By Daniel CleryAug. 24, 2016 , 1:00 PM
Say hello to our newest, nearest neighbor.
After years of scrutinizing the closest star to Earth, a red dwarf known as Proxima Centauri, astronomers have finally found evidence for a planet, slightly bigger than Earth and well within the star’s habitable zone—the range of orbits in which liquid water could exist on its surface.
Researchers have already found hundreds of similarly sized planets, and many appear to be far better candidates for hosting life than the one around Proxima Centauri, known as Proxima b. But researchers are excited because the planet is just a stone’s throw away from Earth, cosmically speaking. At 4.25 light-years distant, Proxima b may be within reach of telescopes and techniques that could reveal more about its composition and atmosphere than that of any other exoplanet discovered to date. “This is the best planet we have [for study] right now, no question,” says planetary scientist Sara Seager of the Massachusetts Institute of Technology in Cambridge, who was not involved in the finding.
Rumors had been circulating for weeks about the discovery, announced on 24 August in Nature. The planet was found using the radial velocity method: Telescopes scrutinize a star’s light to see if its frequency is periodically stretched and squeezed by the Doppler effect as the star is tugged, first away and then toward us, by an orbiting planet. The task was especially difficult for Proxima Centauri, which tends to flare up dramatically, obscuring the planet-induced signal.
The exoplanet next door
Astronomers have discovered an Earth-like planet around Proxima Centauri, the closest star to the sun.
Source: NASA; Graphic: A. Cuadra/Science
Since 2000, a number of groups had found tantalizing hints of a planet around Proxima Centauri, but nothing conclusive. “We designed an experiment to confirm what we suspected was there,” says team leader Guillem Anglada-Escudé of Queen Mary University of London, who used the European Southern Observatory’s High Accuracy Radial velocity Planet Searcher (HARPS) spectrograph on a 3.6-meter telescope in Chile. Their Pale Red Dot campaign used HARPS to observe the star for 20 minutes every night for 60 nights in a row, beginning this past January. When combined with earlier observations, these new data clinched it: There was a planet at least 1.3 times the mass of Earth orbiting the star every 11.2 days. “This solidifies our view that rocky planets are everywhere, ubiquitous, around all kinds of stars,” says astrophysicist Nikku Madhusudhan of the University of Cambridge in the United Kingdom, who was not involved in the project.
A planet so close to its star—just 5% of the Earth-sun distance—might be expected to be a red-hot cinder, but Proxima Centauri is just one-eighth the mass of the sun and burns much less brightly. The total energy hitting Proxima b is only 65% of what Earth gets from the sun, so liquid water could easily exist there so long as the planet has some sort of atmosphere to trap heat.
Nevertheless, the planet isn’t particularly welcoming for life. It’s probably tidally locked, meaning that it always presents the same face to the star, resulting in permanent day and night sides with huge differences in temperature. Thanks to its closeness to Proxima Centauri, the planet also receives 100 times as much high energy radiation as Earth, in the form of ultraviolet light and x-rays. And during stellar flare-ups, Proxima b is blasted with high-energy particles, too—unless it has a protective magnetic field like Earth’s. Nevertheless, Anglada-Escudé says, there is “a reasonable range of parameters that could make it a comfortable planet.”
Proxima b’s discovery has sparked a race to see if it passes across the face of its star as viewed from Earth. Detecting such “transits” would pin down the planet’s size and mass, which would allow researchers to calculate its density. Knowing that would confirm the planet’s rocky nature and give hints about what those rocks are made of. And starlight passing through the planet’s atmosphere during a transit can reveal its composition. “That would be amazing, a dream come true,” Seager says.
But there is only a 1.5% chance that the orbit is aligned for scientists to witness a transit. The star’s tendency to flare isn’t making things easy, either. “The star is tricky,” says astronomer David Kipping of Columbia University. Kipping’s team studied Proxima Centauri in 2014 using Canada’s orbiting Microvariability & Oscillations of Stars Telescope. They are now reanalyzing their data based on the Pale Red Dot findings and hope to have an answer in a few weeks. “Even if we succeed, the follow up is not trivial,” Kipping says. “We’re not used to stars like this. Life will be harder.”
If Proxima b doesn’t transit, there is still hope. Next decade, a new generation of extremely large telescopes or space-based observatories may be able to blot out the light of Proxima Centauri and gather the planet’s light directly, producing our first direct look at another potentially habitable world.
And if sending a probe across the trillions of kilometers of space to visit a nearby star system ever becomes feasible, engineers have their first target.
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