A sky full of super-Earths

exoplanet orbiting sun
M. Weiss/CfA
An artist's impression of LHS 1140b

On the one hand, it's hard to get excited about every single Earth-ish exoplanet that astronomers discover when it seems like we've got a new one (or seven) on the books every month or so. On the other hand, uh, we're basically discovering new Earth-ish exoplanets every month or so.

April's "is it aliens?" entry is LHS 1140b, a "super-Earth" described Wednesday in the journal Nature. The basic stats are these: it's just 39 light-years away in the constellation Cetus (The Sea Monster, rawr), it's just under one-and-a-half times the size of Earth (though it's a lot denser, with over six times our mass), and it orbits a faint red dwarf star.

"This is the most exciting exoplanet I've seen in the past decade," lead author Jason Dittmann of the Harvard-Smithsonian Center for Astrophysics said in a statement. "We could hardly hope for a better target to perform one of the biggest quests in science—searching for evidence of life beyond Earth."

Little red dwarfs have become a big focal point for astronomers hunting for habitable planets. For a long time, planet-hunters focused on bigger stars similar to our own sun. After all, we only know of one place where life successfully evolved—so it makes sense to try to find solar systems as close to our own as possible. That's why "Earth-like" exoplanets get so much buzz, too: these are worlds that researchers estimate to be rocky, based on their mass and their distance from their host star (as opposed to massive planets made of gas), so they have surfaces not totally unlike the own found on our own planet. In theory, rocky worlds that sit in their star's "habitable zone"—not so close as to turn into a sizzling rocky like Mercury, but not so distant as to freeze over like Pluto—could maintain liquid water, given the proper atmosphere. And we know water is a key ingredient for life, at least as we know it.

But in recent years, red dwarfs have gotten more attention. The "dim bulbs" of the Universe are more plentiful in our neck of the woods than sun-like stars, and they're also older—which means any rocky planets they host have had plenty of time to evolve a microbe or two. Most rocky planets around red dwarfs are thought to be tidally locked (one side of the planet always faces the sun, while the other sits in perpetual darkness) but hey, beggars can't be choosers.

LHS 1140b is thought to be at least five billion years old (about 500,000,000 years older than Earth), and it's right smack dab in the middle of its stars habitable zone: it's 10 times closer to its star than the Earth is to the sun, but because of the star's diminutive nature it only receives about half as much light. Young red dwarf stars emit violent shots of radiation, which can bake their closely-held planets beyond any hope of habitability. But because of LHS 1140b's size and density, researchers believe it would have cooled down slowly enough after its formation to maintain a large ocean of magma for millions of years. The constant output of steam from this roiling lava may have been enough to keep the planet's atmosphere moist during its star's wild youth.

Dittmann and his colleagues think the newly-discovered exoplanet is the perfect target for the soon-to-launch James Webb Space Telescope. Scientists will be able to examine light as it passes through LHS 1140b's atmosphere (assuming it indeed has one), which will allow them to analyze its chemical composition. The team has already secured time on the telescope, though they'll continue to observe the planet using data from the Hubble and other already functioning scopes in the meantime.

We'll have to wait a while before we know if LHS 1140b has any atmosphere to speak of, let alone one full of the life-giving molecules that astrobiologists covet. And even if all the ingredients are there—even if all of the proverbial lights are on—it's quite possible that no one is home. And while 39 light-years is basically our backyard on a cosmological scale, there's no way a probe will ever visit this alien world in our lifetime. Luckily, some of the best habitable world candidates are actually moons within our own solar system. Exoplanets like LHS 1140b can reveal invaluable secrets about how planets form and how often they come with a habitability starter pack, but you can be pretty sure that we'll track down our first alien lifeforms a lot closer to home.