TRAPPIST-1 b: A Barren Rock or a World Wrapped in Haze?

New observations with the James Webb Space Telescope (JWST) have sparked renewed debate over the nature of TRAPPIST-1 b, one of the rocky planets orbiting the ultracool red dwarf star TRAPPIST-1. Two conflicting yet fascinating possibilities have emerged: a planet with a fresh, bare rocky surface or one cloaked in a thick, CO₂-rich atmosphere laced with photochemical hazes.

Published in Nature Astronomy | Estimated reading time: 5 minutes

Two Faces of TRAPPIST-1 b: Bare Rock or Hazy Atmosphere?

TRAPPIST-1 b, located about 40 light-years from Earth, has intrigued astronomers with its thermal emissions measured using JWST’s Mid-Infrared Imager (MIRI). These observations focused on two wavelengths: 12.8 μm and 15 μm. In last year’s findings, researchers concluded the planet likely lacked an atmosphere, resembling a barren, rocky body with no significant heat redistribution.

However, a deeper analysis of five new occultations, combined with previous data, has led to a more nuanced picture. According to the study led by Elsa Ducrot from the Commissariat à l’Énergie Atomique (CEA), two scenarios align with the current measurements:

• A bare rock model: TRAPPIST-1 b could have a young, geologically active crust made of ultramafic material—rocks typically found in Earth’s mantle. The surface, estimated to be only about 1,000 years old, may undergo dramatic changes due to extreme volcanism or tidal forces.
• A CO₂-rich atmosphere: An alternative explanation involves a thick carbon dioxide atmosphere. In this case, photochemical hazes, similar to those on Saturn’s moon Titan, could create an unexpected temperature inversion, where the upper atmosphere warms and emits detectable infrared radiation.

“The idea of a rocky planet with a heavily weathered surface without an atmosphere is inconsistent with the current measurement,” said Jeroen Bouwman from the Max Planck Institute for Astronomy (MPIA).

Geological Activity: A Fresh Surface

The possibility of a geologically fresh surface raises exciting questions. Under normal conditions, stellar radiation and meteorite impacts gradually weather planetary crusts. Yet TRAPPIST-1 b’s surface appears unusually young. This could be explained by residual heat from the planet’s formation, tidal forces exerted by its star and neighboring planets, or even magnetic interactions—similar to the volcanic activity seen on Jupiter’s moon Io.

A Hazy Atmosphere: Complex but Plausible

While atmospheric models suggest that a CO₂-rich atmosphere with photochemical hazes can explain the data, it is considered less likely. The primary challenge lies in the vulnerability of atmospheres around red dwarf stars. TRAPPIST-1 emits significant radiation and stellar winds, which can erode planetary atmospheres over billions of years. Yet, as Thomas Henning, emeritus director of MPIA, notes, under certain conditions, haze can reverse the typical atmospheric temperature gradient, leading to emissions detectable in the thermal infrared range.

The Challenges of Detection

Even with JWST’s unprecedented capabilities, determining the atmospheric composition of rocky planets remains difficult. The current observations relied on secondary eclipse measurements, which detect the thermal emissions from the planet’s dayside as it disappears behind its host star. This technique circumvents some issues posed by the star’s activity but requires extensive observation time—nearly 48 hours in this case.

Future observations, including a planned phase curve analysis of TRAPPIST-1 b, aim to resolve these uncertainties. By mapping temperature distributions across the planet’s orbit, researchers hope to confirm whether TRAPPIST-1 b possesses an atmosphere capable of transporting heat from its star-facing side to its night side.

Glossary

• Occultation: The event when a planet passes behind its host star, blocking its thermal emission from view.
• Photochemical Hazes: Smog-like particles that form in an atmosphere when UV radiation breaks down gases, creating complex hydrocarbons.
• Tidal Heating: Heating of a planet’s interior caused by gravitational interactions with its star or nearby planets.
• Ultramafic Rocks: Igneous rocks rich in magnesium and iron, often associated with geological activity.
• Phase Curve: A measure of the brightness changes of a planet over its orbit, revealing temperature distributions.

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