Exoplanets: Astronomers Discover a Cold Super Earth in Our Neighborhood

"It's kind of like the great white whale of planet hunting."

There are a lot of reasons to be excited about the recent discovery of an Earth-like, frozen planet orbiting one of our neighboring stars. For one, it represents the culmination of years of searching for exoplanets, and two, as one scientist involved in the search tells Inverse, it may open the floodgate to finding more potentially habitable planets in the future.

Let’s be clear: This planet, which Paul Butler, Ph.D., and his colleagues at the Carnegie Institution for Science found orbiting Barnard’s Star (the nearest single star to Earth, at six light years away) is frozen solid — in other words, it’s not habitable. But Butler explains that the planet designated Barnard’s Star b, which is about 3.2 times the size of Earth and is now the second closest known exoplanet to Earth, very well could be if Barnard’s Star wasn’t a red dwarf, a small, low-wattage version of a star. If Barnard’s Star was hot enough to melt the planet’s ice, then the planet’s orbit, which lasts around 233 days, would suggest it could support life. Butler and his more than five dozen co-authors published their paper describing their work Tuesday in Nature.

“This is the central star. It’s kind of like the great white whale of planet hunting,” he says. “But what’s really exciting is that this planet is probably very cold. If there’s any water it’s probably liquid ice. But if this were a more sun-like star, then this would be the orbital distance where you would expect potential habitable planets.”

A rendition of a sunset on a planet orbiting Barnard's Star.

ESO - M. Kornmesser

Unfortunately, it’s impossible to turn up the heat in a red dwarf, which means that this planet won’t be supporting life as we know it. But the important silver lining here is that the technique used to find this planet will likely yield many others in the future. We can’t see planets that orbit nearby stars, Butler says, so instead we infer that they’re there based on how the sun the planet is (probably) orbiting behaves.

Normally when we picture planets, we imagine them orbiting the sun in perfect little planetarium-style ellipses. But both planets and suns center these orbits around what’s called the common center of mass, a sort of balancing point between the light little planet and the dense star that it orbits.

“It’s the exact same thing if you had two kids on a teeter-totter, a fat kid and a skinny kid. If you just put them on a normal teeter-totter the fat kid will go to the bottom and the skinny kid will go up,” Butler explains. “But if you move the fulcrum close to the fat kid, then it will come to a balance point. That’s the exact same mathematical balance point between a planet and a star.”

Artist’s impression of Barnard’s Star b reflecting the orange-red light from the star.

Credit: IEEC/Science-Wave - Guillem Ramisa

Since we understand this about planets and stars, Butler says, we can work out how their orbits should be, relative to this point. By observing those orbits — specifically, the astronomers look at the wavelength of light emitted from stars as they go around their orbits — they can estimate that there might be a skinny kid somewhere on the other end of the teeter totter.

“We can see the effect that the planet has on the star through gravity. From the motion of the star we can work out all the orbital elements of the planet,” he explains.

In this case, the skinny kid was an exoplanet planet roughly 3.3 times the size of Earth with an orbit that would be potentially hospitable if that star was a bit hotter. So we may not be colonizing it soon, but Butler notes that precedent suggests there are more exoplanets are waiting to be discovered.

“We’re getting closer to being able to find potentially habitable Earth-like planets around sun-like stars,” he says. “This is one of the key stepping stones to that.”

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