 230 degrees Celsius.

The measurement is based on the planet’s thermal emission - heat energy given off in the form of infrared light detected by Webb’s Mid-Infrared Instrument (MIRI). 

First detection of light from small rocky exoplanet

The reading is the first detection of light from an exoplanet as small and potentially cool as the rocky planets in our own solar system - an important step in determining whether planets orbiting small active stars could sustain atmospheres needed to support life.

The researchers also say it is a good demonstration of Webb’s ability to study Earth-sized exoplanets using 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 on the study published on Tuesday in the journal Nature. “No previous telescopes have had the sensitivity to measure such dim mid-infrared light”.

The TRAPPIST-1 planets were discovered in 2017, orbiting the ultracool red dwarf star which lies 40 light-years from Earth. They garnered particular interest because of their apparent similarity to the inner planets of our solar system.

While they all orbit much closer to their star than any of the planets in our solar system (within the orbit of Mercury, the closest to our Sun), they receive comparable amounts of energy from their tiny star.

While TRAPPIST-1 b is not in the habitable zone, it gives researchers important information about its neighbouring planets - and others around similar stars.

“There are ten times as many of these stars in the Milky Way as there 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 Alternative Energies and Atomic Energy Commission (CEA) in France, who was on the team that conducted the initial studies of the TRAPPIST-1 system, added that “it’s easier to characterise 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”.

Detecting an 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.

TRAPPIST-1 b is not hot enough to give off its own light, but it does have an infrared flow. By subtracting the brightness of the star from the brightness of the star and planet combined, the researchers could successfully calculate how much infrared light is being given off by the planet.

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The findings show how powerful Webb is. The star is more than 1,000 times brighter than the planet, and a change in brightness when the planet is lined up in eclipse is less than 0.1 per cent.

“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 analysed 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”.

Analysis of data from five separate secondary eclipse observations indicates that TRAPPIST-1 b has a dayside temperature of roughly 230 C. The team thinks the most likely interpretation is that the planet does not have an atmosphere.

“We compared the results to computer models showing what the temperature should be in different scenarios,” explained Ducrot.

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“The results are almost perfectly consistent 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 carbon dioxide, which would be apparent in these measurements”.

‘Hot Jupiter’ gives unexpected results

Another paper just published in the journal Nature also made breakthroughs regarding planetary atmospheres, using JWST - but this time it was looking at gas giants. 

Gas giants orbiting our sun have a clear pattern, the researchers state: the more massive the planet, the lower the percentage of heavy elements (anything other than hydrogen or helium) in the atmosphere.

Using Webb, they were able to confirm however that gas giants out in other star systems don’t conform to this pattern.

Looking at a so-called “hot Jupiter” orbiting a star comparable to our sun, they found the atmosphere to be super-abundant in heavier elements such as carbon and oxygen.

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