Whether confirmed or not, the subject offers insight into how rare - or how common - our own solar system might be.
The exomoon, which is a satellite that orbits a planet outside our solar system, appears to be revolving around the Jupiter-sized gas giant Kepler-1625b.
"The first exomoon is obviously an extraordinary claim and it requires extraordinary evidence", said David Kipping, an astronomer at Columbia University in NY who has spent more than a decade hunting for far-flung moons.
Kipping has spent a decade working on the "exomoon hunt". Assuming they have discovered an exomoon, it will be the first of its kind ever found, marking a new milestone in the exploration of the universe around us. But the results of those Hubble observations, published today in Science Advances, bring the putative moon back in play; the team finds two new lines of evidence for a moon. "We'd be very grateful if we had the means to use James Webb, because then we could really clean up", Kipping said.
However, the finding is both promising and intriguing.
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. The newly discovered exomoon and the planet it orbits, estimated to be several times the mass of our solar system's largest planet Jupiter, are both gaseous, an unexpected pairing. And past year, they announced that they might have found one. Another place to look is the many gas giants identified during the Kepler mission. They want some more observational data to be certain what they're looking at is indeed an exomoon.
An artist's conception of a star with the planet Kepler-1625b and exomoon in the foreground.
Kipping and his colleague Alex Teachey made the discovery after analyzing data from almost 300 distant planets discovered using the Kepler space telescope.
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Hubble's data, along with fine-tuned data from Kepler, strengthened the case for claiming Kepler-1625b had an exomoon.
The astronomers were allocated 40 hours of observation time on the Hubble telescope.
After watching the planet, the researchers noticed that shortly after it passed in front of its star, there was a second dip of light, indicating that there was "a moon trailing the planet like a dog following its owner on a leash". These observations ended before a full transit of the moon could be measured.
"An extraterrestrial civilization watching the Earth and Moon transit the Sun would note similar anomalies in the timing of Earth's transit", Kipping explained.
Perhaps that wobble could be due to the presence of a second planet, the researchers thought. The planet and moon are therefore likely to be in the "habitable zone" of the star, with equilibrium temperatures likely reaching 300-350 Kelvin (27-227°C). Panel 4 shows the secondary transit showing up in the light curve as the large candidate moon passes in front of the star. "It was a shocking moment to see that light curve".
"That's been a key driver for us for a while, just trying to understand the cosmic habitats out there that we might look for, for life", says Kipping.
He added: "We hope to re-observe the star again in the future to verify or reject the exomoon hypothesis". This value is close to the mass-ratio between the Earth and its moon. It makes sense, Teachey said, that the first moon scientists spot would also be a giant.
After analysing their data, they found that the planet transit began 77.8 minutes earlier than predicted. The moons of Jupiter and Saturn on the other hand coalesced from the debris left behind after the planets formed. There are three primary theories about how moons form. Another is capture, when objects are captured and pulled into orbit around a large planet - like Neptune's moon Triton, which is believed to be a captured Kuiper Belt object. So it remains a mystery - for now.
Study co-author Alex Teachey, also of Columbia, admitted that the potential exomoon discovery is far from a slam dunk: "We are urging caution here". It's been hard enough to find planets around distant stars, let alone moons. But for now, the researchers welcome comment and criticism of their hypothesis from other astronomers as part of the scientific process. They determined it wasn't a result of activity on the star's surface, and there was no evidence that another planet was tugging on Kepler 1625b and messing with its orbit. "It's sort of raising new questions and just having a moon of this size, has so far not really been anticipated much in the literature", Teachey said.