The exoplanet zoo just got a truly spectacular addition. Astronomers have found evidence for an Earth-mass planet in our own backyard, about 4.4 light years away in the Alpha Centauri system, the star system nearest to our own.
The planet, which is at least 1.1 times the mass of the Earth, orbits Alpha Centauri B (α Cen B), one of two sun-like stars in the three-star system. With a year lasting just 3.2 days, the planet speeds around α Cen B in a scorching orbit, one that's far too close to be considered habitable. "If there are any inhabitants there, they're made of asbestos," SETI Institute astronomer Seth Shostak quipped in an interview with NPR.
Still, the discovery has astronomers excited because low-mass planets often seem to come in groups. It may be that there is an Earth-like planet just waiting to be found in α Cen B's 'habitable zone', with temperatures in just the right range to support the presence of liquid water and life as we know it.
That doesn't mean such a planet will be easy to find. This discovery was already pushing the limits of existing instruments. Xavier Dumusque, a Ph.D. student affiliated with the Geneva Observatory, and colleagues found the planet by scouring starlight coming from α Cen B using the High Accuracy Radial velocity Planet Searcher (HARPS) instrument, an ultra-precise spectrograph mounted on a 3.6-meter telescope at the La Silla Observatory in Chile. HARPS is designed to pick up tiny shifts in spectral lines that arise when planets tug on their host stars and cause them to move back and forth along the line of sight with Earth.
Finding the stellar wobbles created by the planet around α Cen B required detailed modeling of stellar activity. Although α Cen B is a relatively calm star, it still has weather that can mask the presence of planets. In the end, the team uncovered a signal that, according to an article accompanying the team's paper, is just one third as strong as α Cen B's stellar activity. The signal is puny compared to previous finds, corresponding to stellar motion of just 50 centimeters per second, or about as fast as a baby crawls, according to a press release put out by the European Southern Observatory. That's about 1% the speed generated by the tug of 51 Peg b, a planet about half the mass of Jupiter that was discovered in 1995 (it was the first to be found orbiting an ordinary star).
"The Geneva team has done something very difficult," says Debra Fischer, an astronomy professor at Yale University. "There is not a more exciting result for an individual star, even with the long line of spectacular results from the last 2 decades." Fischer has been conducting a search for planets around α Cen B using a new detector on the 1.5-meter telescope at the Cerro Tololo Inter-American Observatory in Chile and says her team is well poised to follow-up on the HARPS measurements.
Finding habitable planets around α Cen B could be trickier. At a press conference on Tuesday, Dumusque said that, at its current sensitivity, HARPS could turn up a planet in the habitable zone of α Cen B with a mass as low as four times the mass of Earth. Finding a true Earth twin, an Earth-mass planet on an Earth-like orbit, would require measuring stellar motions of just 9 centimeters per second, says Nature News.
There is some hope that we'll get there. Astronomers are testing out new laser-based calibration systems that could help boost the sensitivity of HARPS and other such instruments by a factor of 10 (although astronomers will still have to contend with stellar activity). And there are other ways to detect planets. If the alignment is right, we could potentially see α Cen B planets as they pass in front of the star in the course of their orbit, temporarily blocking out a fraction of the light that reaches Earth.
Either way, maybe it's time to start contemplating a mission to Alpha Centauri to see what there is to find. We'll probably want something a bit speedier than our interstellar pioneer, Voyager 1. If the spacecraft were headed in that direction, it would take more than 70,000 years to reach the system.
(Image: G. Hüdepohl/ESO)
Rachel Courtland, an unabashed astronomy aficionado, is a former senior associate editor at Spectrum. She now works in the editorial department at Nature. At Spectrum, she wrote about a variety of engineering efforts, including the quest for energy-producing fusion at the National Ignition Facility and the hunt for dark matter using an ultraquiet radio receiver. In 2014, she received a Neal Award for her feature on shrinking transistors and how the semiconductor industry talks about the challenge.