Scientists use space telescope to step up hunt for first ‘exomoon’

There are eight planets in our solar system, which formed out of the disc of dust and gas left over from the birth of our sun. It is thought that most stars and planets form in a similar way, prompting a long-held belief among astronomers that other stars are also very likely to have planets.

It took until 1992, however, for the first so-called “exoplanet” to be spotted. Since then a further 5,600 have been detected by the Hubble, Kepler, Tess and James Webb space telescopes.

An array of weird and wonderful planets — small and large, rocky and gaseous, hot and cold, arid and watery — have been found in the Milky Way, with one planet even spotted in orbit around a star in another galaxy.

There are at least 288 moons in orbit around the planets of our solar system, but to date no moons have been confirmed in orbit around any exoplanets.

The process most commonly used to detect an exoplanet is called the “transit method”. This is where telescopes can detect a slight dip in the brightness of a star as a planet passes across its face like a tiny eclipse.

Ben Cassese, a graduate student at Columbia University in the United States who is leading the study with David Kipping, an exoplanets expert, said they were now “pushing the limits” of the James Webb Space Telescope in trying to detect a much smaller second dip in a star’s brightness caused by a moon trailing along next to a planet.

“[The planet Kepler-167e] is one of the most favourable targets to have a Galilean-like moon system,” he said, referring to the four largest moons of Jupiter — Ganymede, Io, Europa and Callisto — that were first spotted by Galileo in 1610.

“It’s one of the closest comparison planets we have to Jupiter, it’s almost exactly Jupiter’s radius and mass and almost the same temperature. And Jupiter has a whole bunch of moons and we think that planets like Jupiter will naturally form moons like that, with a disc of material around it that collapses into moons.”

The planet has a “nice wide orbit” of about 1,000 days and has no other planets in its vicinity. It is in orbit around its star, Kepler-167, which is about 75 per cent the size of our sun, at a distance about twice that of Earth’s orbit. It is very cold, with an average temperature of less than -73C.

There are three much smaller planets, probably rocky and between 1.2 and 1.7 times the size of the Earth, in very close and very hot orbits around the star, but they are not near enough to Kepler-167e to risk their gravity scattering any moons that it may have.

The heavens will have to play ball and align nicely for astronomers to be certain a moon is present, Cassese said. He will need a moon to appear at a good distance either side of the planet to create a discrete signal that the telescope can detect and must hope that it is not obscured by its planet or by one of the smaller planets passing by at the same time. But with the right conditions he thinks there is an “80-something per cent chance of detecting it”.

“Finding exomoons would give a lot of context to the formation of our own solar system,” he explained. “If we search around these Jupiter-like planets and none of them have moons, that means there’s something weird about Jupiter and, by extension, the rest of our solar system and maybe we’d need to rethink our models of planet formation.”

He added that understanding more about exomoons would help in the search for alien life.

If astronomers detect “biosignatures” — possible signs of life — in the atmosphere of a distant world, they will need to be sure whether these are all coming from a single planet or whether some might be coming from a nearby moon.

The James Webb telescope, which launched on Christmas Day 2021 and now sits at a fixed spot a million miles from Earth, will also feed back information about Kepler-167e’s atmosphere and whether it is perfectly spherical or slightly flattened at the poles, known as “oblate”.

Two possible candidates for exomoons have previously been spotted, around planets called Kepler-1625b and Kepler-1708b, but both are deemed far from conclusive. Cassese will now spend months poring through the telescope’s data to find the first incontrovertible evidence.

“The fact that you can, in theory, detect something this small is already amazing to begin with,” Cassese said. “It’s a good first step and we can worry about characterising them later.”