Astronomers have discovered an Earth-sized planet orbiting the star that is nearest to our own solar system – a tantalizingly close find that presents an ideal target for further study and an attractive destination for a proposed interstellar mission many years from now.
Measurements suggest the newfound planet is a rocky world with at least 1.3-times Earth's mass. Based on its orbital characteristics, it could maintain water at its surface in liquid form, a precondition for life as we know it – providing it has an atmosphere.
"Whether there is water or not we do not know. That entirely depends on the history, on the formation of the planet," said Ansgar Reiners, an astrophysicist at the University of Göttingen in Germany and a co-discoverer of the planet.
The star that the planet circles is Proxima Centauri, part of the Alpha Centauri system, a group of three stars that look like one bright star to the unaided eye and that together are the Sun's nearest stellar neighbours. Of the three, Proxima is by far the smallest and coolest. It is a red dwarf star that puts out only a tiny fraction of the Sun's energy.
Astronomers have previously detected hints of something in orbit around Proxima but have been unable to confirm the existence of a planet until now. The new find emerged when researchers combined recent data with previous studies conducted at the European Southern Observatory in Chile.
The planet, dubbed Proxima b, is too close to its star to be seen separately. Instead, astronomers discerned its existence by measuring the slight wobble in the star's motion as the orbiting planet's gravity tugs it back and forth with an 11-day cycle.
"Statistically there's no doubt that it's a very periodic signal," said Guillem Anglada-Escudé, an astronomer at the Queen Mary University of London and lead author of the investigation, published in the journal Nature. Astronomers then ruled out the possibility that the signal could be the result of internal fluctuations in the star.
In our solar system, any planet with an 11-day orbit would be baked to a cinder by the Sun. But because its star is so much smaller and dimmer, Proxima b receives only about 65 per cent of the solar energy that Earth does. Without an atmosphere, its average surface temperature would be about -40° Celsius. But if an atmosphere is present, its insulating effects could raise the temperature enough to allow for liquid water, the discoverers note.
But, in many ways, Proxima b must be far different from our own world.
Because it is so close to its star, the planet is bathed in high-energy radiation about 100 times stronger than Earth experiences. Its spin is likely to be locked, so that one hemisphere is permanently illuminated while the other stuck in an eternal night, creating huge temperature extremes.
These and other factors make prospects for life on such a planet highly uncertain, notes Sara Seager, an astrophysicist who specializes in exoplanets at the Massachusetts Institute of Technology. "The habitability of planets like Proxima b – in the sense of sustaining an atmosphere and liquid water on its surface – is a matter of intense debate," Dr. Seager said.
The good news, she added, is that the planet is so close to our solar system. If its orbit is oriented in a way that causes it to regularly cross in front of its star, astronomers can use such crossings to glean additional information, including the presence of an atmosphere. The crossings should be detectable by the Transiting Exoplanet Survey Satellite, a NASA mission that Dr. Seager is helping to lead and that is slated for launch in 2017.
Even if Proxima b does not cross in front of its star, it may be observable directly by the next generation of giant telescopes currently scheduled for completion a decade or so from now.
And then there's the more tantalizing possibility of a future visit to Proxima b by a robot probe. Such a voyage would take tens of thousands of years using current space technology. But earlier this year, the Russian venture capitalist Yuri Milner announced the Breakthrough Starshot project, endorsed by cosmologist Stephen Hawking.
The project aims to demonstrate that a computer chip-sized spacecraft attached to a thin sail-like membrane could be pushed by a laser beam and accelerated to 20 per cent the speed of light. If the technology proves feasible, such a tiny craft could reach Proxima b after a flight of two to three decades.