- Astronomers think they have found a “cold super-Earth” exoplanet orbiting Barnard’s star.
- Barnard’s star, less than six light-years from us, is the closest single-star system to our sun.
- The new world, called Barnard’s star b, or GJ 699 b, is at least 3.2 times as massive as Earth.
- It orbits in its parent star’s “snow line,” a region at the edge of a star’s habitable zone where scientists suspect most rocky planets form.
- This roughly Earth-size exoplanet may be the first to be photographed by a new generation of powerful telescopes.
Astronomers have for decades suspected that a nearby red dwarf star, called Barnard’s star, may be hiding a roughly Earth-size planet.
On Wednesday, researchers said they’d discovered the first such exoplanet, with about 99.2% certainty.
A team of dozens of scientists published the finding in the journal Nature and said there were even hints that a second world might lurk nearby.
Barnard’s star is 5.87 light-years from Earth, making it the closest one-star system to us. Only Proxima Centauri, a three-star system, is closer.
What’s more, the newly discovered world is close enough to Earth – yet far enough from its blindingly bright star – to be photographed by a new generation of giant telescopes.
“This is probably the first Earth-sized planet we will directly image by future missions,” Abel Méndez, an astrobiologist at the University of Puerto Rico at Arecibo who wasn’t involved in the study, told Business Insider.
The new world is known as Barnard’s star b or, in other circles, GJ 699 b. The team that found it used more than 20 years’ worth of telescope observations, and the data suggests the planet is at least 3.2 times as massive as Earth and has a 233-day year.
The world appears to orbit in Barnard’s star’s “snow line,” a region just on the edge of the habitable zone, where liquid water can exist on the surface of a planet.
For that reason, scientists consider the possible planet to be a “cold super-Earth,” and some are wondering whether alien life might exist there.
What it might be like on Barnard’s star b
Barnard’s star is not like the sun. It’s what astronomers call an M dwarf, meaning it’s smaller, cooler (and redder), less massive, and billions of years older.
“M dwarfs are prime targets for planetary searches, because they favour the detection of small companions,” or planets, Rodrigo F. Díaz, an astrophysicist at the University of Buenos Aires who wasn’t part of the research team, wrote in a Nature “News and Views” piece.
This is because M-dwarf stars are small enough to “wobble” when rocky planets orbit them. Such wobbling happens because the gravity of the planet and the star mutually tug at each other. (A planet’s orbit is rarely perfectly circular.) Some telescopes can detect this behaviour.
Barnard’s star b took a long time to find because red dwarf stars often have sunspots, which can throw off sensitive measurements. Pooling together more than two decades’ worth of telescope observations, however, helped the research team find the signal of a presumable planet in the noise of Barnard’s star’s light.
The newly discovered planet is about as far from its star as Mercury is from the sun. That’s fairly close. But being next to a smaller and cooler star puts Barnard’s star b at the edge of the habitable zone.
Practically, this means the surface temperature of Barnard’s star b is most likely -150 degrees Celsius. That’s cold enough to freeze carbon dioxide into dry ice, let alone water into ice.
But bone-chilling temperatures don’t mean the exoplanet is a dead world bereft of liquid water.
Can a cold super-Earth be habitable?
The surface of Europa, an icy moon that orbits Jupiter, is 10 degrees colder than Barnard’s star b. And Ganymede, a smaller icy moon around Saturn, is about 20 degrees colder.
The researchers’ estimate of Barnard’s star b’s temperature also assumes it has no atmosphere. But it very well could.
“Since the planet is more massive than Earth, it may retain a hydrogen atmosphere,” Sara Seager, the deputy science director for NASA’s TESS mission and an astrophysicist at MIT who wasn’t involved in the study, told Business Insider. “Hydrogen is a potent greenhouse gas and could conceivably keep the surface temperature warm enough for life, if the atmospheric pressure is high enough.”
It’s not a far-fetched idea. Seager said all planets – even Earth – are born with a hydrogen atmosphere. This is because hydrogen is the dominant material in nebulas, the clouds of gas and dust out of which stars (and their planets) form.
“Assuming it has the estimated minimum mass, then it is probably a rocky or frozen ocean world with a thick atmosphere, maybe like Titan in the solar system,” Méndez said.
On the other hand, 3.2 Earth masses is a minimum estimate for the new world’s size. Méndez added that “its mass could be larger and therefore a mini-Neptune.”
The first photo of a rocky exoplanet
Assuming the exoplanet does indeed exist, it may look like a tiny dot or blob in an image. But the data trapped in such a picture could tell scientists whether it’s small and rocky like Earth, or big and gassy like Neptune, or somewhere in between.
What’s more, some of the light that passes through or bounces off the world’s atmosphere might even be sampled for indirect signs of life. Such gases might include methane, oxygen, carbon dioxide, and perhaps 14,000 other so-called biosignatures.
Red dwarf stars typically blast out a lot of flares and solar storms, which can harm or destroy planetary atmospheres. They’re also very bright in ultraviolet light, which can break down biosignatures. But Seager said Barnard’s star, as an M dwarf, is calmer and gives off less UV light – so its presumable planet may be able to accumulate biosignatures to detectable levels.
But even if Barnard’s star b is a bust for biosignatures, observations may find that it isn’t alone.
“I don’t discard the possibility of smaller Earth-sized planets in the habitable zone of Barnard’s star,” Méndez said. “Now we know that Barnard has planets, and there is plenty of space between the star and this new planet for a few small ones.”
This story has been updated.