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TRAPPIST-1 is a faint and cool red dwarf star in the eastern parts of the constellation Aquarius. With a visual magnitude of just +18.8, the star is far too faint to be seen with the naked eye. It has only about a tenth of the Sun's mass, and emits only a fraction of the Sun's energy. It is nonetheless important as host to a system of seven known planets, all of which are rocky terrestrial worlds, and TRAPPIST-1 h is the outermost of these planets.

Imagery provided by Aladin sky atlas

The TRAPPIST-1 system is quite different in its arrangement from the Solar System. It has no known giant planets, and the seven known small rocky planets are crowded into orbits extremely close to their parent star. TRAPPIST-1 h is the outermost of these, but even as the outer planet, it orbits the star TRAPPIST-1 at a distance of just 0.06 AU (that is, about a sixth of the distance of Mercury from the Sun). This close orbit means that TRAPPIST-1 h orbits its star rapidly, completing a circuit in a period of a little under nineteen days.

Physically TRAPPIST-1 h is the smallest and least massive of the planets in its system. Most of these planets approach or exceed Earth's mass, but TRAPPIST-1 h has only a third of the mass of Earth. Its diameter is roughly three quarters that of Earth, showing that it must be considerably less dense (for reasons that are not clearly understood, but may reflect the compounds that make it up, or possibly its internal physical structure).

Because the red dwarf TRAPPIST-1 is comparatively cool by stellar standards, its habitable zone (the zone in which the heat provided by the star is suitable for liquid water to exist) is a narrow region very close to the star. Despite its close orbit, however, TRAPPIST-1 h lies beyond the outer fringes of this zone. This means that (unless unforeseen effects are active on the planet) its temperature will be an estimated -100°C, and its surface is therefore likely to be covered in ice.


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