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27 August 2016
What the new planet Proxima b says about life in the universe
The newly discovered planet Proxima b is about to change the focus of astronomy for decades to come — and maybe longer, if it reveals signs of life. The planet, some 25 trillion miles away from our own, is like a twin to Earth, but one separated at birth and living a very different kind of life.
The discovery, which was announced in the journal Nature, represents by far the closest habitable planet to Earth — near enough that humans could take pictures of it, if not with today’s telescopes, then with ones that will come online soon. If the planet is like Earth, these near-future telescopes could pick up hints of vegetation and sunlight glinting off the ocean.
And Proxima b looks a lot like Earth. It’s only a little bigger than our planet, according to scientists’ calculations, and it sustains about the same average temperature. The star it orbits, Proxima Centauri, is the sun’s closest neighbor. Significantly, astronomers believe the planet, like ours, is in the “habitable zone” of its star, which means liquid water could exist on its surface. And liquid water could mean life.
Studying this planet could reveal something important about the timeline of life in the universe, and whether we earthlings are early to the party. That’s because stars like Proxima Centauri are the future of the universe. Called red dwarfs, these make up the majority of stars in the galaxy, and they live about 1,000 times as long as our sun.
In a paper made public last month, Harvard astronomer Avi Loeb looked at the cosmic implications of life surrounding red dwarfs. Loeb calculated that if life is just as likely to form around these stars as sun-like ones, then the vast majority of life in the universe has yet to be born, and we earthlings are not just early, he said, but “premature.”
That’s because scientists believe eventually, all the raw materials for star formation will be used up and all the stars will die, leaving a dark universe of dust and black holes. For most of the trillions of years stretching between now and cosmic darkness, red dwarfs will be the only game in town.
Why wouldn’t red dwarfs be just as good at nurturing life as stars like the sun? There are many hazards. The main requirement scientists use for habitability is the potential for water to remain in liquid form, since all life on Earth requires it. But because red dwarfs are dim, their planets would have to orbit so close that they get “tidally locked,” with one side of the planet roasting in permanent daylight and the other trapped in perpetual darkness. This is what happened to Mercury, the planet closest to our sun.
Still, while planets like Mercury are hellish, tidal locking isn’t necessarily the end of the world, according to planetary scientist James Kasting of Penn State University. With enough air and water circulation, even a locked planet might sustain life.
The other, slightly more serious problem is that red dwarfs start out very hot when they are young and then cool off. As an analogy, University of Washington astronomer Victoria Meadows says, think of turning on a gas stove. That initial burst of high heat would likely boil off any water and destroy any atmosphere on nearby planets.
Still, that doesn’t mean life couldn’t exist on a planet orbiting a red dwarf. Planets can move around in the early stages of a forming solar system, she said, so it’s possible Proxima b was at a safer distance when the star was firing up.
A pair of papers on the potential habitability of Proxima b came out alongside the announcement of its discovery. One author, Villanova University astronomer Edward Guinan, said he was surprised life looks to be possible there. “When they first told me about it, I thought 'This doesn’t have a chance,'” he said. Not only is the star hot in its first billion years, it would bombard the planet with hundreds of times the intensity of X-rays and ultraviolet light that our planet gets from the sun. But scientific models showed that a protective magnetic field could preserve an ocean and an atmosphere on the planet.
The scientists say that until they look closer, they won’t know if Proxima b is a desert planet, barren as Mars, or perhaps overheated as Venus. But they can’t rule out the possibility that it’s lush, watery and blue. Scientists may learn more from the successor to the Hubble, called the James Webb Space Telescope, to be launched later this decade. Greater hopes are pinned on a couple of enormous ground-based telescopes, one planned for the high desert in Chile.
If there is life on Proxima b, it has a better future than Earth, which is already more than halfway through its habitable existence. The sun won’t actually die for another few billion years, but scientists say that a billion years from now it may brighten enough to sterilize our poor planet. Proxima b’s star is just a child, by contrast, with trillions of years stretching into its future. If the planet turns out to be a nice place, said Harvard’s Loeb, we may end up moving there.