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. 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.
It's an "extraordinary" find that "defies easy explanation" said co-author Alex Teachey - nothing like it exists in our own solar system. The gas giant it presumably orbits has several times the mass of Jupiter.
"An extraterrestrial civilization watching the Earth and Moon transit the Sun would note similar anomalies in the timing of Earth's transit", Kipping said.
With the Hubble, they monitored the exoplanet Kepler-1625b as it passed between the star it orbits - Kepler-1625 - and Earth, and they looked at how the brightness of that star dimmed as the planet passed in front of it.
Still, it is possible that there is a second planet in the system external to Kepler-1625b whose transit we can't see, Teachey said. Current theories cannot therefore explain how a Neptune-sized moon could have formed in orbit around a Jupiter-sized planet.
While it definitely looks likely that the newfound moon is there, researchers will still need to do a bit more work to confirm it.
This still doesn't prove the existence of an exomoon around Kepler-1625b. The Hubble observations showed that the planet's transit occurred 75 minutes earlier than predicted based on earlier transits observed by the Kepler spacecraft. The moon, which orbits a giant exoplanet called Kepler-1625b, is incredibly large, comparable to the size of the gas giant Neptune in our solar system. They observed the system throughout a predicted transit of the planet Kepler-1625b over the course of 40 hours.
Although moons are common in our solar system, which has almost 200 natural satellites, the long search for interstellar moons has been an empty one.
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The exomoon is exponentially larger than our solar system's biggest moon.
Kipping said there's already another potential planet-moon combination worth looking at.
The host planet and its moon lie within the solar mass star's (Kepler 1625) habitable zone, where moderate temperatures allow for the existence of liquid water on any solid planetary surface. After it ended, Hubble detected a second and much smaller decrease in the star's brightness 3.5 hours later, consistent with "a moon trailing the planet like a dog following its owner on a leash", Kipping said.
Although the researchers can not say with certainty that an exomoon caused these effects, Kipping argues that the moon hypothesis offers the best single explanation for both anomalies.
"We hope to re-observe the star again in the future to verify or reject the exomoon hypothesis".
If indeed a moon, it would be about 2 million miles (3 million kilometers) from its planet and appear twice as big in its sky, as the moon does in ours. Of the eight planets in our solar system, only Mercury and Venus have none. The moons of Jupiter and Saturn on the other hand coalesced from the debris left behind after the planets formed. It makes sense, Teachey said, that the first moon scientists spot would also be a giant.
KIPPING: So when we look for an Earth twin, I think one of the most obvious things you might ask is, does it have a moon twin, because that seems to have a large influence. To further confirm the exomoon, the team will need to continue to observe the transit events in this system.
"If this does pan out and turn into a true discovery, it would be really revolutionary, but I don't think we're quite there yet", says Megan Bedell, an astronomer at the Flatiron Institute in NY. Once the James Webb Space Telescope launches in 2021, the search for exomoons may be full of possibilities.