Earth-size Planets Around Nearby Dwarf Star

Astronomers just discovered three planets, two of which are roughly the size of Earth, orbiting a dim nearby dwarf star. Their proximity makes their atmospheres ripe for observing.

TRAPPIST-1 planet system artist's illustration

This artist’s impression shows an imagined view of the three planets orbiting the red dwarf star 40 light-years from Earth. They are the first planets ever discovered around such a tiny, dim star.
ESO / M. Kornmesser / N. Risinger (skysurvey.org)

Forty light-years away, a star just a tenth the size of the Sun holds three Earth-size planets in orbit, where none were expected. Though they’re probably not habitable, they’re so nearby we could one day soon glimpse water and other molecules in their atmospheres. And they hold the promise of many more such worlds around other dim, nearby stars.

The discovery is a first for the exoplanet community, which for a long time has sought exoplanets around far brighter stars. The ultimate goal for the Kepler team, for example, was to find an Earth-size planet in an Earth-like orbit around a Sun-like star.

But the robotic 0.6-meter telescope at La Silla Observatory in Chile known as TRAPPIST (TRAnsiting Planets and PlanetIsimals Small Telescope) homes in on ultracool dwarfs, stars far cooler and dimmer than the Sun. (Technically, the term even includes failed stars known as brown dwarfs that can’t maintain fusion in their core.)

Smaller Stars, Smaller Planets?

Ultracool dwarf star size comparison

This illustration shows the Sun and the red dwarf star TRAPPIST-1 to scale. The faint star has only 11% of the diameter of the sun, making it only slightly larger than Jupiter.
ESO

Scientists had good reason to believe that the smallest of stars might not host planets. The smaller the star, the less planet-forming material it holds in reserve. Some simulations showed that such dwarf stars shouldn’t be able to host Earth-size planets at all. Other studies disagreed. But according to Michaël Gillon (University of Liège, Belgium), who led the study, the controversy was enough to dissuade time-consuming observations of the smallest, faintest stars — until the TRAPPIST telescope began its work in 2010.

“These tiny stars and brown dwarfs were just overlooked,” Gillon adds. “As there are only about 1,000 of them in the sky that are bright enough for planet detection and detailed characterization, you need to observe them one by one.”

In the case of the M8-class star now known as TRAPPIST-1, the telescope snapped near-infrared images every 1.2 minutes for 245 hours over 62 nights between September and December 2015. That’s a lot of time spent on a tiny star: TRAPPIST-1 spans 11.5% of the Sun, contains 8% of the Sun’s mass, and emits only 5% of the Sun’s luminosity.

But the close attention paid off. In the May 2nd online issue of Nature, the authors report 11 signatures of planets passing in front of the star. Because the dwarf is so faint, the transits made a decent impression, blocking 1% of the star’s light.

Additional observations came from UKIRT in Hawai'i, the Very Large Telescope in Chile, and the Himalayan Chandra Telescope in India.

TRAPPIST-1 Transits: Habitable Planets?

TRAPPIST-1d artist's illustration

An artist's illustration presents a possible view from TRAPPIST-1d, the planet farthest from the cool red dwarf star.
ESO / M. Kornmesser

Analysis of all the observations revealed that two planets, TRAPPIST-1b and TRAPPIST-1c, had made nine of the 11 transits, orbiting their star every 1.5 and 2.4 days, respectively. The two planets are so close to the star, they’re probably tidally locked, keeping one face toward the star at all times. The third planet, 1d, only made two transits and the team couldn’t pin down its exact orbit.

The transits show that 1b and 1c have Earth-like radii, but they’re not in their star’s habitable zone, that artificially defined region around a star where a rocky planet might keep liquid water on its surface.

The authors briefly speculate that the western terminator on each planet’s permanent dayside could have just the right temperature for water — any life there would exist in permanent twilight. But for this scenario to work out, the planets’ nightsides would need to trap sufficient atmosphere, and even the authors admit the planets are too hot all around for this to be likely.

The third planet might lie in the habitable zone or beyond it — there isn’t enough information on its orbit yet to tell.

But habitable or not, what’s so fun about these planets is that they’re only 40 light-years away. That means they’re close enough that astronomers can study their atmospheres in detail. Giant planets and even super-Earths have revealed glimpses of their atmospheres, but Earth-size planets have generally been thought to be out of range for current telescopes.

Gillon says he and his colleagues have already been granted observing time with the Hubble Space Telescope to take a closer look at TRAPPIST-1’s planets. And that’s only the first step. “Later, with JWST, we will really probe the atmosphere in detail,” Gillon says. “The study of these fascinating planets is just beginning!”

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