Innermost TRAPPIST-1 exoplanet is hot and airless

NASA’s Webb space telescope has been taking its first close look at the TRAPPIST-1 planetary system in recent months. Astronomers and the public alike have been eagerly awaiting the results, the first of which were published last December. The system contains seven Earth-sized planets, of which at least three are in the habitable zone. On March 27, 2023, an international group of scientists announced more detailed findings about the innermost planet, TRAPPIST-1 b. While the TRAPPIST-1 system overall has intriguing potential for habitability, this world, it turns out, is a bit too hot for comfort.

The researchers published their peer-reviewed findings in the journal Nature on March 27, 2023. You can also read a preprint version of the paper on arXiv.

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Focusing on a small exoplanet

Webb used its Mid-Infrared Instrument (MIRI) to measure the infrared light coming from TRAPPIST-1 b. This is also the first time that anyone has observed such light from an exoplanet this small. It’s much easier with larger gas giant planets similar to Jupiter or Saturn. Lead author Thomas Greene at NASA’s Ames Research Center said:

These observations really take advantage of Webb’s mid-infrared capability. No previous telescopes have had the sensitivity to measure such dim mid-infrared light.

With MIRI, the researchers used a technique called secondary eclipse photometry. This technique measured the change in brightness of the planet as it moved behind its star from our viewpoint on Earth. As with the planets in our solar system, TRAPPIST-1 b doesn’t generate its own visible light. It reflects light from its star. It does, however, emit an infrared glow, which infrared telescopes can detect.

Atmosphere or no atmosphere?

As with the other six planets in the system, scientists didn’t know if TRAPPIST-1 b had an atmosphere or not. NASA’s Hubble and Spitzer space telescopes had observed the planet before and found no sign of a puffy atmosphere. That means a deep, extended atmosphere more like the ones of Uranus or Neptune. It was still possible, however, that the planet possessed a thinner atmosphere, more like Earth’s.

Webb measures heat from TRAPPIST-1 b

Webb can take more sensitive measurements of TRAPPIST-1 b. It looked for evidence of a thinner atmosphere around the planet by measuring the heat of the planet’s surface. The planet is also tidally locked, meaning it always keeps the same side facing its star. That affects the planet’s temperature, depending on whether it has an atmosphere or not. As co-author Pierre-Olivier Lagage from the Alternative Energies and Atomic Energy Commission (CEA) in France noted:

This planet is tidally locked, with one side facing the star at all times and the other in permanent darkness. If it has an atmosphere to circulate and redistribute the heat, the dayside will be cooler than if there is no atmosphere.

By measuring the amount of infrared light from the planet, the researchers could estimate its surface temperature. And it’s hot! The calculated temperature is about 450 degrees Fahrenheit (230 degrees Celsius) on the planet’s dayside.

A difficult-to-see secondary eclipse

Being able to observe the secondary eclipse wasn’t guaranteed. It was difficult to detect, due to the difference in brightness between TRAPPIST-1 b and its star. The planet is about 1,000 times fainter than its star. The detection was essential in being able to determine the planet’s temperature and atmosphere (or lack thereof), so it is fortunate that it worked. And it turned out to be quite accurate, too. Taylor Bell, a post-doctoral researcher at the Bay Area Environmental Research Institute, said:

There was also some fear that we’d miss the eclipse. The planets all tug on each other, so the orbits are not perfect. 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.

In fact, the research team was able to observe five separate secondary eclipses altogether. The results showed that TRAPPIST-1 b doesn’t have any noticeable atmosphere. Co-author Elsa Ducrot at CEA explained:

We compared the results to computer models showing what the temperature should be in different scenarios. 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.

6 more TRAPPIST-1 planets to go

For those hoping that TRAPPIST-1 b may be habitable, it’s a disappointing result. It shouldn’t be too surprising, however. The planet orbits much closer to its star than Mercury does to our sun. And like most red dwarfs, the star emits powerful bursts of solar flares. Those flares can strip the atmosphere off a planet that is too close.

The results for TRAPPIST-1 b may not be what many hoped for, but there are still several more planets in the system to study. In fact, the system contains six altogether, and all of them are rocky and close to Earth in size. TRAPPIST-1 b was always considered likely to have lost its atmosphere, being so close to its star. All seven planets orbit their star at a distance that would easily fit within the orbit of Mercury. But remember, the star is much smaller and cooler than our sun. So the habitable zone, where water could exist, is correspondingly smaller as well.

Water-rich TRAPPIST-1 planets?

The most distant of the planets, TRAPPIST-1 h, may, in fact, be frozen. It’s the middle planets that are potentially the most interesting in terms of habitability. Indeed, previous studies suggest that at least some of these planets could well be water-rich. Scientists don’t know yet, though, whether that would be lakes, oceans or subsurface water. Of course, that largely depends on whether these planets have atmospheres, and what kind.

Lagage for one is happy with the results, saying:

There was one target that I dreamed of having. And it was this one. This is the first time we can detect the emission from a rocky, temperate planet. It’s a really important step in the story of discovering exoplanets.

It will be very interesting to see what Webb reveals about these intriguing worlds!

Bottom line: NASA’s Webb space telescope has found that TRAPPIST-1 b, the innermost planet in the TRAPPIST-1 system, is airless and too hot for life.

Source: Thermal Emission from the Earth-sized Exoplanet TRAPPIST-1 b using JWST

Source (preprint): Thermal emission from the Earth-sized exoplanet TRAPPIST-1 b using JWST

Via Webb Space Telescope