By Jason Palmer
Science and technology reporter, BBC News
The first planet to be seen outside the Milky Way may lie in Andromeda
Astronomers believe they have seen hints of the first planet to be spotted outside of our galaxy.
Situated in the Andromeda galaxy, the planet appears to be about six times the mass of Jupiter.
The method hinges on gravitational lensing, whereby a nearer object can bend the light of a distant star when the two align with an observer.
The results will be published in Monthly Notices of the Royal Astronomical Society (MNRAS).
The team, made up of researchers from the National Institute of Nuclear Physics (INFN) in Italy and collaborators in Switzerland, Spain, and Russia, exploited a type of gravitational lensing called microlensing.
The effect of large, massive objects between an observer and a distant planet or star can cause distortion or multiple images as the intermediary object's gravity bends the passing light. Microlensing, by contrast, occurs when a less massive object lies in the middle.
There is a noticeable increase in the observed intensity of light coming from the aligned pair as the intermediate object focuses the distant one's light.
Because the effect depends on smaller objects that will be moving quickly relative to one another, microlensing events are fleeting, happening over the course of minutes or hours. Moreover, the mutual alignment of two small, far-flung objects with an observer on the Earth is exceptionally rare.
For that reason, dense collections of millions of stars, such as the Andromeda galaxy, are surveyed in order to detect them.
Francesco De Paolis of the INFN and his colleagues developed a computer model to determine the likelihood of detecting an exoplanet via a microlensing event in the Andromeda galaxy.
They modelled the "light curve", the variation in light that a microlensed star would exhibit if it were being orbited by a companion - another star or a planet.
Having determined the clues that a planet in Andromeda would show, they returned to a survey completed in 2004 by the Point-Agape collaboration of astronomers that showed an unusual light curve.
That event, the group says, matches up to its theory and can be attributed to a companion of a mass about six times that of Jupiter.
That suggests either a planet, or a small companion star such as a brown dwarf.
Stronger gravitational lensing results in multiple images
Unfortunately, given that microlensing events from a given pair of objects happen just once, astronomers cannot return to the planet candidate to confirm the idea.
But Dr De Paolis is encouraged by the possibility of detecting planets at such phenomenal distances.
"The interesting thing is that the technology is in place to truly see planets of Jupiter's mass and even less in other galaxies," he told BBC News. "It's an exceptional thing."
Armed with the new theory, the authors of the work are looking to secure time on a larger telescope to continue with their observations in the hope of finding more candidates.
With about 350 extra-solar planets already found in our galactic neighbourhood, Dr De Paolis said, it was likely that such candidates were abundant. The difficulty is in catching sight of one through a gravitational lens.
"It's not easy, obviously," he said. "The problem is that we don't know when a gravitational microlensing event is going to happen."