In twelve years time the universe looks set to offer us an exceptionally rare opportunity to discover if there are any planets orbiting Alpha Centauri A, one of the two nearest Sun-like stars to our own.
General relativity predicts that gravity can distort the path of light. When correctly positioned, a heavy object bends light around it to act as a lens, giving us a better view of whatever is behind it. The nature of the bending also tells us a lot about the distorting object. One of the ways we have discovered planets around other stars is by watching the parent star act as a gravitational lens for objects towards the center of the galaxy, and noticed the extra blip provided by the planet itself.
Usually, however, even the closer star is a long way from Earth. Discovering their planets has been useful in building statistics on planetary size and locations, but interest in individual discoveries was more muted. One of the closest stars to Earth passing in front of an object that would allow it to act as a gravitational lens would be a different matter entirely.
Kervella’s team plotted the movements of both stars in the double system across our field of view. They made an extensive study of the background objects they will pass, and concluded that in May 2028 Alpha Centauri A will pass almost directly in front of the star 2MASS 14392160-6049528 (nicknamed S5). The timing is particularly fortuitous, since Alpha Centauri will be high in the sky at night, and the two stars will be widelyseparated, so that Alpha Centauri B’s light will not interfere.
The paths of Alpha Centauri A (orange) and B (red) across the sky, with the objects one or the other star will pass in front of marked. Kervella et al Astronomy and Astrophysics
Little is known about S5, a previously disregarded star whose magnitude of 7.8 makes it visible in binoculars or small telescopes. It is thought to be a red giant at a distance of several thousand light-years.
As seen from Earth, Alpha Centauri A is predicted to pass just 0.015 arc seconds from S5, creating a lensing effect sharp enough that we should be able to detect any large planets near it at the same time. Kervella estimates a 45 percent chance of the approach being close enough to produce an Einstein Ring, the most dramatic form of lensing.
There will be four previous times when one of Alpha Centauri’s stars pass in front of a background object, and many more before 2050. These are at least a thirty times fainter, greatly reducing the chance of detecting planets, although some may prove useful as test runs.
Trajectories of Centauri A (orange curve) and B (red curve) superimposed on an image Alpha Centauri. Kervella et al/Astronomy and Astrophysics
The recent discovery of a planet orbiting Proxima Centauri has put the focus on our nearest stellar neighbor, but the twin stars of Alpha Centauri A and B are almost as close, and bear a much greater resemblance to the Sun.
Kervella’s technique could be applied to other nearby stars. However, Alpha Centauri’s location near the galactic plane means that it will pass in front of stars that are bright enough to be useful unusually often. For planet-hunting astronomers, 2028 could be a true once in a lifetime opportunity.