Just as the first alien planets were unexpected "hot Jupiters" that revolved close to their stars, this first reported "exomoon" is also odd: a Neptune-size megamoon, some 8000 light-years away, that looms over a giant planet, twice as large in the sky as Earth's moon.
But they say that further observations are needed to understand the distant planetary system.
"We've tried our best to rule out other possibilities such as spacecraft anomalies, other planets in the system or stellar activity, but we're unable to find any other single hypothesis which can explain all of the data we have", co-author David Kipping, an astronomer at Columbia University in NY, told reporters earlier this week.
To date, astronomers have discovered more than 3,500 exoplanets - worlds orbiting stars other than the Sun. In the case of Kepler-1625b, however, 1.5 per cent of its mass makes its exomoon a giant, roughly the same mass as the planet Neptune!
In addition to this dip in light, Hubble provided supporting evidence for the moon hypothesis by measuring that the planet began its transit 1.25 hours earlier than predicted.
This 19-hour event, known as a transit, blocked out some of the light coming from the star, which lies at a distance of 8,000 light-years from Earth.
Kipping and Teachey discovered it among 300 exoplanets in Kepler's catalogue, all of which produce predictable dips in starlight that occur as an orbiting body passes in front of its sun - a phenomenon called a "transit".
The first of these signals was a dip in the parent star's brightness as the exomoon passed in front.
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One of the planets, Kepler 1625b, caught their attention because of "little deviations and wobbles in the light curve", said Kipping.
The light of a star known as Kepler-1625 dims slightly as a large planet - Kepler-1625b - moves across the star's disk as viewed from Earth.
In a paper published Wednesday in Science Advances magazine, researchers at Columbia University outline evidence supporting the existence of a moon orbiting the exoplanet Kepler-1625b, using data from the U.S. space agency NASA's Hubble Space Telescope.
"The first is that the planet appears to transit one and a quarter hours too early; that's indicative of something gravitationally tugging on the planet. Our analysis of the new light curve reveals two substantial anomalies", Kipping explained during a press briefing. The simulations closely matched the observed data.
Still, he and lead author Alex Teachey, also a Columbia astronomer, stressed that the observations don't constitute a definitive detection. "Unfortunately, the scheduled Hubble observations ended before the complete transit of the moon could be measured". David Kipping, assistant professor at Columbia University, has kept at it for nearly a decade.
Several promising exomoon candidates have cropped up in the past, only to be debunked as further data became available.
Teachey and his co-author David Kipping found the moon - named Kepler-1625b-i - by using the intrepid Hubble Space Telescope. And the tenuous nature of the supporting data, combined with the freaky properties of the system, give us reasons to take that skepticism seriously, even if a moon is still the best explanation for them. That latter number explains why the duo refers to the big candidate object as a satellite, Teachey said. No one, however, has ever conclusively found an alien moon. There are no indications of tidal capture among our Solar System's moons.
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