Kipping and Teachey discovered it among 300 exoplanets in Kepler's catalogue, all producing predictable dips in starlight that occur as an orbiting body passes in front of its sun - a phenomenon called a "transit". They succeeded in observing another transit in late October 2017 - and saw two "substantial anomalies", according to Kipping. The Kepler outcomes were sufficient enough for the group to achieve 40 hours of time with Hubble to vehemently study the planet acquiring the data four times more accurate than that of Kepler. And they noticed weird deviations in the "light curve" generated by the 19-hour-long transit of Kepler-1625b, a planet about three times heftier than Jupiter that orbits a star about as massive as our own sun. Simply based on this example, exomoons should be abundant in the galaxy, yet evidence for them has remained elusive. "When we look for an Earth twin, I think one of the most obvious things you might ask is, 'Does it have a moon twin, ' because that seems to have a large influence", he notes. "It was definitely a shocking moment to see that light curve - my heart started beating a little faster and I just kept looking at that signature", David Kipping described his feelings. "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".
One of the authors behind the study - David Kipping from Columbia University in the USA - has been speculating about the possible existence of exomoons, and describing how they might be detected, for the last ten years.
Accepting that the data do indeed show evidence for an additional body co-orbiting the star with the planet Kepler-1425b, the question arises - is this really a moon? However, the star itself, as the team have ascertained from Kepler data, is so quiet they can't even detect its rotation.
The study, titled "Evidence for a Large Exomoon Orbiting Kepler-1625b", is freely available at the Science Advances website. Or perhaps, like Earth's moon, it is a product of its planet, formed in some catastrophic collision. If established by pursuing Hubble observations the discovery could throw light upon important signals about the progress of planetary systems and may originate specialists to frequent conjecture of how moons form around planets.
The gas giants in our own solar system have rocky and icy moons, but no examples of having gas-giant moons.
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But the research group also says there is still a possibility that they found a planet, not a satellite, so they say they will continue monitoring to confirm their findings. Other moons, such as Neptune's largest moon Triton, may have been captured from the Kuiper belt. Or maybe its origin story resembles that of the moons of Jupiter, which are thought to have coalesced from a ring of gas and dust that circled the planet.
This graphic depicts how the astronomers believe the Kepler-1625 system works, with the exomoon orbiting along a wide, highly inclined path around its huge parent world. Another dip could well be a moon - known as an exomoon outside our solar system. "It's exciting to see the hunt for the first exomoon continue, and with what would be a shockingly large moon", wrote Sara Seager, an astrophysicist at the Massachusetts Institute Technology in Cambridge who was not involved with the work, in an email. Jupiter's moon Ganymede has a diameter of about 5,260km. The challenge with these is that other planets in the same exosolar system can also cause transit-timing variations.
Artist's impression of the exoplanet and exomoon transiting the star.
Those observations could come from another go-round with Hubble, or they could come from NASA's James Webb Space Telescope, which is due for launch in 2021.