TRAPPIST-1 d

TRAPPIST-1 d is the third planet in the world-famous TRAPPIST-1 system, located 40 light-years away in the constellation Aquarius. It is one of the most scrutinized planets in Lisa Kaltenegger’s research because it sits on the inner edge of the system’s habitable zone. 

Key Physical Characteristics

  • Planet Type: Terrestrial (rocky) world.
  • Mass: It is a “lightweight” compared to Earth, with only about 30% of Earth’s mass (0.39 Earth masses).
  • Orbit: It orbits its ultra-cool dwarf star every 4.05 Earth days.
  • The Star: TRAPPIST-1 is a tiny, dim red dwarf. Because it is so cool, all seven of its planets orbit closer to it than Mercury does to our Sun. 

Habitability & Environment

In Kaltenegger’s 2026 study, TRAPPIST-1 d is a critical case for atmospheric loss:

  • Energy Intake: It receives about 1.14 times the starlight that Earth gets. This makes it slightly warmer than Earth, putting it in a climate category similar to Venus or a very hot Earth.
  • Atmosphere: Recent observations from the James Webb Space Telescope (JWST) are trying to determine if it has a thin atmosphere like Earth, a thick one like Venus, or no atmosphere at all due to early stellar flares.
  • Tidal Locking: Like its siblings, it is tidally locked. However, because it is so small, it may have a harder time holding onto a thick atmosphere than the larger planets further out in the system. 

Scientific Significance

  • The “Sibling” Comparison: TRAPPIST-1 d is often compared to planets e, f, and g (which are also in the habitable zone). By studying “d,” Kaltenegger and her team can see where the “boundary of life” begins in this system.
  • Composition: Despite its low mass, it is confirmed to be a rocky world. Some models suggest it could have a significant amount of water (up to 5% of its mass), which would make it a “water-rich” world compared to Earth (where water is less than 0.1% of the mass).
  • Observation Target: Because it transits its star so frequently (every 4 days), it provides many opportunities for telescopes to “sniff” its air as starlight passes through its limb.

Published @ March 24, 2026 9:21 am