Exomoons: astronomers report first ever discovery

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Past year a team of researchers led by Columbia University astronomer Alex Teachey said they think they discovered a moon orbiting the planet Kepler-1625b, which is located 2.4 kiloparsecs (roughly 8,000 light-years) from our planet and surrounded by the constellation Cygnus. An ageing sun-like star in the constellation Cygnus is known to host a huge, gas giant planet the size of Jupiter, known as Kepler 1625b.

Alex Tichy and David Kipping of Columbia University studied 300 extrasolar planets discovered by the telescope "Kepler". "And if validated, the planet-moon system-a Jupiter with a Neptune-sized moon-would be a remarkable system with unanticipated properties, in many ways echoing the unexpected discovery of hot Jupiters in the early days of planet hunting". If confirmed, this would be the first discovery of a moon outside our Solar System. Its planet orbits Kepler-1625, a star 70% larger than the sun.

Although the object itself can not be seen, there are hints it exists, according to the researchers: The planet moves around its star in a way that indicates something else is pulling on it gravitationally, probably a moon.

To find an exoplanet, astronomers rely on spotting the tiniest dips of light as a planet passes in front of its host star (the transit method) - which themselves are only small pinpricks of light when viewed with even the largest telescopes.

To gather more precise data on the unusual system the researchers monitored in October 2017, during Kepler 1625b's next transit for 40 hours.

There are no moons anywhere near this size in our solar system.

A moon "is an excellent explanation" for these observations, he said.

"The first exomoon is obviously an extraordinary claim and it requires extraordinary evidence", Teachey said. Whether future observations confirm the existence of the Kepler-1625b moon, NASA's James Webb Space Telescope will be used to find candidate moons around other planets, with much greater detail than Kepler. All the planets in our solar system except Mercury and Venus have moons. "But we knew our job was to keep a level head and essentially assume it was bogus, testing every conceivable way in which the data could be tricking us". It could be rogue planet caught in the gravity of the Kepler 1625b or, like the moons of Jupiter, it coalesced out of gas, dust and other space debris.

Now the researchers say that what Hubble saw seems to confirm the idea that this planet has a moon. In this search, the Neptune-sized moon would have been among the easiest to detect because of its large size, scientists said.

Because their transit signals are weaker than those of planets and their positions change as they orbit their parent planets. However, both bodies are considered to be gaseous and, therefore, unsuitable for life as we know it.

The researchers note that in principle, this anomaly could be caused by the gravitational pull of a hypothetical second planet in the system, although Kepler found no evidence for additional planets around the star during its four-year mission. These observations have to be done from space; the rotation of Earth means that ground-based telescopes spin away from their targets before they can capture a whole event. They chose to look at exoplanets with the widest orbits, or those that take about 30 days to circle their stars.