LIVERPOOL, U.K.—Alpha Centauri, a three-star system just 4 light-years away that is the sun's nearest neighbor, ought to be a great place to look for Earth-like planets. But last week, at a meeting of the European Astronomical Society (EAS) here, astronomers lamented the way the system has thwarted discovery efforts so far—and announced new efforts to probe it. "It's very likely that there are planets," says Pierre Kervella of the Paris Observatory in Meudon, France, but the nature and positions of the stars complicate the search. "It's a little frustrating for planet searchers."
The system's two sunlike stars, Alpha Centauri A and B, orbit each other closely while Proxima Centauri, a tempestuous red dwarf, hangs onto the system tenuously in a much more distant orbit. In 2016, astronomers discovered an Earth-mass planet around Proxima Centauri, but the planet, blasted by radiation and fierce stellar winds, seems unlikely to be habitable. Astrobiologists think the other two stars are more likely to host temperate, Earth-like planets.
Maksym Lisogorskyi, an astronomer at the University of Hertfordshire in Hatfield, U.K., tried to find them with an instrument on the European Southern Observatory's (ESO's) 3.6-meter telescope in Chile. He and his colleagues looked for Doppler shifts in the spectral lines of the stars' light that would be caused if a planet tugged them back and forth. But Lisogorskyi told the meeting that the stars' surfaces are turbulent, and prone to flares that also jiggle the spectral lines, masking the subtle signals from any Earth-size planets. "The lines do all kinds of things," he says. Although Alpha Centauri has been a primary target for the planet-finding instrument since it was inaugurated in 2005, it has seen nothing so far.
Also hampering observations are the current positions of the two stars. As viewed from Earth, they are very close together, making them harder to study individually, Lily Zhao of Yale University told the meeting. More precise observations should become possible as their 80-year orbit carries them farther apart. In the meantime, Zhao and her colleagues have succeeded in ruling out the presence of giant planets around either star, based on a decade's worth of data from three instruments on different telescopes. "There are no Jupiters in the system, but there may be plenty of Earth-sized planets still to discover," she said.
In a binary system like Alpha Centauri the lack of giant planets in Jupiter-like orbits is no surprise, because the gravity of each star would tend to kick any such planets orbiting the other star out of the system, Kervella says. But he says that temperate planets in the habitable zone, closer in, would be immune to these perturbations. A chance to get a close look is coming soon: Kervella's team mapped out the system's trajectory and found that in a decade, Alpha Centauri A will pass in front of a more distant star and act as a gravitational lens, distorting the light of the star behind it. How the light from the distant star flickers and mutates over time will provide a wealth of information about any inner planets. By that time, ESO's 39-meter Extremely Large Telescope is expected to be operating and capable of observing the distortion in detail. "We will see all the planets, big and small," says astronomer Hans-Ulrich Käufl of ESO in Garching, Germany.
The privately funded Breakthrough Initiatives wants an even closer look. In 2016, the organization announced its Starshot program, a $100 million effort to equip a microchip-size spacecraft with a camera and light-sails. A blast of photons from a giant ground-based laser would accelerate the craft to 20% of the speed of light, allowing it to make the 4-light-year trip in 20 years. During a flyby that might last only seconds, it would snap close-ups of the Alpha Centauri planets—assuming they exist.
Finding targets for the Starshot is one aim of a Breakthrough-funded effort that ESO announced last year: adapting an existing instrument on the Very Large Telescope in Chile to directly image possible planets. Called VISIR, the instrument will be equipped with a coronagraph—a mask to block out the light of the star so that the much fainter planets can be seen. VISIR observes in the midinfrared, an advantage for imaging a temperate planet because the disparity in brightness between the dim planet and its brilliant parent star is smaller in this part of the spectrum. The ESO team is testing the hardware and hopes to start observing in mid-2019 with 100 hours of dedicated telescope time.
Others at the EAS meeting think the fastest and cheapest way to detect an Earth-like planet around either of the sunlike stars is with a space telescope. A privately backed organization called Project Blue is seeking $70 million to build and launch a 50-centimeter telescope that would stare at Alpha Centauri. Last year, the project raised $150,000 through crowdfunding to design the spacecraft. Franck Marchis, an astronomer at the SETI Institute in Mountain View, California, a partner with Project Blue, says such a telescope, outfitted with a coronagraph, would be able to obtain an image. "It's doable. The technology is there," Marchis said. "The goal is to image a pale blue dot."