Year in review: A planet lurks around the star next door

Worlds in the Alpha Centauri system — the trio of stars closest to our sun — have been a staple of science fiction for decades. From Star Trek to Avatar, writers have dreamed up exotic landscapes (and inhabitants) for interstellar explorers to encounter. Now a planet around one of those stars is no longer fiction.

In August, breathless headlines heralded the discovery of a small, potentially habitable planet orbiting Proxima Centauri, a dim red dwarf star just 4.24 light-years away (SN: 9/17/16, p. 6). The planet, Proxima b, isn’t the first roughly Earth-mass planet discovered. It’s not the first seen in a star’s habitable zone, the region where temperatures are just right for liquid water. Nor is it the first found around a red dwarf, the most common type of star in the galaxy.

Proxima b got special attention for one reason: It’s the closest known exoplanet to us. “This is a game changer in exoplanetary science,” says Rory Barnes, an astronomer at the University of Washington in Seattle. “The fact that it’s so close means we have the opportunity to follow up on it better than any other planet discovered so far.”

When it comes to interstellar distances, “so close” is still incomprehensibly far. Proxima Centauri is a roughly 40 trillion kilometer jaunt. The fastest spacecraft to leave Earth — the New Horizons probe that zipped past Pluto in 2015 (the mission took the top spot in Science News’ 2015 Year in Review) — would need nearly 80,000 years to get there, traveling at its launch speed of roughly 58,000 kilometers per hour. But if Earthlings ever do venture beyond the solar system, Proxima Centauri is likely to be the first stop.

Astronomers found Proxima b by looking for a tiny wobble in the speed of its parent star, the sign of a gravitational tug from an orbiting planet. Observations from telescopes in Chile confirmed its existence. But not much is known about the planet. Researchers have determined, based on that tiny wobble, that the planet is at least 1.3 times as massive as Earth and it travels along an 11.2-day orbit. Its habitability is speculative. The planet basks in enough light to sustain liquid water, but other factors might foil the possibility of life. The planet’s climate depends strongly on the characteristics of its atmosphere — if it even has one. No one knows if Proxima b has a solid surface where water could pool and critters could crawl. No one knows the planet’s size. Even its mass is just a minimum estimate.

With a near-term voyage to Proxima b unlikely, the best chance to learn more about the planet might come from a transit, when a planet periodically slips between us and its sun. If the planet transits, it would block a smidgen of starlight from reaching Earth, subtly dimming Proxima Centauri. Astronomers could then estimate the size of Proxima b by measuring how much light the planet intercepts. A transit could also pin down the true mass by removing some ambiguity in the details of the planet’s orbit. By considering the size of the planet and its mass, researchers could calculate the planet’s density, revealing whether Proxima b is really a rocky world like Earth (as a lot of the initial news coverage jumped to proclaim) or a gassy one.

During a transit, a sliver of light would also have to pass through Proxima b’s atmosphere. Molecules in the atmosphere would block specific wavelengths of starlight, allowing astronomers to deduce the chemical makeup of the atmosphere and hunt for any by-products of living organisms.

The odds of a transit are slim — there’s just a 1.5 percent chance the orbit’s orientation is right — and one early investigation is not promising. The Canadian Microvariability and Oscillations of Stars satellite, MOST for short, monitored Proxima Centauri for hints of a transiting planet in May 2014 and May 2015 for about 44 days and turned up nothing, astronomer David Kipping of Columbia University and colleagues reported online at arXiv.org in September.

But researchers aren’t giving up yet. Several groups are keeping an eye on Proxima Centauri in the hopes of catching the telltale dip from a transit. “We should know whether there is a transit or not before the end of the year,” says Guillem Anglada-Escudé, the astronomer at Queen Mary University of London who led the Proxima b discovery team. Seeing the planet directly could also reveal new details, though that is probably beyond the capability of any current or planned telescopes.

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Even if astronomers can’t learn anything more about Proxima b, there will be plenty of other nearby worlds to study. Late in 2017, NASA plans to launch the Transiting Exoplanet Survey Satellite, or TESS, a telescope on a two-year mission to monitor about 200,000 stars for transiting exoplanets. Many of these stars are among the sun’s closest neighbors.

Astronomers estimate that TESS will turn up about 1,700 worlds in addition to the more than 3,500 already discovered. That haul could include more than 500 planets less than twice the size of Earth, roughly 50 of which might lie within the habitable zone of their stars, Peter Sullivan, now an engineer at NASA’s Jet Propulsion Laboratory in Pasadena, Calif., and colleagues reported in 2015 in the Astrophysical Journal. And because TESS will add only transiting planets, many will be ripe for follow-up investigations by future observatories such as the James Webb Space Telescope, which will view the cosmos in the infrared and is scheduled to launch in 2018 (SN: 4/30/16, p. 32).

More planets means more chances of finding life beyond the solar system. But finding aliens isn’t the only goal. Astronomers want “to understand how our solar system — and how Earth — fits into the universe,” Barnes says. “What’s special and what’s not special about our solar system?”

And interstellar travel will continue to capture imaginations. In the 1935 short story Proxima Centauri, published just 18 years after astronomers measured the distance to the star, Murray Leinster wrote of Earth’s first interstellar spaceship closing in on an imagined planet orbiting our stellar neighbor. In Leinster’s story, an alien race of mobile carnivorous plants ends up devouring most of the crew. Hopefully our first ambassadors to an exoplanet, whether Proxima b or elsewhere, will fare better.