Astronomers have reported the existence of a second large moon orbiting a Jupiter-sized planet beyond our solar system. If confirmed, the sighting could mean that extrasolar moons are as common in the universe as exoplanets, and that, large or small, these moons are a characteristic of planetary systems.
But the wait could be long since the first sighting of an exomoon, four years ago, confirmation is still pending, and the verification of this new candidate could be just as long and controversial.
The discovery, published in the magazine ‘Nature Astronomy’, was led by David Kipping and his Cold Worlds Laboratory at Columbia University in the United States, which reported the first ex-moon candidate in 2017. “Astronomers have found more than 10,000 exoplanet candidates so far, but exomoons are a lot more challenging, “explains Kipping, who has spent the last decade hunting extrasolar moons.” They are uncharted territory. “
The team discovered the candidate for ex-moon that orbits the planet Kepler 1708b, a world located 5,500 light years from Earth in the direction of the constellations of Cygna and Lyra. This new candidate is about a third smaller than the Neptune-sized moon that Kipping and his colleagues previously found orbiting a similar Jupiter-sized planet, Kepler 1625b.
Both candidates for supermoons are probably made up of gas It has accumulated under the gravitational pull caused by its enormous size, Kipping said. If an astronomer’s hypothesis is correct, the moons could even have started life as planets, only to be dragged into the orbit of an even larger planet like Kepler 1625b or 1708b.
The two moons are located far from their host star, where there is less gravity to pull on the planets and strip them of their moons. In fact, the researchers looked for cold, gaseous planets in wide orbits precisely because our solar system analogs, Jupiter and Saturn, have more than a hundred moons between them.
If there are other moons, they are likely to be less monstrous, but also more difficult to spot, Kipping notes. “The first detections in any study will generally be the strangest,” he continues. “The big ones that are simply easier to detect with our limited sensitivity.”
The fascination of exomoons
Exomoons fascinate astronomers for the same reasons as exoplanets. They have the potential to reveal how and where life may have arisen in the universe. They’re also curiosities in their own right, and astronomers want to know how these exoplanets form, if they can support life, and what role, if any, they play in making their host planets habitable.
In the study, the researchers examined the sample of the coldest gas giant planets captured by the NASA Kepler spacecraft for the search for planets. After analyzing 70 planets in depth, they found only one candidate – Kepler 1708b – with a signal similar to that of a moon. “It’s a stubborn sign,” says Kipping. “We’ve thrown the whole kitchen sink into this thing, but it won’t go away.”
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Observations from other space telescopes will be needed, such as the Hubble, to verify the discovery, a process that could take years. Four years later, Kipping’s first discovery of an exomoon is still hotly debated. In a recent article, he and his colleagues showed how a group of skeptics may have missed Kepler’s moon 1625b in their calculations. Meanwhile, Kipping and his colleagues continue to investigate other lines of evidence.
Eric Agol, a professor of astronomy at the University of Washington, admits that he has doubts that this last signal turns out to be real. “It could just be a fluctuation in the data, either due to the star or the instrumental noise,” he admits. But others are more optimistic. “This is the best of science,” says Michael Hippke, an independent astronomer from Germany. “We find an intriguing object, make a prediction, and confirm the ex-moon candidate or rule it out with future observations.” “I am very excited to see a second candidate for exomoon, although it is a shame that only two transits have been observed,” he added. “More data would be very interesting.”
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