The Suspense is Killing Us. The Next Planet in the TRAPPIST System Gets the JWST Treatment

The TRAPPIST-1 system is easily the most exciting collection of exoplanets ever discovered by astronomers. The system contains seven rocky planets orbiting an ultracool red dwarf star 40 light-years from Earth. Several of the planets are in the star’s habitable zone.

With the James Webb Space Telescope’s ability to detect and study the atmospheres of distant planets orbiting other stars, data on the TRAPPIST planets have been highly anticipated. Astronomers have now released detailed information about the second planet, TRAPPIST-1 c, theorized to be a Venus-like world. Unlike Venus, however, JWST failed to detect any trace of a thick carbon dioxide atmosphere.

“I was a little sad that we didn’t see a thick CO2 atmosphere, but I’m mostly just amazed that JWST can detect signals like this at all,” said Dr. Laura Kreidberg, on Twitter. She is the director of APEx (Atmospheric Physics of Exoplanets) at the Max Planck Institute for Astronomy in Germany, and co-author on a new paper published today in Nature. “We are truly entering the era of rocky exoplanet characterization! … This planet is the same size and irradiation as Venus, but its atmosphere is *not* Venus-like. It could have a thin atmosphere without much CO2, or could be a bare rock like T1b [TRAPPIST 1 b].”

In March 2023, astronomers shared the JWST data on TRAPPIST-1 b, the innermost planet. It has an orbital distance about one hundredth that of Earth’s, and so is not within the system’s habitable zone. JWST detected no atmosphere at all, which was not unexpected due to the hellish conditions of being so close to the star.

All planets in the TRAPPIST-1 system have been observed previously with the Hubble and Spitzer Space Telescopes, and so far, no atmospheric features have been detected. But still, astronomers haven’t been able to rule out the possibility. With JWST’s infrared capabilities, it has the power to detect ‘heavy’ molecules such as carbon dioxide, oxygen, and methane, and so has the potential to determine whether or not the TRAPPIST-1 planets have atmospheres, and if so, what they are made of.

TRAPPIST-1 c orbits its star at a distance of 0.016 AU (about 2.4 million km, 1.5 million miles), completing one orbit in just 2.42 Earth-days. TRAPPIST-1 c is slightly larger than Earth, but has around the same density, which indicates that it must have a rocky composition. JWST’s measurement of 15-micron mid-infrared light emitted by TRAPPIST-1 c suggests that the planet has either a bare rocky surface or a very thin carbon dioxide atmosphere.

“We want to know if rocky planets have atmospheres or not,” said Sebastian Zieba, a graduate student at from Max Planck and first author on the new paper, in a NASA press release. “In the past, we could only really study planets with thick, hydrogen-rich atmospheres. With Webb we can finally start to search for atmospheres dominated by oxygen, nitrogen, and carbon dioxide.”

This light curve shows the change in brightness of the TRAPPIST-1 system as the second planet, TRAPPIST-1 c, moves behind the star. This phenomenon is known as a secondary eclipse. Astronomers used Webb’s Mid-Infrared Instrument (MIRI) to measure the brightness of mid-infrared light. When the planet is beside the star, the light emitted by both the star and the dayside of the planet reach the telescope, and the system appears brighter. When the planet is behind the star, the light emitted by the planet is blocked and only the starlight reaches the telescope, causing the apparent brightness to decrease. Credits: NASA, ESA, CSA, Joseph Olmsted (STScI)

Zieba and team used MIRI (JWST’s Mid-Infrared Instrument) to observe the TRAPPIST-1 system on four different occasions (on October 27th and 30th, and November 6th and 30th, 2022) as the planet 1 c moved behind the star, a phenomenon known as a secondary eclipse. By comparing the brightness when the planet is behind the star (starlight only) to the brightness when the planet is beside the star (light from the star and planet combined) the team was able to calculate the amount of mid-infrared light with wavelengths of 15 microns given off by the dayside of the planet.

NASA said that the amount of mid-infrared light emitted by a planet is directly related to its temperature, which is in turn influenced by atmosphere. Carbon dioxide gas preferentially absorbs 15-micron light, making the planet appear dimmer at that wavelength. However, clouds can reflect light, making the planet appear brighter and masking the presence of carbon dioxide.

In addition, a substantial atmosphere of any composition would redistribute heat from the dayside to the nightside, causing the dayside temperature to be lower than it would be without an atmosphere. Because TRAPPIST-1 c orbits so close to its star – about 1/50th the distance between Venus and the Sun – it is thought to be tidally locked, with one side in perpetual daylight and the other in endless darkness.

“Our results are consistent with the planet being a bare rock with no atmosphere, or the planet having a really thin CO2 atmosphere (thinner than on Earth or even Mars) with no clouds,” said Zieba. “If the planet had a thick CO2 atmosphere, we would have observed a really shallow secondary eclipse, or none at all. This is because the CO2 would be absorbing all of the 15-micron light, so we wouldn’t detect any coming from the planet.”

This graph compares the measured brightness of TRAPPIST-1 c to simulated brightness data for three different scenarios. The measurement (red diamond) is consistent with a bare rocky surface with no atmosphere (green line) or a very thin carbon dioxide atmosphere with no clouds (blue line). A thick carbon dioxide-rich atmosphere with sulfuric acid clouds, similar to that of Venus (yellow line), is unlikely. Credits: NASA, ESA, CSA, Joseph Olmsted (STScI).

In their paper, the team said that “The absence of a thick, CO2-rich atmosphere on TRAPPIST-1?c suggests a relatively volatile-poor formation history… if all planets in the system formed in the same way, this would indicate a limited reservoir of volatiles for the potentially habitable planets in the system.”

Kreidberg said on Twitter that the amount of water when TRAPPIST-1 c formed would be less than 10 Earth oceans. “That would suggest a mode of planet formation that isn’t hugely water-rich (though no guarantee that c formed in a similar way as the outer planets),” she said.

NASA said that later this year, researchers will conduct a follow-up investigation to observe the full orbits of TRAPPIST-1 b and TRAPPIST-1 c. This will make it possible to see how the temperatures change from the day to the nightsides of the two planets and will provide further constraints on whether they have atmospheres or not. In addition, other TRAPPIST-1 planets will also be observed. So, stay tuned for the next thrilling data release.