Neptune-sized moon orbits Jupiter-sized planet
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. The data indicate an exomoon the size of Neptune, in a stellar system 8000 light-years from Earth. The new results are presented in the journal Science Advances.
The hunt for exoplanets — planets outside our own Solar System — provided its first results only 30 years ago. While astronomers now find these planets on a regular basis, the search for moons orbiting exoplanets wasn’t successful — until today.
In 2017 NASA’s Kepler Space Telescope detected hints of an exomoon orbiting the planet Kepler-1625b. Now, two scientists from Columbia University in New York (USA) have used the incomparable capabilities of the NASA/ESA Hubble Space Telescope to study the star Kepler-1625, 8000 light-years away, and its planet in more detail. The new observations made with Hubble show compelling evidence for a large exomoon orbiting the only known planet of Kepler-1625. If confirmed, this would be the first discovery of a moon outside our Solar System.
The candidate moon, with the designation Kepler-1625b-i, is unusual because of its large size; it is comparable in diameter to the planet Neptune. Such gargantuan moons are unknown in our own Solar System. “This may yield new insights into the development of planetary systems and may cause astronomers to revisit theories of how moons form,” Alex Teachey, a graduate student who led the study, explained excitedly .
Like its moon, Kepler-1625b is also bigger than its counterparts in the Solar System. The exoplanet is a gas giant, several times more massive than Jupiter . It orbits its parent star at a distance similar to the distance between the Sun and Earth, which puts it — and its candidate moon — at the inner edge of the habitable zone of the star system .
To find evidence for the existence of the exomoon, the team observed the planet while it was in transit in front of its parent star, causing a dimming of the starlight. “We saw little deviations and wobbles in the light curve that caught our attention,” David Kipping, second author of the study, said.
The planet was observed by Hubble before and during its 19-hour-long transit. After the transit ended, Hubble detected a second and much smaller decrease in the star’s brightness approximately 3.5 hours later, consistent with the effect of a moon trailing the planet. “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. Unfortunately, the scheduled Hubble observations ended before the complete transit of the moon could be captured.
In addition to this second dip in the light curve, Hubble provided compelling supporting evidence for the moon hypothesis by detecting the planet’s transit more than an hour earlier than predicted. This is consistent with a model of the system in which the planet and its moon orbit a common centre of gravity, causing the planet to wobble away from its predicted location .
In principle this anomaly could also be caused by the gravitational pull of a hypothetical second planet in the system, but the Kepler Space Telescope found no evidence for additional planets around the star during its four year mission. Still, further observations by Hubble are needed to fully confirm the existence of Kepler-1625b-i.
“If confirmed, Kepler-1625b-i will certainly provide an interesting puzzle for theorists to solve,” said Kipping. Teachey concluded: “It is an exciting reminder of how little we really know about distant planetary systems and the great spirit of discovery exoplanetary science embodies.”Notes
 The moons of Jupiter and Saturn likely formed through the agglomeration into a disc of material orbiting the planets, so it is possible that this exomoon also formed in a circumplanetary disc. Another possibility is that a passing object was captured by the planet’s gravity. Tidal forces between the two objects would rob momentum from the less massive companion and eventually pull it into a permanent orbit. There are no indications of tidal capture among our Solar System’s moons. In the case of the Earth–Moon system, an early collision with a larger body is hypothesised to have blasted off material that later coalesced into a moon. However, Kepler-1625b and its candidate moon are gaseous, not rocky, so such a collision would not have led to the condensation of a satellite.
 Despite its size, the mass of the candidate moon is estimated to be only 1.5 percent of the mass of its companion planet. This value is close to the mass ratio between Earth and the Moon.
 While both the planet and its candidate moon are within the habitable zone, where moderate temperatures allow for the existence of liquid water, both bodies are considered to be gaseous and therefore unsuitable for life as we know it.
 A distant observer watching the Earth and Moon transit the Sun would note similar anomalies in the timing of Earth’s transit.
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