TRAPPIST-1d

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. Orbit TRAPPIST-1d is a closely orbiting planet, with one full orbit taking just to complete. and has a temperature of . The star is metal-rich, with a metallicity ([Fe/H]) of 0.04, or 109% the solar amount. This is particularly odd, as such low-mass stars near the boundary between brown dwarfs and hydrogen-fusing stars should be expected to have considerably less metals than the Sun. Finally, its luminosity is . Stars like TRAPPIST-1 have the ability to live up to 4–5 trillion years, 400–500 times longer than the Sun will live (the Sun only has about 5 billion years of lifespan left, slightly more than half of its lifetime). Because of this ability to live for long periods of time, it is likely TRAPPIST-1 will be one of the last remaining stars when the universe is much older than it is now, when the gas needed to form new stars will be exhausted and the remaining ones begin to die off. The star's apparent magnitude, or how bright it appears from Earth's perspective, is . Therefore, it is too dim to be seen with the naked eye (the limit for that is 6.5). The star is not just very small and far away, it also emits comparatively little visible light, mainly shining in the invisible infrared. Even from the close-in proximity of TRAPPIST-1d (about 50 times closer than Earth is from the Sun), the planet receives less than 1% the visible light Earth sees from the Sun. This would probably make the days on TRAPPIST-1d never brighter than twilight is on Earth. However, that still means that TRAPPIST-1 could easily shine at least 3000 times brighter in the sky of TRAPPIST-1d than the full moon does in Earth's night sky. ==Atmosphere==

In 2025, the planet was observed by the James Webb Space Telescope. Study results determined with greater than 95% confidence that thick and cloud-free atmospheres could be ruled out (analogous to a clear Venus, early Mars, Titan and Archean Earth). This leaves two main possibilities consistent with the data: either the planet has no atmosphere, similar to Mercury or the Moon, which was termed the "satisfactory fit" for the data by the study's authors, or there is a "marginal possibility" that TRAPPIST-1d has an atmosphere with high-altitude clouds or aerosols that block starlight from passing through, thus muting any molecular signatures from detection. 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. == Discovery ==

A team of astronomers headed by Michaël Gillon of the Institut d'Astrophysique et Géophysique at the University of Liège in Belgium used the TRAPPIST (Transiting Planets and Planetesimals Small Telescope) telescope at the La Silla Observatory in the Atacama Desert, Chile, The team made their observations from September to December 2015 and published its findings in the May 2016 issue of the journal Nature. The original claim and presumed size of the planet was revised when the full seven-planet system was revealed in 2017: :"We already knew that TRAPPIST-1, a small, faint star some 40 light-years away, was special. In May 2016, a team led by Michaël Gillon at Belgium's University of Liege announced it was closely orbited by three planets that are probably rocky: TRAPPIST-1b, c and d... :"As the team kept watching shadow after shadow cross the star, three planets no longer seemed like enough to explain the pattern. "At some point we could not make sense of all these transits," Gillon says. :"Now, after using the space-based Spitzer telescope to stare at the system for almost three weeks straight, Gillon and his team have solved the problem: TRAPPIST-1 has four more planets. :"The planets closest to the star, TRAPPIST-1b and c, are unchanged. But there's a new third planet, which has taken the d moniker, and what had looked like d before turned out to be glimpses of e, f and g. There's a planet h, too, drifting farthest away and only spotted once." == See also ==