In a rare find, a rocky “super-Earth” planet has been located orbiting around a cool dwarf star — and it’s just a mere 39 light years away. The world gets its “super” nickname because it’s a bit bigger and slightly more massive than our own planet. But just like Earth, this planet sits in the coveted habitable zone, the region around a star where temperatures are just right for liquid water to pool on a planet’s surface. That make this place an exciting candidate in the search for life outside our Solar System.
Dubbed LHS 1140b, the planet was first spotted in September 2014 by a group of telescopes in the mountains of southern Chile. The telescopes, part of the MEarth-South telescope array, saw the planet as it passed in front of its host star. During these transits, the world slightly dims the star’s light, making enough of a change that can be detected from Earth. Based on this dimming and further scrutiny of the star, scientists were able to figure out the planet is about 40 percent larger than Earth and about 6.6 times more massive, according to a study published today in Nature. Those dimensions strongly suggest that the world is rocky like Earth too.
Just because LHS 1140b shares a few key traits with Earth doesn’t mean this planet is exactly like ours, though. But finding a super-Earth with a rocky composition is pretty rare, and the fact that it’s in the habitable zone makes the discovery even sweeter. Water is a key ingredient for Earth life, so if liquid H2O is present on this planet, it could up the chances that life exists there as well. And the conditions of this planetary system mean astronomers are well poised to figure out if water might be there. “We’ll be basically trying to [study] this planet with everything we have,” Jason Dittmann, an astronomer at Harvard and the study’s lead author, tells The Verge.
When it comes to the search for planets outside our Solar System, also called exoplanets, astronomers have been particularly interested in super-Earths like LHS 1140b. These planets are more massive than Earth but lighter than gas giants like Neptune or Uranus, and they’re also fairly common throughout our galaxy. But they’re not always ripe for life. In fact, these planets can be made of rocky surfaces, just like Earth, or thick hydrogen and helium gases with no surface at all.
Dittmann and his team were able to figure out that LHS 1140b is rocky by measuring how it tugged on its star. A planet’s gravitational pull can cause its host star to slightly wobble, giving astronomers an idea of just how massive the world is. That, combined with the planet’s estimated size, indicates this world is rocky. Not a lot of super-Earths have been found to sit in the habitable zone, so this rare find raises the possibility that there could be a lot more planets like this one. “Now we can say rocky Earth-size planets are probably fairly common,” Bruce Macintosh, an exoplanet expert at Stanford who was not involved in this study, tells The Verge.
LHS 1140b is a great world to study also because of the star it orbits: a red dwarf — a cool star that’s less than one-third the size of our Sun. These star types have become popular targets for exoplanet hunters, since they’re also common throughout the galaxy and it’s easier to spot planets around them. The recent seven-planet system found by NASA’s Spitzer Telescope all circle around a dwarf star known as TRAPPIST-1. Those planets are smaller than this one, but they’re thought to be rocky too. And a good chunk of them also lie in the habitable zone. The TRAPPIST-1 system is about as far from Earth as LHS 1140b is, so its seven planets are also in a prime position for further study.
But the star that Dittmann found is a bit more intriguing since it’s fairly quiet. The TRAPPIST-1 star and other similar red dwarfs are usually pretty active, meaning they send out high intensity solar flares quite frequently. Such flaring can strip nearby planets of their atmospheres and douse them with solar radiation, making them inhospitable to biological life. That’s not the case for this star. The distance between LHS1140b and its host star is about 8 percent of the distance between Earth and the Sun, but the amount of high-energy radiation it receives is still relatively low. And that means there’s a better chance that this planet may be holding onto some valuable chemicals, like organic molecules and water. “We don’t have atmospheric measurements right now, but the star behaves nicely so that it’s not ruling out anything,” says Dittman.
The star that LHS 1140b orbits is both small and faint. That makes it much easier to see and study this planet, since it’s not completely overpowered by intense starlight. “You want to get as much light as possible, but if it’s too bright it overwhelms the camera,” Dittmann says. The host star is also relatively close to Earth, which means its light is just bright enough to be used as a tool to peer into the planet’s atmosphere. Figuring out the gases in that atmosphere could paint a picture of what conditions are like on the surface below. For instance, astronomers may be able to measure oxygen molecules, which are key for life on our planet, Dittmann says. The star is “almost as bright as you could possibly get without making life difficult,” he says. TRAPPIST-1 is just a little too dim to do this, Dittmann argues.
Given all these possibilities, Dittmann and his team are eager to keep studying this planet with more ground-based telescopes, as well as the space-based Hubble telescope. But they’re particularly excited to use NASA’s powerful James Webb Space Telescope, which is supposed to launch late next year. That space-based observatory is meant to study the earliest galaxies and stars of the Universe, but it will also have enough precision to peer into LHS 1140b’s atmosphere. Dittmann and his team have been working with the JWST teams, who have agreed to give some observation time to LHS 1140b when the telescope first launches. “We’ll basically be taking a look at this planet right out of the starting gate,” says Dittmann.