A moon the size of NEPTUNE that orbits a giant planet 8,000 light-years away


Astronomers have found evidence of the first moon to exist beyond our solar system.

The gargantuan ‘exomoon’ is similar in size to the gas giant Neptune and orbits a planet 8,000 light years away called Kepler-1625b.  

More than 3,000 exoplanets – worlds orbiting stars other than the sun – have been found so far but no ‘exomoons’ have ever been confirmed.


‘This would be the first case of detecting a moon outside our solar system,’ said  Dr David Kipping, an assistant professor of astronomy at Columbia University. 

The researchers analysed data from 284 planets discovered by the Kepler telescope that orbit their host star in at least 30 days.

NASA’s Kepler space observatory launched in 2009 on a mission to discover Earth-size planets orbiting other stars.

Kepler looks for the momentary dimming of brightness caused by the transit of a planet and is often a telltale sign of a planet’s existence.

When studying the data from the transit of a planet called Kepler-1625b, the team of researchers noticed some anomalies in the data.

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

As a result of these 2017 findings the team secured a 40-hour window with the Hubble telescope, which is four times more precise than Kepler, to study the potential moon in greater detail.  

Using Hubble, the team observed the entire 19-hour journey of the distant planet across the surface of its star.

Three and a half hours after the completion of the event the team then saw a smaller dip in the star’s brightness, indicating the passing of a smaller object in front of the star.

The astronomers believe that this was the planet’s moon following slightly behind the large planet. 

Dr Kipping said of the findings: ‘[It looked like] a moon trailing the planet like a dog following its owner on a leash.’ 

‘Unfortunately, the scheduled Hubble observations ended before the complete transit of the moon could be measured.’ 

The study, published in the journal Science Advances, also found that the enormous gas giant planet began its transit 75 minutes earlier than predicted.

This, the scientists claim, further solidifies the theory that that there is a moon orbiting the planet as it implies both celestial bodies are orbiting a common centre of gravity.

This is known in astronomy as the ‘barycentre’ and would cause the planet to wobble from its expected location.

Dr Kipping noted: ‘An extraterrestrial civilisation watching the Earth and Moon transit the sun would note similar anomalies in the timing of Earth’s transit.’

This mild deviation in the planet’s orbit could be explained by the gravitational tug from another nearby planet, but astronomers believe there are no other planets in this particular star system.  

Lead author Mr Teachey, a graduate fellow in astronomy, said: ‘A companion moon is the simplest and most natural explanation for the second dip in the light curve and the orbit-timing deviation.

‘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.

‘But we knew our job was to keep a level head testing every conceivable way in which the data could be tricking us until we were left with no other explanation.’

The moon is estimated to be only 1.5 per cent the mass of its companion planet – which itself is estimated to be several times bigger than Jupiter. 

Despite both the moon and the host planet are far larger than Earth and the moon, but the scientists say the mass-ratio between the two is similar.   

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

Ideal planets for observation for potential exomoons are in large orbits and have long and infrequent transit times.

In this search, the Neptune-sized moon would have been among the easiest to first detect because of its large size.

The host planet and its moon lie within the habitable zone of the star Kepler-1625 where moderate temperatures allow for the existence of liquid water on solid surfaces.

But Dr Kipping said: ‘Both bodies, however, are considered to be gaseous and therefore unsuitable for life as we know it.’

Future searches will target Jupiter-sized planets that are farther from their star than Earth is from the sun. There are just a handful of these in the Kepler database.

Nasa’s upcoming James Webb Space Telescope could really ‘clean-up’ in the satellite search.

Professor Kipping said: ‘We can expect to see really tiny moons.’


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