Such big moons are not present in our own solar system where 200 natural satellites have been indexed.
Using the NASA/ESA Hubble Space Telescope and older data from the Kepler Space Telescope two astronomers have found the first compelling evidence for a moon outside our own Solar System. "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".
"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", Columbia Professor David Kipping, the study's other author, told reporters during a teleconference in advance of the study's release.
Now the researchers say that what Hubble saw seems to confirm the idea that this planet has a moon.
Kipping has spent a decade working on the "exomoon hunt". As with most "first" discoveries, this first exomoon detection is not yet absolutely conclusive, as the signals are at the limit of what is now measurable, but I am hopeful that its existence will be confirmed with subsequent observations. "Including rocky exomoons in our search for life in space will greatly expand the places we can look".
However, the finding is both promising and intriguing.
The candidate moon is also a whopper - according to the astronomers' calculations, it's roughly the size of Neptune, and therefore also a gas body, orbiting its planet at a distance of about 3 million kilometres.
Indeed, the mass ratio is similar for our own Earth-moon system - 1.2 percent. When the moon isn't eclipsing or being eclipsed by its planet, it should block a small bit of additional light during the transit. They found no evidence for exomoons in the vast majority of the systems, but Kepler-1625 showed tantalising signs of the tiny dip in brightness that constitutes an exomoon signature. It's comparable to so-called hot Jupiters, gas giant exoplanets that are closer to their stars than Jupiter is to its own, and warmer. The researchers' investigations showed that the HST-recorded transit of Kepler-1625b occurred almost 80 minutes earlier than expected, a pattern suggesting the presence of transit timing variations, or TTVs, which are among the first proposed methods to confirm the presence of exomoons. The team then used the Hubble telescope to monitor the planet and detected a second dip in brightness. This catalog included 284 planets found by Kepler with wide orbits around their host stars.
The first hints that there may be an exomoon in the Kepler-1625 system came in a paper by the same authors in 2017. While this is an exciting discovery, it's still not confirmed - it will take more time on the Hubble Space Telescope to confirm this remarkable find.
"A moon is an excellent explanation to the data at hand", Kipping said.
Hubble was also able to measure that the planet began its transit earlier than expected, consistent with the "wobble" that occurs when a planet and moon orbit the same center of gravity. Our moon does the same to Earth.
The researchers identified 121 giant planets that have orbits within the habitable zones of their stars. A second slight drop in the "light curve" indicates the possible presence of a large moon. "It was a shocking moment to see that light curve".
"The search for life as we know it starts with water", Mayorga said.
"If we want to do moon hunting in the future, we will have to look at planets further [than one astronomical unit, or the distance between the sun and Earth]", Teachey said.
The researchers believe the star system to be 10 billion years old, which means it's had time to evolve.
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