Proxima Centauri b and the question of habitability

The discovery of evidence of a planet in orbit around our nearest stellar neighbor, Proxima Centauri, has people all agog and with good reason. It’s something of a misnomer, however, to call the exoplanet Proxima Centauri b “Earth-like.” Rory Barnes, a professor in the Department of Astronomy and the Astrobiology Program at the University of Washington, points out that the planet’s mass is probably somewhere between 1.3 and five times that of Earth.

UW prof. Rory Barnes speaking at an Astronomy on
Tap Seattle event earlier this year. Photo: Greg Scheiderer.

“There’s a lot of excitement about this planet because it is so close in mass to the Earth, but we don’t actually know if it’s even rocky like the Earth,” said Barnes during a recent talk at the Pacific Science Center. Barnes, who uses computer modeling to study the habitability of exoplanets, noted that even though Proxima Centauri is the next closest star, it’s still pretty far away at 4.24 light years. If the Sun were the size of a baseball resting on home plate at Safeco Field, Barnes said Proxima Centauri b would be a grain of sand in New York City. Still, he noted there’s understandable excitement about the discovery.

“The reason why I think that this is the biggest exoplanet discovery since the discovery of exoplanets is because it is still very close, at least relatively speaking,” Barnes said. “We really have a chance, with this planet, to potentially observe its atmosphere and its surface and maybe start to try and sniff out the presence of life on that planet. Or not. We don’t know yet. But being so close, it gives us a shot.”

Not really “like” Earth

While Proxima Centauri b is about the mass of Earth, plenty else is different. It’s twenty times closer to its home star than Earth is to the Sun, and goes around that star in just 11.2 days. We know little else about it. The star has just 12 percent of the mass and 14 percent the radius of the Sun, and its brightness is just one one-thousandth that of the Sun.

“This is a small, dim star,” Barnes said.

Is there life there?

Life requires energy, some bioessential elements, and liquid water. The energy and elements are abundant in the universe, so Barnes says the key to finding life elsewhere is liquid water.

“When we think about exoplanets, we’re really going to focus, at least for now, on surface water,” Barnes said. “Not only is it going to be easier to see, but it’s going to be more similar to the Earth and that gives us a better shot at maybe being able to interpret the observations that we’re going to get.”

The desire to find liquid surface water on a planet led us to the concept of the “habitable zone” around a star, an area where the temperature would be right for liquid water to exist. Barnes said Proxima Centauri b is smack in the middle of the habitable zone.

“This is a dream planet for those of us who study this field,” he said, but added a caveat: “Being in the habitable zone does not mean you’re habitable. It is just the first step we need to get to.”

“The habitable zone is jargon, and it’s really misleading,” Barnes added. “I apologize for my field for inflicting it on you!”

Barnes said there are several threats to habitability for planets orbiting M dwarf stars like Proxima Centauri. With the habitable zone so close to the star, there is potential that stellar flares could blow away the atmosphere of a planet within it. Planets that close are probably tidally locked, too, but this isn’t a deal-breaker; their atmospheres might distribute heat and energy effectively. Tidal heating could cause problematic volcanism.

Barnes showed this chart demonstrating that while
Proxima Centauri b is now within the habitable zone, the
zone was once much further from the star.

The biggest threat to the habitability of Proxima Centauri b, according to Barnes, is that its star was once much bigger and brighter before it contracted into the dim, red phase it is in today. In the early years that would have meant that its habitable zone was out at a distance between .25 and .5 astronomical units, while Proxima Centauri b orbits at a mere .05 AU. Being so far inside the habitable zone after formation means that the planet could have lost all of its water and become a completely uninhabitable place like Venus. On the other hand, if Proxima Centauri b formed as something like Neptune, being so close to the star could have blasted away its hydrogen envelope.

“Maybe that planet could have actually transformed from an uninhabitable Neptune-like planet into a rocky planet like the Earth,” Barnes speculated. “This is what we at the University of Washington think is probably the best bet for how this planet could be habitable.”

Barnes is hopeful that the discovery of Proxima Centauri b will help boost support for the sorts of telescopes and observatories that can make the observations needed to learn more about this intriguing exoplanet and determine if it is habitable, and even inhabited.

While Barnes won’t give the odds of life there—there are way too many variables and so little we know right now—he sounds confident that we’ll find life somewhere. He noted that we’ve found life on Earth in the deep sea, extreme deserts, extreme cold, acidic environments, and under other harsh conditions.

“The realization that extreme life is everywhere is part of the astrobiological revolution that is occurring right now in science,” Barnes said. “This recognition that life finds a way gives us confidence as we go forward.”