For only the second time, astronomers have detected what appears to be a moon orbiting a planet in another solar system.

Just like the first time, this one has traits suggesting that such moons may differ greatly from those populating our solar system.

Data obtained by Nasa's Kepler space telescope before it was retired in 2018 indicated the presence of a moon 2.6 times the diameter of Earth

orbiting a Jupiter-sized gas giant about 5,700 light-years away from our solar system in the direction of the Cygnus and Lyra constellations, scientists said on Thursday.

A light-year is the distance light travels in a year, 5.9 trillion miles (9.5 trillion km).

This moon's diameter would make it larger than any of the roughly 220 ones known to be orbiting planets in our solar system and more than nine times the diameter of Earth's moon.

"We don't know the mass or indeed composition. It could be a rocky core with a light fluffy envelope or a thick atmosphere all the way down to some high-density core," said Columbia University

Our solar system's moons all are rocky or icy objects.

Close to 5,000 planets beyond our solar system, or exoplanets, have been identified, compared to only two such moons, called exomoons.

That is not because moons are thought to be any scarcer in other solar systems but because planets tend to be larger and therefore easier to find, the researchers said.