Astronomers uncover moon orbiting a gas-giant planet 8,000 light-years away

An illustration shows a moon orbiting around a planet as the pair cross the face of their host star

An illustration shows a moon orbiting around a planet as the pair cross the face of their host star

In a stellar system 8000 light-years from Earth, astronomers have found the first compelling evidence for a Neptune-sized moon orbiting a Jupiter-sized planet.

It had all the right numbers: It was several times bigger than Jupiter, orbited a host star that is similar in mass to the sun, and is located at the same distance as the earth from sun.

Moons are certainly plentiful in our solar system - the four giant planets host over 200 moons between them - so it is entirely plausible that many (or even most) of the now known exoplanets - nearly 4,000 - should harbour exomoons. We won't know for sure until researchers can do more tests, of course, but even so, what we do have is very promising.

Astronomers Alex Teachey and David Kipping set out to hunt for exomoons using the Kepler space telescope, and analysed the data of 284 exoplanets discovered by Kepler. It orbits about 1.8 million miles from its Jupiter-size planet.

NASA's Hubble and Kepler space telescopes have uncovered what could be the first moon outside our solar system ever found.

"It was a shocking moment to see that light curve, my heart started beating a little faster and I just kept looking at that signature", said lead author Teachey, NSF Graduate Fellow in astronomy at Columbia.

Only one planet held promise for hosting a moon, one around the star known as Kepler-1625, which is about the size of our sun but older. With his help, they found a second, much smaller hole in the brightness of the star through 3.5 hours after the transit of the planet.

Teachey and Kipping relied on the "transit" method, which is already used by researchers to discover almost 4,000 planets outside our solar system, called exoplanets.

All of this makes sense if a huge Neptune-sized moon is orbiting Kepler 1625b. That behavior is common with a planet and moon orbiting a common center of gravity, which causes the planet to wobble from its previously-expected location. And the third is moons forming from the disc materials that created the planets in the early days of the solar system.

Only two planets in our solar system - Mercury and Venus - don't have moons, Kipping said.




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

Unfortunately, the scientists' time on Hubble ended before they could completely observe the second transit.

Kipping said that's how the Earth and moon would appear from far away.

Exomoons are hard to find because they are smaller than their companion planet and so their transit signal is weak.

The potential moon would be considerably larger than Earth - about the size of Neptune or Uranus.

The planet was observed by Hubble before and during its 19-hour-long transit.

The anomalies were intriguing enough to warrant 40 hours of observation time using the Hubble Space Telescope. The problem is only large planets that orbit close to stars are detectable, and those types of planets typically don't have moons.

Although the planet and its possible moon are within the habitable zone of their star, both are considered to be gas giants and "unsuitable for life as we know it", Kipping said.

"We can expect to see really tiny moons with Webb", Teachey said.

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