Stargazing: 'Exomoons' similar to our own Moon could be discovered orbiting extrasolar planets (like this one, pictured), by watching for telltale peaks in the light emitted by their stars.

Credit: NASA

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SYDNEY: Moons similar to our own could be discovered in distant solar systems by watching the stars for telltale blips of light, says a Korean researcher.

So far, more than 300 extrasolar planets have been discovered, but as yet no moons have been found orbiting those planets. Now, Cheongho Han from Chungbuk National University in Chongju, South Korea, suggests gravitational 'microlensing' could reveal the presence of large moons in other solar systems.

In the past, this technique has been used to detect seven extrasolar planets, including some of near Earth-size. Hans, who published his findings in the Astrophysical Journal this week, thinks it could also reveal the presence of large moons in other solar systems. His research mathematically models how far a moon could be from its planet and still be visible to observers on Earth.

Bending light

"With the increasing number of discovered extrasolar planets, the existence of moons and their characteristics in these exoplanets emerge as new questions," said Han.

In microlensing, a star's light is bent by the gravity of a star in between the source star and an observer. This middle star acts as a lens, effectively 'magnifying' the source star's light. Light waves that would normally spread out to the left and right of the source star are bent by the lensing star's gravity, and we see the source star momentarily brighten.

If the lens star has a planet, it creates a second peak of light. The addition of a moon around the planet should cause a third, even smaller peak in the source star's light.

Microlensing is particularly suitable for finding low-mass planets. It has revealed the smallest extrasolar planets found so far, and is a better bet for finding moons than other methods such as observing the transit of a planet across its star, said Han.

The microlensing method is also more sensitive for detecting the moons of remote stars - the target stars of transit searches need to be bright, which means only nearby stars are candidates. Microlensing searches tend to be made where many stars clump together, such as the centre of the Milky Way Galaxy.

"Microlensing searches are sensitive to stars anywhere along the line of sight toward the galactic bulge. Therefore, the microlensing method can provide a sample of extrasolar moons distributed throughout the galaxy," said Han.

Firming up an idea

Australian astrophysicist Brad Carter, from the University of Southern Queensland said that while Han's research "firms up the idea" of detecting moons using microlensing, the research "isn't groundbreaking".

"This latest paper is one in a series of conceptual papers on this technique," said Carter. He said Han's research gives astronomers some idea of what they'll see if a moon is at the right distance from its planet.

"What you see when you find a moon is an extra blip on the light curve. When a star that has a planet passes in front of another star, it creates a second peak of light - a smaller rise and fall of light. When you are looking for a moon around an exoplanet you're looking for a third peak superimposed on the second."

But he added that gravitational microlensing relied on a sample size of millions of stars. "You need a lot of data to catch an extrasolar planet lensing event. What is extraordinary for us is that microlensing is sensitive to Earth-mass planets. But it is a one-off in terms of getting the alignment of stars and planets such that these blips of light are visible from Earth," Carter said.