Night by night, star by star, astronomers are edging ever closer to learning just how crowded our universe really is—or at least our galaxy, anyway.
A quarter century after the first exoplanets were found orbiting other stars, statistics from the thousands now known have revealed that, on average, each and every stellar denizen of the Milky Way must be accompanied by at least one world.
Look long and hard enough for a planet around any given star in our galaxy and you are practically guaranteed to find something sooner or later.
But even a crowded universe can be a lonely place. Our planet-rich Milky Way may prove to be life-poor. Of all the galaxy’s known worlds, only a figurative handful resemble Earth in size and orbit.
Each occupying a nebulous “Goldilocks” region of just-rightness—a fairy-tale-simple ideal in which a world is neither too big nor too small, neither too hot nor too cold, to sustain liquid water and life on its surface.
Instead, most of the Milky Way’s planets are worlds theorists failed to predict and have yet to fit comfortably in any conception of habitability: “super-Earths” bigger than our familiar planet but smaller than Neptune.
No super-Earths twirl around our sun for solar system–bound scientists to directly study, making it that much harder to know whether any elsewhere are Goldilocks worlds—or, for that matter, whether any one-size-fits-all metric of habitability is hopelessly naive.
A Frozen Super-Earth?
“If you live in a city of millions of people, you are not interested in meeting every one of them—but maybe you want to meet your immediate neighbors,” says lead author Ignasi Ribas, an astronomer at the Institute of Space Studies of Catalonia in Spain.
“This is what we are doing for the planetary systems of the stars that surround us. Otherwise we cannot answer the big questions. How does our solar system and our Earth fit in with the rest of the galaxy?
“Are there other habitable or inhabited planets? Barnard’s Star b is not giving us those answers just yet, but it is telling us part of the story we need to know.”
Located in the constellation of Ophiuchus, Barnard’s Star is so dim in visible light that it cannot be seen with unaided eyes.
Yet it has been a favorite of astronomers since 1916, when measurements revealed its apparent motion across the sky was greater than that of any other star relative to our sun.
A sign of its extremely close cosmic proximity. The star’s nearness to us is only temporary—within tens of thousands of years, its trajectory will have swept it out of our solar system’s list of top five closest stars.
According to Ribas and his colleagues, the candidate planet is at least three times heavier than our own, and circles its star in a 233-day orbit.
That would put it in the torrid orbital vicinity of Venus around our yellow sun, but Barnard’s Star is a comparatively pint-size and dim red dwarf star.
This means its newfound companion is near the “snow line,” the boundary beyond which water almost exclusively exists as frozen ice—a region around other stars thought to be chock-full of planets, but that astronomers have only just begun to probe for small worlds.
Alternatively, the planet might be covered by a thick, insulating blanket of hydrogen leftover from its birth in a spinning disk of gas and dust around its star.
Although hydrogen on smaller, hotter worlds would dissipate into space, super-Earths in frigid orbits might manage to hang on to enough of the gas to build up a massive planet-warming greenhouse effect—a possibility that throws Earth-centric Goldilocks ideas into tumult.
If this mechanism operates on Barnard’s Star b or other cold super-Earths, “our dreams that every star may have a habitable planet could well come true,” says Sara Seager, a planet-hunting astrophysicist at Massachusetts Institute of Technology who was not involved with Ribas’s study.
“There are some crazy worlds out there.”
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