[Fe/H] 0.04 (± 0.08) Discovery status Published | Discovery date May 2, 2016 | |
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TRAPPIST-1d, also designated as 2MASS J23062928-0502285 d, is an exoplanet, likely rocky, possibly orbiting within the habitable zone around the ultracool dwarf star TRAPPIST-1 approximately 40 light-years (12.1 parsecs, or nearly 3.7336×1014 km) away from Earth in the constellation of Aquarius. The exoplanet was found by using the transit method, in which the dimming effect that a planet causes as it crosses in front of its star is measured.
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Radius, mass, and temperature
TRAPPIST-1d is a terrestrial exoplanet with a radius of approximately 0.772 R⊕, intermediate in size between Mars (with a radius about half that of Earth) and Earth. The mass has been estimated to be around 0.41 M⊕. These values allow its standard gravity to be estimated as 6.79 m/s² (69% of Earth value). With these numbers one can calculate a density of 4.95 g/cm^3. It has an equilibrium temperature of 288 K (15 °C; 59 °F).
Host star
The planet orbits an (M-type) ultracool dwarf star named TRAPPIST-1. The star has a mass of 0.08 M☉ (close to the boundary between brown dwarfs and hydrogen-fusing stars) and a radius of 0.11 R☉. It has a temperature of 2550 K and is at least 500 million years old. In comparison, the Sun is 4.6 billion years old and has a temperature of 5778 K. 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 considerly less metals then the Sun. Its luminosity (L☉) is 0.05% of that of the Sun.
Stars like TRAPPIST-1 have the ability to live up to 4–5 trillion years, 400–500 times longer than the Sun will live. 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 18.8. Therefore, it is too dim to be seen with the naked eye.
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 our 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.
Orbit
TRAPPIST-1d orbits its host star with an orbital period of about 4.05 days and an orbital radius of about 0.0214 times that of Earth's (compared to the distance of Mercury from the Sun, which is about 0.38 AU).
Habitability
The exoplanet was announced to be orbiting on the inside part of the expected habitable zone of its parent star (the region where, with the correct conditions and atmospheric properties, liquid water may exist on the surface of the planet). The investigators' three-dimensional climate model resulted in a runaway greenhouse effect. However, a small quantity of water may have persisted in limited regions beyond the early hot phase in the planet's history.
The planet is very likely tidally locked, with one hemisphere permanently facing towards TRAPPIST-1 and the other shrouded in darkness. However, between these two intense areas, there would be a sliver of habitability – called the terminator line, where the temperatures may be suitable (about 273 K (0 °C; 32 °F)) for liquid water to exist.
During formation of the system, it is possible that water loss during its first few million years of existence occurred. This was likely due to photoevaporation.
TRAPPIST-1d may have kept enough water to remain habitable depending on its initial content. The two innermost planets, b and c, probably lost up to four times the amount of Earth's oceans, depending on their composition.
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, to observe TRAPPIST-1 and search for orbiting planets. By utilising transit photometry, they discovered three Earth-sized planets orbiting the dwarf star; the innermost two are tidally locked to their host star while the outermost appears to lie either within the system's habitable zone or just outside of it. 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."