An extrasolar planet may have been detected in nearby Andromeda (Image: Bill Schoening, Vanessa Harvey/REU program/NOAO/AURA/NSF)

We could find planets in other galaxies using today’s technology, according to a new simulation. The study gives credence to a tentative detection of a planet in Andromeda, our nearest large galactic neighbour.

The idea is to use gravitational microlensing, in which a distant source star is briefly magnified by the gravity of an object passing in front of it. This technique has already found several planets in our galaxy, out to distances of thousands of light years.

Extending the method from thousands to millions of light years won’t be easy, says Philippe Jetzer of the University of Zürich in Switzerland, but it should be possible.


Jetzer and five colleagues simulated microlensing from the Andomeda galaxy, which is more than 2 million light years away. They started by populating Andromeda with planets, assuming that they will have the same range of sizes and orbits as known exoplanets in our own galaxy. “These are reasonable guesses, probably right within a factor of two,” Jetzer told New Scientist.

Uneven pattern

They then calculated how these planets might reveal themselves. When a lensing star system drifts into our line of sight, it bends light from the background star, which appears to brighten and then fade again smoothly over a few weeks or months. If the lensing star has a planet in tow, then the planet’s additional gravity can produce an uneven pattern of brightening and fading, or even add a brief flare lasting hours or days.

Looking at Andromeda, telescopes won’t be able to pick out individual background stars, but instead they could see a similar effect in the brightness of individual pixels – which may represent several stars – when a big star is lensed.

How often you see a planet-revealing event depends on how powerful your telescope is. A 4-metre instrument watching Andromeda for 9 months in total might pick up one or two planets – if the assumptions behind the simulation are right.

While it may be difficult to get that much time on a large telescope soon, smaller-scale observations have already been done and will continue. Next year, a group at the University Observatory of Munich will begin looking for microlensing events in Andromeda using a new 2-metre telescope on Mount Wendelstein in Germany.

Possible planet

They might just get lucky and spot the first planet in another galaxy…unless that has already been done.

Jetzer was part of a group that spotted an uneven microlensing event from Andromeda using the Isaac Newton Telescope on the Spanish island of La Palma.

When they reported it in 2004, they suggested that the lens could be a binary star, but according to the new simulation, the lensing pattern fits a star with a smaller companion weighing just 6 or 7 times the mass of Jupiter.

“It plausibly could be a planet,” says Andrew Gould of Ohio State University, who was not part of the team. The matter will probably not be settled, since lensing events occur randomly and do not repeat themselves, and for the foreseeable future, other techniques will be unable to detect planets beyond the Milky Way.

Journal reference: Monthly Notices of the Royal Astronomical Society (forthcoming)