TRAPPIST-1c: Surprising Clues for Potential Habitability

Astronomers are searching for exoplanets that might support life, and TRAPPIST-1c has emerged as an intriguing candidate. Located 40 light-years away in the TRAPPIST-1 system, this rocky planet orbits within a zone where conditions could be suitable for habitability. Recent observations have provided unexpected insights, challenging previous assumptions about planets of its type.

Understanding TRAPPIST-1c’s potential habitability requires examining its physical characteristics, orbit, atmosphere, and surface conditions.

Physical Properties

TRAPPIST-1c is a rocky exoplanet with a radius approximately 1.10 times that of Earth, placing it within the terrestrial planet category. Its mass, estimated at around 1.38 Earth masses, suggests a composition dominated by silicate rock and possibly an iron-rich core. However, recent density measurements indicate a slightly lower value than expected, hinting at a different internal structure or a less substantial metallic core than Earth’s.

The planet’s surface gravity, about 1.1 times that of Earth, could aid in atmospheric retention. However, its close proximity to its host star complicates this, as intense stellar radiation and tidal interactions may have significantly shaped its environment.

The TRAPPIST-1 star, an ultracool red dwarf, emits far less energy than the Sun, yet TRAPPIST-1c receives a stellar flux comparable to Venus. This suggests its surface has undergone extensive modification due to prolonged exposure to high-energy photons. Whether it has a thick, Venus-like atmosphere or a barren, airless landscape depends on how its physical properties interact with external forces such as stellar winds and magnetic field dynamics.

Orbital Features

TRAPPIST-1c orbits its ultracool red dwarf star in just 2.42 Earth days. This close proximity likely results in tidal locking, with one hemisphere facing perpetual daylight while the other remains in darkness. This configuration has profound implications for climate, creating extreme temperature contrasts between the two hemispheres.

Gravitational interactions within the TRAPPIST-1 system further influence TRAPPIST-1c’s orbit. It is in a 3:2 resonance with its inner neighbor, TRAPPIST-1b, meaning that for every three orbits TRAPPIST-1b completes, TRAPPIST-1c completes two. These gravitational effects can induce oscillations in orbital eccentricity, potentially generating internal heating through tidal forces. If substantial, this could drive geological activity, shaping the planet’s surface over millions of years.

Despite its close orbit, TRAPPIST-1c’s position places it in a region where stellar radiation levels are high but not necessarily prohibitive for atmospheric retention. The TRAPPIST-1 star, while dim compared to the Sun, emits strong flares and radiation bursts that can erode planetary atmospheres. Whether TRAPPIST-1c has retained an atmosphere depends on a complex interplay between magnetic shielding, atmospheric composition, and replenishment mechanisms such as volcanic outgassing.

Atmospheric Composition Considerations

Determining TRAPPIST-1c’s atmosphere is challenging due to its exposure to intense stellar radiation. Observations using the James Webb Space Telescope (JWST) suggest that if an atmosphere exists, it is unlikely to be hydrogen-rich, as lighter gases would have been stripped away. This raises the possibility of a secondary atmosphere formed through volcanic outgassing, potentially dominated by heavier molecules such as carbon dioxide, nitrogen, or sulfur compounds.

Spectroscopic data from JWST provide initial constraints on atmospheric properties, though definitive confirmation remains elusive. A dense CO₂-rich atmosphere could create a strong greenhouse effect, leading to extreme surface temperatures similar to Venus. Alternatively, extensive atmospheric loss may have left the planet nearly airless, exposing the surface directly to space. The balance between outgassing and erosion will determine whether any residual atmosphere persists.

Temperature And Surface Conditions

TRAPPIST-1c’s surface environment is shaped by intense stellar irradiation, with models suggesting an equilibrium temperature of around 110°C (230°F) if no atmosphere is present. A thick, CO₂-rich atmosphere could push temperatures even higher, possibly exceeding those of Venus. If atmospheric loss has been significant, surface temperatures would be dictated by direct stellar heating and the efficiency of heat redistribution across its tidally locked hemispheres.

Without substantial atmospheric insulation, extreme temperature differences would exist between the day and night sides. The night side could plunge to well below freezing, while the illuminated side remains scorching. Some models suggest that if a thin atmosphere exists, strong winds might moderate these extremes. In the absence of sufficient pressure, volatile compounds could accumulate as solid deposits in the colder regions, similar to Martian polar caps. Surface materials with high reflectivity, such as silicate-rich regolith, could also influence local temperature variations by affecting how much radiation is absorbed or reflected.

Astrobiological Interest

TRAPPIST-1c’s habitability depends on its ability to retain an atmosphere, sustain liquid water, and maintain stable conditions over time. While it may lack a thick atmosphere like Earth’s, even a tenuous one could create localized environments conducive to chemical processes essential for life. If volcanic activity persists, it could periodically release gases that contribute to a transient atmosphere, supporting prebiotic chemistry.

Given its tidal locking, the terminator zone—the boundary between perpetual daylight and eternal night—could offer a more stable environment for habitability. If conditions allow, liquid water might exist in this region. Some models suggest that even a thin atmosphere could facilitate heat exchange between hemispheres, preventing extreme temperature swings. If TRAPPIST-1c retains subsurface water reservoirs, geothermal heating from tidal forces could sustain liquid water beneath an icy crust, similar to Europa or Enceladus.