3 Earth-Sized and Potentially Habitable Worlds Spotted Orbiting a Nearby Dwarf

May 3, 2016 | Joanne Kennell

3 exoplanets orbit TRAPPIST-1
Photo credit: ESO/M. Kornmesser. Image has been cropped.

This ultra-cool star is now the best place to hunt for alien life!

An international team of astronomers has discovered three Earth-sized planets orbiting near the “habitable zone” of a ultra-cool dwarf star. What’s more, the planets are located so close to Earth — only 40 light-years away — that astronomers should be able to obtain details of each of the planet’s composition, atmosphere, as well as any signs of alien life.

"The kind of planets we've found are very exciting from the perspective of searching for life in the universe beyond Earth," said Adam Burgasser, a professor of physics at UC San Diego's Center for Astrophysics and Space Sciences and a key member of the international team, in a press release.

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The team used the TRAPPIST (TRAnsiting Planets and PlanetesImals Small Telescope) telescope at the La Silla Observatory in Chile to trace the dwarf star’s light moment-to-moment over the course of 62 nights last September through December. After analyzing these changes in brightness, the team saw periodic dips as the three planets casted shadows while crossing in front of the star’s face.

Further observations with larger telescopes, including the HAWK-I instrument on ESO's 8-metre Very Large Telescope in Chile, found that the planets have sizes quite similar to Earth and that two of the planets have orbital periods of roughly 1.5 days and 2.4 days, while the third planet has a period in the range of 4.5 to 73 days.

"With such short orbital periods, the planets are between 20 and 100 times closer to their star than the Earth to the Sun," said Michaël Gillon of the University of Liège, Belgium, and leader of the international team, in the release.

The planets orbit very close to their host star, but since the star is much dimmer than the sun, the inner two planets only receive four and two times, respectively, the amount of radiation received by Earth. This puts them just inside the habitable zone for this system. However, the presence of life will depend on their surfaces and the composition of their clouds and atmospheres — if they even have one.

The third outer planet is currently less understood, but it likely receives less radiation than Earth. However, it may still lie in the habitable zone.

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According to astronomers, the two planets closest to the star are probably tidally locked, where one side of the planet is always facing towards the star. Although this could limit the circulation of water and an atmosphere, the tidal forces could potentially keep the planet surface warm even on the dark side. However, these same forces could also result in extreme geothermal activity, which would make the surfaces unstable.

"Fortunately, we may be able to answer these questions in the near future [...] with the launch of the James Webb Space Telescope," Burgasser said.

These planets orbit a cool, red, dim dwarf star called "TRAPPIST-1." It is a little larger than Jupiter in diameter but is about half the temperature of the sun and almost 2,000 times fainter.

"While such a 'cold' star might sound exotic, many, if not most, of the stars in our Milky Way Galaxy are of this cool, red, small and dim variety," Burgasser said. "If Earth-like planets around these stars turn out to be common, there may be many more habitable planets out there than current estimates predict."

The discovery is detailed in a paper published on May 2 in the journal Nature.

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