What kind of atmosphere could TRAPPIST-1 c have?
TRAPPIST-1 c is slightly larger and more massive than Earth, and receives about as much energy from its star as Venus does from the Sun. It was once discussed as a possible “twin of Venus” with a thick carbon dioxide atmosphere and clouds. But measurements of its dayside heat in 2023 and its day-to-night brightness changes in 2026 do not fit an atmosphere with surface pressure greater than about 1 bar, similar to pressure at Earth’s surface. Researchers still cannot distinguish a thin oxygen atmosphere from a bright, airless surface that reflects much of the starlight.
- About 2.4218 days Time required to orbit the star
- About 2.36 million km Average distance between the planet and star
- About 1.31 Earth masses Mass derived from changes in transit times
- About 1.10 Earth radii Radius derived from the fraction of starlight blocked
What can we measure when a planet passes behind its star?
When the planet appears beside the star, the telescope receives both starlight and infrared light from its dayside. Once the planet disappears behind the star, only the starlight remains. Webb’s MIRI instrument observed four such secondary eclipses. The total brightness fell by about 0.04% at an infrared wavelength of 15 μm, revealing the heat emitted by the planet’s dayside.
That brightness corresponded to a brightness temperature of about 380 K, or 107°C. A thick, carbon-dioxide-rich atmosphere like Venus’s would absorb 15 μm infrared light and carry daytime heat to the night, making the dayside appear dimmer. Planet c was much brighter than that. It is not a Venus-like world with a thick carbon dioxide atmosphere and sulfuric-acid clouds, but this observation alone could not show whether it has no atmosphere at all.
How can we compare heat on the day and night sides?
The moment when a planet hides behind its star reveals only its dayside heat. Webb continuously recorded its infrared brightness while different sides faced Earth over one full orbit. This is called a thermal phase curve. With a thick atmosphere, heat would travel into the night and produce nightside infrared light, while the hottest point could shift away from the point facing the star.
In the 2026 observations, c’s dayside brightness temperature was 369±23 K, or about 96±23°C. Heat from the nightside was not clearly detected, and the time of peak brightness did not shift noticeably. These results greatly reduced the likelihood of a thick atmosphere that broadly carries heat into the night, especially one with surface pressure greater than about 1 bar.
Planet c does not have a thick atmosphere like Venus. But researchers still do not know whether it has a thin atmosphere or a bright surface that reflects much of its starlight.
Why is c’s dayside cooler than expected?
Planet c is farther from the star than b and receives less energy. Yet distance alone does not explain the full difference between their phase curves. Its dayside is cooler than b’s, and airless-surface models require a higher reflectivity. A bright surface sends more incoming starlight back to space and remains cooler.
Another possible explanation is a very thin atmosphere made mainly of oxygen. It would be too weak to trap much heat or carry it into the night, but could still affect dayside infrared brightness. Models also show that intense starlight might once have broken water apart, allowing light hydrogen to escape and leaving oxygen behind. This is a calculated possibility, not a direct detection of oxygen.
- Size and mass were determined Blocked starlight gave the size, while changes in neighboring planets’ transit times gave the mass.
- 2023 Dayside 15 μm infrared light ruled out a thick Venus-like carbon dioxide atmosphere.
- Starlight during transit was analyzed A thick hydrogen-rich atmosphere was absent, but changes in the star contaminated the measurement.
- 2026 Day and night heat across a full orbit further reduced the likelihood of an atmosphere thicker than 1 bar.
- Today More observations are needed to separate a thin oxygen atmosphere from a highly reflective airless surface.
How do changes on the star’s surface affect atmosphere measurements?
Analyzing starlight that passes through the atmosphere during a transit can reveal which gases absorbed it. Near-infrared observations of c ruled out a hydrogen-rich atmosphere and some thick atmospheres rich in water, ammonia or carbon monoxide. But the spots and bright regions on TRAPPIST-1 also change the color of its light substantially, preventing a clear detection of the small marks from a thin atmosphere.
The infrared brightness change over a full orbit is affected by starspots in a different way, but it cannot directly identify a gas such as oxygen that traps little heat. Several infrared wavelengths and the starlight recorded during transit must all point to the same explanation before a thin atmosphere can be distinguished from a bright surface.
How similar is TRAPPIST-1 c to Venus?
Planet c’s mass and radius fit a rocky world, but do not separately determine the size of its iron core, its surface minerals or its atmospheric pressure. Receiving a similar amount of energy to Venus also does not mean their present environments are alike. Their stars’ spectra and flares, their building materials and their histories of atmospheric loss differ.
The value 369±23 K, or about 96±23°C, is a dayside brightness temperature calculated from 15 μm infrared light. It is neither the planet’s average temperature nor the actual temperature at every point on its surface. A thin oxygen atmosphere and a highly reflective surface are two models that can explain the measurement; oxygen, clouds and minerals have not been detected directly.
Sources
- NASA Exoplanet Archive — TRAPPIST-1 c
- NASA Science — Thick Carbon Dioxide Atmosphere Ruled Out
- Nature — No Thick Carbon Dioxide Atmosphere on TRAPPIST-1 c
- Nature Astronomy — Thermal Phase Curves of TRAPPIST-1 b and c
- The Astrophysical Journal — TRAPPIST-1 c Transmission and Stellar Contamination
- The Planetary Science Journal — TRAPPIST-1 Masses and Radii
- NASA Science — Webb Results for the TRAPPIST-1 System