"If confirmed by follow-up Hubble observations, the finding could provide vital clues about the development of planetary systems and may cause experts to revisit theories of how moons form around planets", Kipping said.
But these extrasolar satellites had lingered at the limits of detection with current techniques. The team used the telescope with the aim of finding evidence to either validate or refute the existence of the first exomoon. They believe it's orbiting a gas giant.
Whether confirmed or not, the subject offers insight into how rare - or how common - our own solar system might be.
The potential first exomoon that the astronomers have observed is thought to orbit the exoplanet Kepler-1625b.
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To spot exoplanets, astronomers look at their host stars, and wait for a planet to pass between the starlight and observers on Earth.
"We saw little deviations and wobbles in the light curve that caught our attention", Kipping said, in the paper reported in the journal Science Advances. The scientists needed more time and data to confirm the discovery though, and they got it through the Hubble telescope. They watched the planet during its 19-hour transit. With his help, they found a second, much smaller depressions in the brightness of the star through 3.5 hours after the transit of the planet.
Dr Kipping said this was consistent with "a moon trailing the planet like a dog following its owner on a leash".
Unfortunately, the scientists' time on Hubble ended before they could completely observe the second transit.
After analysing their data, they found that the planet transit began 77.8 minutes earlier than predicted.
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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? In the case of earth and sun, for example, since both bodies have mass, both exert gravitational pull on each other.
One of the authors behind the study - David Kipping from Columbia University in the United States - has been speculating about the possible existence of exomoons, and describing how they might be detected, for the last ten years. My heart started beating a little faster, and I just kept looking at that signature. Its moon alone is estimated to be roughly the size and mass of Neptune, which has a radius four times that of Earth. This mass-ratio is similar to the one between Earth and the Moon. Other moons, such as Neptune's largest moon Triton, may have been captured from the Kuiper belt.
Neither Kepler 1625b nor its moon are likely to be habitable, the scientists said, because both are made mostly of gas. They also shift position with each orbit. Exomoons also shift position with each transit because the moon is orbiting the planet.
Hubble watched the star as the planet passed in front of it, blocking some of its light.
The Columbia astronomers directed the Hubble Space Telescope in October in an attempt to verify or rule out their findings. "We didn't think we had enough to claim a discovery", says Kipping. Many publications facing an uncertain future can no longer afford to fund it.
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