Exoplanet TRAPPIST-1d

TRAPPIST-1d (also known as 2MASS J23062928-0502285 d) is an exoplanet, likely rocky, orbiting within or slightly outside the habitable zone around the ultracool dwarf star TRAPPIST-1 approximately 40 light-years (12.1 parsecs) 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.

Mass, radius, and temperature

TRAPPIST-1d is an Earth-sized exoplanet, meaning it has a mass and radius close to that of Earth. It has an equilibrium temperature of 288 K (15 °C; 59 °F). It has a radius of 1.16 R⊕. The mass has not yet been estimated, but based on its size, a mass of around 1.7 M⊕ is possible.

Host star

The planet orbits an (M-type) ultracool dwarf star named TRAPPIST-1. The star has a mass of 0.08 M☉ 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 0.04% of that of the Sun.

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.

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 either orbiting within or slightly outside of the 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. TRAPPIST-1d has a radius of around 1.16 R⊕, so it is likely rocky. Its host star is a red ultracool dwarf, with only about 8% of the mass of the Sun (close to the boundary between brown dwarfs and hydrogen-fusing stars). As a result, 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 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. Additionally, a much larger portion of the planet may be habitable if it supports a thick enough atmosphere to transfer heat to the side facing away from the star.

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.

Image Credit: NASA/R. Hurt/T. Pyle
Explanation from: https://en.wikipedia.org/wiki/TRAPPIST-1d