Ultra-cool dwarf throws planetary party

Three Earth-sized bodies spied circling diminutive star

TRAPPIST-1 as seen from one of its planets. Pic: ESO / M. Kornmesser

Astroboffins have discovered a trio of roughly Earth-sized planets orbiting an ultra-cool dwarf star, the first time such a system has been identified.

Using the Belgian TRAnsiting Planets and PlanetesImals Small Telescope (TRAPPIST) 'scope at The European Southern Observatory's La Silla Observatory in Chile, a team led by Michaël Gillon, of the Institut d’Astrophysique et Géophysique at the University of Liège, observed the transition of the bodies as they passed between Earth and star 2MASS J23062928-0502285.

The star, now handily renamed TRAPPIST-1, lies in the constellation of Aquarius, some 40 light years from Earth. It's "much cooler and redder than the Sun and barely larger than Jupiter", falling into the ultra-cool dwarf star group "not massive enough to sustain hydrogen fusion" and with a temperature of "less than 2,700 kelvin".

View of TRAPPIST-1 and its three planets. Pic: ESO / M. Kornmesser / N. Risinger (skysurvey.org)

Ultra-cool: The Dwarf and its planetary system. Pic: ESO / M. Kornmesser / N. Risinger (skysurvey.org)

ESO says of the newly-spied planetary system: "Two of the planets have orbital periods of about 1.5 days and 2.4 days respectively, and the third planet has a less well determined period in the range 4.5 to 73 days."

The planets are "between 20 and 100 times closer to their star than the Earth to the Sun", Gillon explained, adding: "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."

Regarding the now-obligatory speculation as to whether the three might lie in the "habitable zone", where liquid water could exist, ESO comments: "Although they orbit very close to their host dwarf star, the inner two planets 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.

"The third, outer, planet’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."

Artist's impression of the TRAPPIST-1 system seen from one of the planets. Pic: ESO / M. Kornmesser

Come on in, the water's fine. Pic: ESO / M. Kornmesser

Time will tell. Julien de Wit of MIT, co-author of the team's paper published in Nature, teased: "Thanks to several giant telescopes currently under construction, including ESO’s E-ELT and the NASA/ESA/CSA James Webb Space Telescope due to launch for 2018, we will soon be able to study the atmospheric composition of these planets and to explore them first for water, then for traces of biological activity. That's a giant step in the search for life in the Universe." ®

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