Astronomers have spotted what could be the first known moon to orbit an exoplanet. Earlier observations last year with NASA’s Kepler telescope had hinted at its existence, but with new and better data from the powerful Hubble Space Telescope, astronomers are now more confident that the exomoon, a moon orbiting a planet outside the solar system, is real. Evidence suggests that it is as big as Neptune, orbiting a gas-giant planet roughly the size of Jupiter (called Kepler-1625b), which, in turn, orbits a star called Kepler-1625. Researchers say that the moon hypothesis is tentative and must be confirmed by follow-up Hubble observations. The moon has been designated as Kepler-1625b-i. The new results are presented in the journal “Science Advances”. Exomoons cannot be imaged directly. Their presence is inferred by their transit in front of a star, momentarily dimming its light, a technique used to detect many of the exoplanets catalogued to date. Being smaller than their companion exoplanets, exomoons are tougher to detect. Searching for exomoons, Mr Alex Teachey and Mr David Kipping, astronomers at Columbia University, analysed data from 284 Kepler-discovered exoplanets with relatively wide orbits—longer than the 30-day orbit period—around their host star. They found a transit signature with intriguing anomalies in one instance in planet Kepler-1625b, suggesting the presence of a moon. The researchers observed the planet for 40 hours using Hubble to study the planet intensively, also using the transit method, obtaining more precise data on the dips of light. They monitored the planet before and during its 19-hour transit across the parent star. But a second, and much smaller, decrease in the star’s brightness was also seen by Hubble about 3.5 hours later. They determined that this small dip was consistent with a gravitationally bound moon trailing the planet. In addition to the second dip, Hubble provided supporting evidence for the moon hypothesis by finding the planet transit occurring more than an hour earlier than predicted. This is consistent with a planet and moon orbiting a common centre of gravity that would cause the planet to wobble from its predicted location, much the way the earth wobbles as its moon orbits it. “A companion moon is the simplest and most natural explanation for the second dip in the light curve and the orbit-timing deviation,” Mr Kipping explained. Large moons do not exist in our own solar system.The researchers say this may yield new insights into the development of planetary systems. Kepler-1625b-i is estimated to be only 1.5 per cent the mass of Kepler-1625b, which itself is several times the mass of Jupiter. This mass-ratio is similar to the earth-moon system. In the case of earth-moon and Pluto-Charon systems, the moons are thought to be created through dust left over after rocky planetary collisions. However, Kepler-1625b and its possible satellite are gaseous. So the moon may have formed through a different process. Kepler-1925b orbits its parent star at a distance similar to the sun-earth distance. However, since both are gaseous, they would be unsuitable for life as we know it.