TRAPPIST-1 d

TRAPPIST-1d is a small exoplanet (about 40 percent the mass of the Earth), which orbits on the inner edge of the habitable zone of the ultracool dwarf star TRAPPIST-1, located away from Earth in the constellation of Aquarius. The exoplanet was found by using the transit method. The first signs of the planet were announced in 2016, but it was not until the following years that more information concerning the probable nature of the planet was obtained. TRAPPIST-1d is the second-least massive planet of the system. It receives just 4.3% more sunlight than Earth, placing it on the inner edge of the habitable zone. A 2025 study, based on a James Webb Space Telescope observation, found that the data was "satisfactorily" consistent with TRAPPIST-1d having no atmosphere at all. Nevertheless, there is still a "marginal possibility" the planet has an atmosphere, but determining this with greater confidence would require a different detection technique.

==Physical characteristics== ===Radius, mass, and temperature=== TRAPPIST-1d was detected with the transit method, allowing scientists to accurately determine its radius. The planet is about with a small error margin of about 70 km. Transit timing variations and complex computer simulations helped accurately determine the mass of the planet, which led to scientists being able to calculate its density, surface gravity, and composition. TRAPPIST-1d is a mere , making it one of the least massive exoplanets yet found. Initial estimates suggested that it has 61.6% the density of Earth () and just under half the gravity. Compared to Mars, it has nearly three times that planet's mass but was thought to still be significantly less dense, which would indicate the presence of a significant atmosphere; models of the low density of TRAPPIST-1d indicated a mainly rocky composition, but with about of its mass in the form of a volatile layer. The volatile layer of TRAPPIST-1d may consist of atmosphere, ocean, and/or ice layers. However, refined estimates show that the planet is more dense, closer to 79.2% of Earth's bulk density (). TRAPPIST-1d has an equilibrium temperature of , assuming an albedo of 0. For an Earth-like albedo of 0.3, the planet's equilibrium temperature is around , very similar to Earth's at .