TRAPPIST-1, also known as 2MASS J23062928-0502285, is an ultracool dwarf star in the constellation of Aquarius.

The star is 38.8 light-years distant, close enough to be considered part of Earth’s neighborhood.

It is much cooler and redder than our Sun, with an age constrained to be 500 million years old, and with a luminosity, mass, and radius of 0.05%, 8% and 11.5% those of the Sun, respectively.

The planetary system TRAPPIST-1 hosts at least three planets, which are designated TRAPPIST-1b, c, and d.

The planets have been detected by the TRAPPIST (TRAnsiting Planets and PlanetesImals Small Telescope), a Belgian robotic 0.6-m telescope based at ESO’s La Silla Observatory in Chile.

According to Dr. Gillon and his colleagues from the United States, Belgium, the UK and India, ultracool dwarf stars, such as TRAPPIST-1, are both very common in the Milky Way Galaxy and very long-lived, but this is the first time that planets have been found around one of them.

“This really is a paradigm shift with regards to the planet population and the path towards finding life in the Universe,” said co-author Dr. Emmanuël Jehin, an astronomer with the University of Liège.

“So far, the existence of such ‘red worlds’ orbiting ultra-cool dwarf stars was purely theoretical, but now we have not just one lonely planet around such a faint red star but a complete system of three planets.”

Follow-up observations with larger ground-based telescopes, including the HAWK-I instrument on ESO’s 8-m Very Large Telescope in Chile, have shown that the planets around TRAPPIST-1 have sizes very similar to that of Earth.

TRAPPIST-1b and TRAPPIST-1c have orbital periods of 1.51 days and 2.42 days, respectively, and the third planet, TRAPPIST-1d, has a less well determined period in the range 4.5 to 72.8-days.

“With such short orbital periods, the planets are between 20 and 100 times closer to their star than the Earth to the Sun,” Dr. Gillon explained.

“The structure of this planetary system is much more similar in scale to the system of Jupiter’s moons than to that of the Solar System.”

Although they orbit very close to their host dwarf star, TRAPPIST-1b and TRAPPIST-1c only receive four times and twice, respectively, the amount of radiation received by the Earth, because their star is much fainter than the Sun.

That puts them closer to the star than the habitable zone for this system, although it is still possible that they possess habitable regions on their surfaces.

TRAPPIST-1d’s orbit is not yet well known, but it probably receives less radiation than the Earth does, but maybe still enough to lie within the habitable zone.

The masses of these planets are still unknown, making their internal composition as yet undetermined.

“We intend to measure their masses during the coming months, to determine if these planets are mainly rocky, like the Earth, or if they are water-rich, like icy moons of the giant planets,” Dr. Gillon said.

Source: Three Earth-Sized Exoplanets Found around Ultracool Dwarf TRAPPIST-1 | Astronomy |