Exomoons: astronomers report first ever discovery

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The new results come from a concerted exomoon hunt Teachey and Kipping conducted using data from Kepler, which has discovered about 70 percent of the 3,800 known exoplanets to date. Researchers stumbled upon one example in Kepler 1625b, that had conspiring inconsistencies. 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.

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 United States space agency NASA's Hubble Space Telescope. 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.

This "exomoon" is not like any in our cosmic neighbourhood: it's the size of Neptune and orbits a planet the size of Jupiter - but with 10 times the mass. In some configurations, that tug will be in the direction of the planet's orbit around its star, causing the transit to occur sooner than expected. The pair would have a similar mass and radius ratio to the Earth and its moon but scaled up by a factor of 11.

"We saw little deviations and wobbles in the light curve that caught our attention", Kipping said. "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.

"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", said Dr Kipping, from Columbia University in NY.

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?

After analysing their data, they found that the planet transit began 77.8 minutes earlier than predicted.

Their findings appear in the journal Science Advances.

Kepler-1625b-i, as the candidate exomoon has been named, orbits an exoplanet called Kepler-1625b, which in turn orbits a yellow, Sun-like star called Kepler-1625.

Astronomers know of three major moon-formation mechanisms: There's gravitational capture (which appears to be the case with Neptune's biggest moon, Triton); powerful impacts (as happened with Earth's moon, which formed from material blasted into space by a long-ago collision); and the merging of material from a disk of material surrounding a newborn planet. With a radius that's around a third that of its parent planet, this object is unlike any moon of a giant planet in the solar system.

This still doesn't prove the existence of an exomoon around Kepler-1625b. 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.

It's an "extraordinary" find that "defies easy explanation" said co-author Alex Teachey - nothing like it exists in our own solar system.

He added: "We hope to re-observe the star again in the future to verify or reject the exomoon hypothesis".

While the researchers have yet to confirm the presence of an exomoon, they estimate that this moon would be about the size of Neptune. The James Cameron movie Avatartook place on Pandora, the moon of a gas giant in the Alpha Centauri star system. Another dip could well be a moon - known as an exomoon outside our solar system. She says moons are kind of the next frontier when it comes to understanding alien solar systems. "We can expect to see really tiny moons", Kipping said. That makes it plausible that the new exomoon is just as off-the-menu as the "hot Jupiters" that surprised early exoplanet hunters, says Stephen Kane, an astrophysicist at the University of California, Riverside, who was not part of the research team.

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