Exomoons have long been predicted to exist, offering the tantalizing possibility that some of them may be habitable worlds. However, there has been no confirmed discovery of an exomoon yet. (Agencies)
Astronomers discovered the new moon and its exoplanet adrift in the cosmos, far from any star.
While most of the 1,000 or so exoplanets discovered to date were found by analysing changes in the light of their star, a select few have been seen using a technique called gravitational microlensing.
When an object passes in front of a distant star as seen from Earth, the object's gravity bends the light from the background star, focusing it like a lens - and making the star temporarily appear brighter if observed from a particular angle.
David Bennett of the University of Notre Dame, Indiana, and colleagues said they spotted a microlensing event in 2011, using a number of telescopes around the world.
First they saw the distant star's light amplified to 70 times its normal brightness. An hour later came a second, smaller increase in brightness.
That suggests that a large object passed in front of the star, followed by a smaller one. However, it is unclear whether these two objects are a planet and its moon as the team came up with two possible scenarios that fit the microlensing data.
In the first case, the pair of objects is relatively near to our solar system, at a distance of about 1,800 light years, and consists of a planet around four times the mass of Jupiter and a moon about half the mass of Earth - and thus many times more massive than our Moon. If this is true, then the team has discovered the first exomoon.
However, in the other scenario, the pair of objects is much further away and consists of a very small star or a failed star known as a brown dwarf, orbited by a Neptune-mass planet.
Exomoons have long been predicted to exist, offering the tantalizing possibility that some of them may be habitable worlds. However, there has been no confirmed discovery of an exomoon yet.