An Earth-Sized Neighbour May Be The Most Important Exoplanet Yet

An Earth-Sized Neighbour May Be The Most Important Exoplanet Yet

Astronomers have discovered a rocky, Earth-sized planet 39 light years from home — right in our cosmic backyard. With a surface temperature of 226C, GJ 1132b is more of an intergalactic furnace than a vacation prospect. Yet some are hailing it “the most important planet ever found outside our solar system.”

That’s because GJ 1132b, which orbits the dim M-dwarf star Gliese 1132, lies close enough to Earth that new telescopes will soon allow us to peer into its atmosphere. What we learn from doing so could pave a path toward finding a second Earth.

“This is our best chance to study the atmosphere of a rocky planet outside of our solar system,” lead study author Zach Berta-Thomson, a post-doc at MIT’s Kavli Institute for Astrophysics and Space Research, told Gizmodo. “By studying this much hotter version of Earth, I think we’ll learn quite a bit about what makes a cooler planet around an M-dwarf habitable.”

Over the past decade, NASA’s Kepler mission has uncovered thousands of worlds beyond our solar system, by spotting the faint dips in starlight caused planetary transits. But all of the stars Kepler’s eyes are trained on lie hundreds to thousands of light years away — not exactly easy places for us to get up close and personal with.

This animation shows the distances to known transiting exoplanet systems in lightyears. GJ 1132b is much closer than most other transiting planets. The proximity and small size of its host star will allow astronomers to spectroscopically study its atmosphere. Credit: Zach Berta-Thompson

We know much, much less about the planets in our immediate stellar neighbourhood. GJ 1132b is one of a select handful that’s both nearby and Earth-sized; Berta-Thomspon calls it the “slosest” (smallest close) exoplanet yet. (Another recently discovered planet is even closer to us, but somewhat larger.) The planet’s transit was first spotted by the MEarth-South Observatory, an array of 40 centimeter wide robotic telescopes designed to study dim, M-dwarf stars. Other scopes were used to confirm the find.

Given the planet’s radius (16% larger than Earth’s), its orbital distance, and its weight of 1.2 Earth masses (inferred from the gravitational “wobble” of its star), the team was able to work out GJ 1132b’s density. At 6 grams per cubic centimeter, GJ 1132b is a perfect match to Earth, telling us it’s probably a rocky world with a similar composition. Although GJ 1132b is too hot for life as we know it, the planet may be cool enough to retain an atmosphere.

And that fact places GJ 1132b on a shortlist of targets for the James Webb Space Telescope (JWST), the successor to Hubble that’s slated to launch in 2018. “Any type of atmosphere is interesting for us to understand how atmospheres work,” Cornell exoplanet researcher Lisa Kaltenegger told Gizmodo. “This one is not in the habitable zone, but it will be interesting to understand how hot planets work too.”

Cartoon showing GJ 1132b to scale against its star. Image Credit: Zach Berta-Thompson

If and when we train JWST on the rocky, Earth-like world that sits in its star’s Goldilocks zone, we’ll want to probe its atmosphere for things like oxygen and methane — so-called “biosignatures” that could indicate the presence of life. We’ll do so by looking at the spectral signature of starlight as it filters through the planet’s air. We can think of GJ 1132b as a training world, one that can help us hone our methods and figure out exactly what we’re looking for.

But what if it turns out that GJ 1132b doesn’t have an atmosphere? That’s also going to be a critical insight for alien-hunters, Berta-Thompson says.

Based on Kepler’s cosmic census, we know that rocky, Earth-sized planets are very common around M-dwarf stars. But it’s still unclear whether an Earth-like world orbiting an M-dwarf could hold onto an atmosphere long enough for life to evolve.

“As these [M-dwarf] stars are young, they’re a lot bigger and much more active,” Berta-Thompson said. “They shoot off UV light and particle storms that bombard the planet, and could, in some cases, erode the atmosphere. If a young M-dwarf can erode the atmosphere of a habitable planet, that same star will definitely erode the atmosphere of a planet closer in.”

So if GJ 1132b has had its atmosphere flayed off by an unsupportive star (our own Sun did something similar to Mars), it could have sweeping implications for the discovery of habitable worlds around M-dwarfs in general.

We simply won’t know until we get a better look at GJ 1132b, and that may not be for another few years. In the meantime, Berta-Thompson and his colleagues are using the Spitzer Space Telescope to scour the same star system for planets in more distant orbits. Maybe there will turn out to be an entire family of rocky planets sitting out there, a cosmic hop-skip away, just waiting to have their atmospheres probed by our telescopes.

“[This discovery] really gives us an opportunity to not only imagine what planets around other stars are like, but to have the hope of observing them directly,” Berta-Thompson said. “To learn what in our imagination comes true and what doesn’t.”

[Read the full scientific paper at Nature]

Top: Artist’s rendering of GJ 1132b. Image Credit: Dana Berry