It’s the second extraterrestrial body discovered that could be an exo moon or a moon outside our solar system. The giant moon has been found orbiting a Jupiter-sized planet called Kepler 1708 b, located 5,500 light-years from Earth.
The newly discovered celestial body is 2.6 times larger than the Earth. There is no analogue of such a large moon in our system. For reference, our Moon is 3.7 times smaller than the Earth.
“Astronomers have discovered more than 10,000 exoplanets so far, but exomoons present a much greater challenge,” Kipping said in a statement. “They are uncharted territory.”
Understanding more about moons, such as how they formed, whether they can support life, and whether they play a role in the potential habitability of planets, can lead to a greater understanding of how planetary systems form and evolve.
Kipping and his team are still working to confirm that the first candidate they found is in fact an exomoon, and this latest discovery will likely face the same uphill battle.
Moons are common in our solar system, which contains more than 200 natural satellites, but the long search for interstellar moons has been largely unproductive. Astronomers have successfully determined the positions of exoplanets around stars outside our solar system, but exomoons are difficult to locate due to their small size.
More than 4,000 confirmed exoplanets have been discovered across the galaxy, but that doesn’t mean finding them was easy. Many have been discovered using the transit method, or looking for dips in starlight when a planet passes in front of its star. Smaller moons that cause more and more tiny dips in starlight are much more difficult to detect.
To find this potential second moon, Kipping and his team used data from NASA’s Kepler Retired Planet Search mission to survey some of the coldest gas giant exoplanets the telescope has found. The researchers used these criteria in their research because in our solar system, the two gas giants Jupiter and Saturn have most of the moons orbiting around them.
Of the 70 planets they studied, only one revealed an accompanying signal that looked like a moon, with only a 1% chance that it was something else.
“It’s an intractable sign,” Kipping said. “We threw the kitchen sink on this thing but it won’t go away.”
3 ways the moon can be formed
The newly discovered candidate shares similarities with the first possible discovery of the outer moon. Both are likely gaseous, which accounts for their enormous size, and are far from their host stars.
There are three primary theories about how the moons formed. One is when large space objects collide and the explosive material turns into a moon. Another is capture, when objects are picked up and pulled into orbit around a large planet – such as Neptune’s moon Triton, which is believed to be a captured object in the Kuiper Belt. The third is the formation of moons from the material, such as the gas and dust that orbits stars, that created the planets in the early days of the solar system.
It is possible that both candidates for the outer moon may have started out as planets that were eventually dragged into orbit around larger planets such as Kepler 1625b and Kepler 1708b.
Giant moons may be an anomaly
Kipping thinks it’s unlikely that all moons outside our solar system are as large as these two candidates, which might make them odd, rather than standard. “The first discoveries in any survey will generally be rather eccentric,” he said. “Big things are easy to detect simply with our limited sensitivity.”
To confirm that the two candidates are exomoons, follow-up observations by the Hubble Space Telescope and the James Webb Space Telescope in 2023 will be required. Meanwhile, Kipping and his team continue to gather evidence to support the outer moons.
The fact that each associated planet takes more than one Earth year to complete an orbit around its star slows the discovery process.
“Confirmation requires seeing the moon repeated several times,” Kipping said. “The long-period nature of the target planets means we only have two transits on hand here, and that’s not enough to see the series of moon transits necessary to claim a confirmed discovery.”
If confirmed, it could be the start of a new acceptance that exomoons are as common as exoplanets outside our solar system.
The first exoplanet was not discovered until the 1990s, and the bulk of the known exoplanets today were not revealed until the launch of Kepler in 2009.
“These planets are strange compared to our internal system,” Kipping said. “But they have revolutionized our understanding of how planetary systems form.”
Correction: An earlier version of this story misspelled the year the outer moon was first discovered.