The TRAPPIST-1 system is 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, the TRAPPIST planet data was long-awaited. Astronomers have already published detailed information about the second planet, TRAPPIST-1 c, which is theorized to be a Venus-like world. Unlike Venus, however, JWST failed to detect any traces of a dense carbon dioxide atmosphere.
“I was a little sad that we didn’t see a thick CO2 atmosphere, but mostly I’m just amazed that JWST can detect signals like this at all,” said Dr. Laura Kreidberg, on Twitter. She is director of APEx (Atmospheric Physics of Exoplanets) at the Max Planck Institute for Astronomy in Germany and co-author of a new paper published today in Nature. “We are truly entering the era of rocky exoplanet characterization! … This planet is the same size and radiation as Venus, but its atmosphere is *not* Venus-like. It could have a thin atmosphere without much CO2, or it could be bare rock like T1b (TRAPPIST 1 b).”
In March 2023, astronomers shared JWST data on TRAPPIST-1 b, the innermost planet. It has an orbital distance of about one-hundredth that of Earth and is therefore not in the system’s habitable zone. JWST detected no atmosphere at all, which was not unexpected given the hellish conditions of being so close to the star.
All planets in the TRAPPIST-1 system have been previously observed with the Hubble and Spitzer space telescopes, and no atmospheric features have been detected so far. However, astronomers have not been able to rule out this possibility. With JWST’s infrared capabilities, it has the power to detect “heavy” molecules like 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’re 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 about the same density, indicating 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 atmosphere of carbon dioxide.
“We want to know whether rocky planets have atmospheres or not,” said Sebastian Zieba, a Max Planck graduate student and first author of the new paper. in a NASA press release. “In the past, we could only really study planets with dense, hydrogen-rich atmospheres. With Webb, we can finally start looking 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 the Webb Mid-Infrared Instrument (MIRI) to measure the brightness of mid-infrared light. When the planet is next to the star, light from both the star and the dayside of the planet reaches 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. Courtesy: 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 27 and 30 and November 6 and 30, 2022) as planet 1c moved behind the star, a phenomenon known as a secondary eclipse. By comparing the brightness when the planet is behind the star (starlight only) with the brightness when the planet is next to the star (combined light from the star and planet), the team was able to calculate the amount of mid-infrared light with wavelengths 15 microns separated from daylight side of the planet.
NASA said that the amount of mid-infrared light emitted by a planet is directly related to its temperature, which in turn is affected by its atmosphere. Carbon dioxide gas preferentially absorbs 15-micron light, making the planet appear darker at this 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 day side to the night side, causing the daytime temperature to be lower than it would be without an atmosphere. Because TRAPPIST-1 c orbits so close to its star—about 1/50 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 either the planet being bare rock with no atmosphere or the planet having a really thin CO2 atmosphere (thinner than Earth or even Mars) with no clouds,” Zieba said. “If the planet had a thick CO2 atmosphere, we would see a really shallow secondary eclipse or none at all. This is because CO2 will absorb all light from 15 microns, so we will not detect any light coming from the planet.
This plot compares the measured brightness of TRAPPIST-1 c with simulated brightness data for three different scenarios. The measurement (red diamond) is consistent with a bare rocky surface with no atmosphere (green line) or with a very thin cloudless carbon dioxide atmosphere (blue line). A dense, carbon dioxide-rich atmosphere with sulfuric acid clouds similar to that of Venus (yellow line) is unlikely. Courtesy: NASA, ESA, CSA, Joseph Olmsted (STScI).In their newspaper, the team said that “The lack of thick CO2-rich atmosphere of TRAPPIST-1?c suggests a relatively variability-poor formation history … if all the planets in the system formed in the same way, this would imply 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. “This suggests a mode of planet formation that is not very water-rich (although there is no guarantee that c formed in a similar way to the outer planets),” she said.
NASA said researchers will conduct a follow-up study later this year to observe the full orbits of TRAPPIST-1 b and TRAPPIST-1 c. This will make it possible to see how temperatures change from the day to night sides of both planets and provide further constraints on whether or not they have atmospheres. Additionally, other TRAPPIST-1 planets will be observed. So, stay tuned for the next exciting data release.