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Leiden PhD student discovers thin atmosphere on exoplanet TRAPPIST-1 c with JWST

A group of astronomers led by Leiden PhD student Sebastian Zieba has discovered using the James Webb Space Telescope (JWST) that the rocky exoplanet TRAPPIST-1 c has a thin atmosphere. Although the planet is nearly identical to Venus in size and temperature, and was expected to have a thick atmosphere, the atmosphere turns out to be quite different. By analyzing the heat ray of the planet, they concluded that the planet only has a thin atmosphere with minimal carbon dioxide. The results appeared in the journal Nature.

'The nearby planetary system TRAPPIST-1 is currently the best candidate to study the atmospheres of rocky, Earth-like planets around a red dwarf star,' said Sebastian Zieba, who is pursuing his PhD at Leiden University and at the Max Planck Institute for Astronomy (MPIA) in Heidelberg, Germany.

TRAPPIST-1 is located about 40 light years away from us and has seven Earth-sized rocky planets, three of which are in the "habitable" zone. This means that radiation from the central star produces enough heat to allow water in liquid form. Because TRAPPIST-1 c is not in this habitable zone, astronomers suspected it would be a Venus-like planet.

Eroding planetary atmospheres

While many low-mass stars are relatively cool on the outside, they exhibit strong stellar winds and intense UV radiation over an extended period of their lives, which can damage and erode the atmospheres of their planets. 'We wanted to find out whether TRAPPIST-1 c could have escaped that fate and retained a substantial atmosphere, and perhaps even be similar to the planet Venus in the solar system,' Zieba explained. The planet's gravity at its surface, which is ten percent of Earth's gravity, should help maintain its atmosphere. As is the case with Venus, the diameter and mass of TRAPPIST-1 c are close to those of Earth. Moreover, the radiation the planet receives from its central star is nearly identical to that of Venus.

A challenging task for the JWST

Characterizing the atmospheres of Earth-sized rocky planets is a challenging task for Webb, even for the mid-infrared instrument MIRI. Therefore, the team combined observations with model calculations to find the most likely set of atmospheric properties. The size, pressure and composition of an atmosphere determines the temperature of a planet depending on the light it receives from its star. Conversely, temperature determines the amount of infrared light the planet emits. In this way, infrared measurements combined with models provide clues about the atmosphere and its composition.

TRAPPIST-1 c looks nothing like Venus

'We can certainly rule out a thick and Venus-like atmosphere,' says second author Laura Kreidberg, lead scientist on this Webb observing program, professor in Leiden and director at MPIA. Contrary to expectations, temperatures reach "only" 110 degrees Celsius, which is up to 390 degrees lower than on Venus. The infrared light emitted by TRAPPIST-1 c does not match a Venus atmosphere, which is rich in carbon dioxide that causes a strong greenhouse effect. While results published earlier this year on TRAPPIST-1 b showed that this planet has no atmosphere similar to Mercury, TRAPPIST-1 c suggests that this planetary system has no analog to our solar system at all.

Thin atmosphere

To answer the question of whether TRAPPIST-1 c has at least a thin gas layer, the scientists calculated the statistical probabilities of a set of atmospheric parameters corresponding to the observations. The atmospheric model consisted of a set of surface pressures and mixtures of an oxygen (O2) dominated atmosphere with varying traces of carbon dioxide (CO2).

"We expected an abundance of oxygen with some carbon dioxide for hot rocky planets orbiting low-mass stars" says Zieba. Planets like TRAPPIST-1 c should have an atmosphere containing carbon dioxide and water vapour early in their evolution. Over time, stellar radiation breaks the water molecules into hydrogen and oxygen. As the highly volatile hydrogen gradually escapes into open space, the heavier oxygen molecules remain, leading to an oxygen-rich atmosphere with traces of carbon dioxide.

 For TRAPPIST-1 c, the model reflecting this assumption is shown to be consistent with a wide range of oxygen-carbon dioxide mixtures and surface pressures between 1% and 100% of Earth's sea level values. This result raises the hope that TRAPPIST-1 c and other sufficiently heavy rocky planets around cool low-mass stars can maintain an atmosphere for a significant portion of its stellar lifetime, since the star TRAPPIST-1 is at least as old as the Sun.

Challenge for Webb

The results will have to be confirmed with additional data. 'Observations of this kind of atmosphere require the utmost from JWST,' Kreidberg admits. The measured signals are weak and many properties are still unknown, leading to uncertainties. In the case of TRAPPIST-1 c, the atmospheric models are not the only ones that match the data. A bare rock with a surface layer of material weathered by stellar irradiation explains the observations just as well. Additional JWST observations are needed to distinguish between a bare rock planet and a planet with a tenuous atmosphere. By measuring the light that TRAPPIST-1 c emits over a wide wavelength range, astronomers can catch the tiny absorption signatures of gases in the atmosphere. The astronomers are also looking forward to the capabilities of ESO's Extremely Large Telescope (ELT), under construction in Chile's Atacama Desert, which will become operational later this decade.

This post appeared at www.astronomie.nl.

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