Closest rocky exoplanet yet discovered

Its proximity will allow a more detailed study of its atmosphere than before.

by Xaq Rzetelny - Nov 17, 2015 1:00pm EST

  

Artist's rendition of Gliese 1132b with its host star, Gliese 1132.

Dana Berry

It’s a new era in exoplanet science, according to Paul Hertz, director of NASA's Astrophysics Division in Washington. "We're at a point now in exoplanet science where we are moving beyond just detecting exoplanets and into the exciting science of understanding them," he said recently. As if the recent discovery of clouds on an exoplanet (the first time such exo-clouds were observed) wasn’t enough to prove his words true, a new discovery promises to.
Using the MEarth-South telescope array in Chile, a team of researchers has identified the closest rocky exoplanet yet found. Roughly 39 light-years away, the planet Gliese 1132b is close enough that it can be studied in more detail than its next-nearest equivalent. As with many other exoplanets, 1132b was discovered by observing dips in the brightness of its parent star.
1132b is pretty similar to the Earth in a lot of respects. Its mass is somewhere between the Earth’s mass and twice that. It’s only a little bigger than Earth at roughly 1.16 times its size. It orbits a fairly typical star, which is a lot smaller than the Sun, with about 18.1 percent of the Sun’s mass and 21 percent of its radius.
Despite the star’s smaller size, conditions on 1132b are extremely hot—so hot it wouldn’t be habitable but not too hot to have a substantial atmosphere. If atmospheric gas gets too hot, the individual particles can gain enough speed to escape into space, but that’s not the case here.

Exo-Earth or exo-Venus?

The planet's hot surface temperature, about 260 degrees Celsius, gives it a closer resemblance to Venus than Earth if only in that respect. “The temperature of the planet is about as hot as your oven will go, so it’s like burnt-cookie hot,” said Zachory Berta-Thompson of MIT, the paper's lead author. “It’s too hot to be habitable—there’s no way there’s liquid water on the surface. But it is a lot cooler than the other rocky planets that we know of.”
The researchers note that any water that existed early in the planet’s history would have been scorched away, evaporated into the atmosphere, split into its component atoms, and then its hydrogen lost to space.
The planet’s heat is due to its closeness to its star. It's so close that it receives 19 times as much light from its star as Earth does from the Sun. At that distance, it orbits so quickly that it completes its revolution in about 1.63 days—compare that to Mercury, the closest planet to the Sun and hence the fastest, which takes a full 88 days to orbit. The planet is also tidally locked, meaning that one side of it always faces its star, just as the Moon is tidally locked to the Earth.
The researchers wonder if the resemblance to Venus goes deeper than just surface temperature.
“What is tremendously exciting to me is that this planet could be a real ‘cousin’ of Venus and Earth,” says Fortney, who was not involved in the research. “I think that this planet’s atmosphere, when we are able to try to determine what it is made of, will be an interesting data point in understanding the diversity in atmospheric composition for Earth-sized planets. In our Solar System, we only have two data points: Earth and Venus. Before we can understand habitability, I think we need to understand the range of atmospheres that nature makes, and why.”
"Our ultimate goal is to find a twin Earth, but along the way we've found a twin Venus," said astronomer David Charbonneau of the Harvard-Smithsonian Center for Astrophysics, one of the paper's authors. "We suspect it will have a Venus-like atmosphere too, and if it does we can't wait to get a whiff."
1132b’s host star, Gliese 1132, is an M-dwarf star, which means it burns hydrogen but is less than 60 percent the size of the Sun. M-dwarfs are the most common class of star in the galaxy, outnumbering stars like the Sun 12-to-one. And recent results have shown that many of these M-dwarfs are home to rocky planets like 1132b, with an impressive estimated rate of about 1.4 rocky planets per M-dwarf. These rocky planets range from half to one-and-a-half times the size of the Earth.
The researchers also searched for additional planets, finding none orbiting Gliese 1132. They note, however, that future, more detailed observations may turn some up.

A new era

Because of its proximity, future observations by current (such as Hubble) and future (such as the James Webb Space Telescope) telescopes will be able to study 1132b’s atmosphere in greater detail than would be possible on more distant worlds. It's hoped that researchers will be able to figure out not only the atmospheric composition but also the pattern of winds in the atmosphere. They might even be able to deduce the color of the sky on this world.
“We think it’s the first opportunity we have to point our telescopes at a rocky exoplanet and get that kind of detail, to be able to measure the color of its sunset, or the speed of its winds, and really learn how rocky planets work out there in the universe,” said Berta-Thompson. “Those will be exciting observations to make.”
These insights will help researchers understand how the planet’s atmosphere has evolved in such proximity to its star, with such strong radiation, stellar winds, and tidal forces from the star. This, in turn, will help them learn about how other exoplanetary atmospheres evolve, which is an important piece of the puzzle that is the ongoing search for life elsewhere in the Universe.
“If we find this pretty hot planet has managed to hang onto its atmosphere over the billions of years it’s been around, that bodes well for the long-term goal of studying cooler planets that could have life,” said Berta-Thompson. “We finally have a target to point our telescopes at and [can] dig much deeper into the workings of a rocky exoplanet and what makes it tick.”