TRAPPIST-1d is the second-least massive planet of the system and is likely to have a compact hydrogen-poor atmosphere similar to Venus, Earth, or Mars.

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.

[3] For comparison, Mercury, the Solar System's innermost planet, takes 88 days to orbit at a distance of about 0.38 AU.

The size of TRAPPIST-1 and the close orbit of TRAPPIST-1d around it means that the star as seen from the planet appears 5.5 times as large as the Sun from the Earth.

Models and scientists are divided on whether their convergent solutions from the data for TRAPPIST-1d indicates Earth-like habitability or a severe greenhouse effect.

[3] However, some three-dimensional modeling solutions have little water surviving beyond the early hot phase in the planet's history.

[14][15] Most models by the University of Washington for TRAPPIST-1d strongly converge on a Venus-like planet (runaway greenhouse effect) with an uninhabitable atmosphere.

Because TRAPPIST-1d is only ~30% the Earth's mass, it, like Venus and Mars, may have no magnetic field, which would allow the parent star's solar wind to strip away the more volatile components of its atmosphere (including water), leaving it hydrogen-poor like those planets.

As a result, it may be very geologically active due to tidal squeezing as happens to Jupiter's moon Io and volcanic gases could replenish the atmosphere lost to the solar wind.

The same researchers point out that such proximity to the host star tends to increase geothermal activity, and tidally heat the bottom of any seas.

A team of astronomers headed by Michaël Gillon of the Institut d’Astrophysique et Géophysique at the University of Liège[17] in Belgium used the TRAPPIST (Transiting Planets and Planetesimals Small Telescope) telescope at the La Silla Observatory in the Atacama Desert, Chile,[18] to observe TRAPPIST-1 and search for orbiting planets.