How close are our nearest extraterrestrial neighbors? They might be right next door, say astronomers.
A new exoplanet candidate, dubbed Proxima b, is just 4.25 light-years from Earth, orbiting our nearest stellar neighbor. And it might be habitable.
The discovery, announced Wednesday in the journal Nature, "shows that we have entered an era in human history where we finally have the instruments available to answer one of humankind's oldest questions," says Daniel Angerhausen, a researcher at NASA's Goddard Space Flight Center who was not involved in the research. Is there life elsewhere in the universe?
While astronomers still don't have an answer, they do have some tantalizing data about Proxima b.
The Earth-like world is about 1.3 times the mass of our planet and orbits its parent star within a range that would make it not too hot and not too cold for liquid water to exist on its surface.
Unlike Earth, Proxima b orbits remarkably close to its host star, Proxima Centauri. Only 4.4 million miles separate the two, making Proxima b more than 20 times closer to its star than Earth is to our sun. At such a distance Earth would be hot, hot, hot, but as a red dwarf star, Proxima Centauri is a much cooler and smaller star than our sun. And with such a tight orbit, Proxima b takes just 11.2 days to revolve around its star.
Of all the stars in the sky, Proxima Centauri is the star closest to Earth, other than the sun.
"The exoplanet couldn't have been closer if it tried," says Lisa Kaltenegger, director of the Carl Sagan Institute at Cornell University, who was not part of the study. This means that the potentially habitable planet is practically on our cosmic doorstep.
Is Proxima b habitable?
Proxima b sits within the habitable zone of its star – an enticing clue that it could indeed host life. But it takes a lot more than temperatures conducive to liquid water to make a place livable.
One challenge is that planets that orbit so closely to a red dwarf star are thought to be tidally locked. That means that one side of the planet always faces its parent star, while the other is perpetually dark. The light side would be constantly bombarded by radiation from the star, while the dark side would freeze.
But that doesn't have to rule out the chance that Proxima b is habitable, says David Charbonneau, an astrophysicist at the Harvard-Smithsonian Center for Astrophysics who was not part of the study.
"Last night when the Sun set in Boston, the temperature did NOT fall to absolute zero," he writes in an email to The Christian Science Monitor. "Any planet with a decent ocean and atmosphere has a means to redistribute energy."
The atmosphere could buffer the sunny side of the planet, while winds may help carry the star's radiation to warm up the dark side.
"Our job now is to try to observe this planet to see what its atmosphere is like and that will tell us more about whether or not it's habitable," says Zachory Berta-Thompson, an exoplanet astronomer at the University of Colorado, Boulder, who was not part of the study, tells the Monitor in a phone interview.
But tidal locking isn't Proxima b's only challenge to habitability.
Proxima Centauri is a flare star, which means that it blasts the exoplanet with huge amounts of X-ray and ultraviolet (UV) radiation every 10 to 30 hours, explains Dr. Kaltenegger in a phone interview with the Monitor.
Even if Proxima b has an atmosphere, it would likely be thin, she says, from being stripped away by this bombardment. So there would likely be extreme conditions down on the surface of the planet.
"By chance, we actually already thought about this before we knew about Proxima b," she says. Kaltenegger and her colleagues explain in their own new paper that some organisms on Earth have a mechanism to protect themselves from a bombardment of UV light: some species of corals.
These corals actually downshift the high energy radiation to a wavelength that doesn't harm them: visible light, she explains. As a result, these corals glow.
And if organisms that bioluminesce like that dominate a planet's biosphere, the whole planet could theoretically light up when a stellar flare hits it – and that distinctive glow might be detectable with the next generation of ground-based telescopes being built right now, like the European Extremely Large Telescope.
Studying Proxima b
Evidence for Proxima b's existence comes from studying its star. When astronomers were peering at Proxima Centauri, they noticed that it was wobbling at regular intervals as if it was being tugged by something.
The star was alternately approaching Earth at about 3 miles an hour and then moving away from our planet at the same speed. And this wobble occurred on consistent time periods of 11.2 days.
Upon closer examination of the tiny Doppler shifts, measured over the course of a decade by two European Southern Observatory telescopes, astronomers realized that they were seeing the effect of an orbiting exoplanet. And Proxima b was born.
Typically, exoplanets are detected by measuring the dip in the light coming from a star when an orbiting body transits across the disc of the star. But astronomers haven't found any evidence that Proxima b transits Proxima Centauri yet.
Studying a transiting exoplanet can help astronomers learn more about its exact mass and provide a direct route to study its atmosphere, says Dr. Berta-Thompson. But even if Proxima b doesn't transit, the next generation of telescopes will likely give scientists the capability to observe the planet directly, particularly because it is so close to us.
So why don't we just rocket off to Proxima b ourselves? In the scheme of our universe, 4.25 light-years is not very far, but with our current space travel technologies, it would still take thousands of years for a human expedition to get there.
"It's only human to wonder about traveling there some day, but for our generation the opportunity is to study this planet telescopically," Dr. Charbonneau tells the Monitor in an email. So for now, "our telescopes are our spaceships, allowing us to know the conditions on distant worlds without the benefit of a direct visit."
How special are Earth-like planets?
Charbonneau says he's very excited about this discovery, but not surprised. That's because his own research suggests that 1 in every 4 red dwarf stars host an Earth-sized planet in the habitable zone. So planets like Proxima b are probably pretty common, and it's not surprising to find one orbiting our nearest neighboring star.
"We don't know yet whether there's life out there in the universe," Berta-Thompson says, "but we're finally getting to know our stellar neighbors in the galaxy and learning really cool things about them."
And although Proxima b might not check all the boxes for habitability as we know it on Earth, we may yet find an extreme form of life there, says Dr. Angerhausen. "Life found ways to cover a lot of Earth," including in some very harsh conditions, he writes in an email to the Monitor. "Maybe there are scientists sitting on [Proxima b] that think life on Earth is impossible because we have that weird 24-hour cycle and seasons…"