To the surprise of all, the first discovered extraterrestrial moon is large and gaseous

WASHINGTON – Astronomers have spotted what appears to be the first moon detected outside our solar system, a vast gaseous world the size of Neptune that does not resemble any other known moon and orbits a much more massive gaseous planet than Jupiter.

The discovery, recently detailed by the researchers, was a surprise and not because it showed that moons existed elsewhere – they felt it was only a matter of time to find one in another star system. Instead, they were astonished by the difference of this moon from the 180 we know in our solar system.

"It's huge and strange by the standards of the solar system," said David Kipping, professor of astronomy at Columbia University, about the moon, known as the exomoon, because it's outside of our solar system.

The moons of our solar system are rocky or icy objects. The newly discovered exomoon and the planet it orbits, estimated several times the mass of the largest planet in our solar system, Jupiter, are both gaseous and unexpected. They are located 8000 light-years from Earth.

Alex Teachey, co-author of the Columbia study, said their observations with the Hubble Space Telescope and NASA's Kepler Space Telescope were the first clear evidence of an exomoon, but that Hubble's subsequent observations in May this year had to confirm these conclusions.

The exomoon is exponentially larger than the largest moon in our solar system. The moon of Jupiter, Ganymede, has a diameter of about 5 260 km. It is estimated that the exomoon is about the size of Neptune, the smallest of the four gas planets in our solar system, with a diameter of about 49,000 km.

The exomoon and its planet revolve around Kepler-1625, a star whose temperature is similar to that of our sun, but about 70% larger. The exomoon gravitates to about 3 million kilometers from its planet. The mass of the exomoon represents about 1.5% of that of its planet.

Kipping and Teachey used the "transit" method already used by researchers to discover nearly 4,000 planets outside our solar system, called exoplanets. They observed a drop in brightness of Kepler-1625b when the planet, then the exomoon passed by.

The size and gaseous composition of the exomoon challenge current theories on the formation of the moon.

"You can argue that because larger objects are easier to detect than smaller ones, it is actually the lowest solution, so it's not totally unexpected that detection the first exomoon is among the greatest possible, "said Teachey.

The results were published in the journal Science Advances.