Astronomers have found another possible ‘Exomoon’ beyond our solar system
And then there were two, maybe. Astronomers say they have found a second plausible candidate for a moon beyond our solar system, an ex-moon, orbiting a world nearly 6,000 light-years from Earth. Called Kepler-1708 bi, the moon appears to be a gas-dominated object, slightly smaller than Neptune, orbiting a Jupiter-sized planet around a sun-like star – an unusual planet-moon configuration but not totally unprecedented. The findings appear in natural astronomy. Confirming or disproving the result may not be immediately possible, but given the expected abundance of moons in our galaxy and beyond, it could additionally herald the tentative beginnings of an exciting new era of extrasolar astronomy. , focused not on alien planets but on nature. satellites that orbit them and the possibilities of life therein.
There are over 200 moons in our solar system, and they have an impressive range of variations. Saturn’s moon Titan has a thick atmosphere and freezing hydrocarbon seas on its surface, perhaps an analogue of early Earth. Icy moons such as Jupiter’s Europa are frozen balls that hide subterranean oceans, and they can be prime habitats for life. Still others, like our own moon, are seemingly barren wastelands, but may have water ice in their shadowy craters and labyrinthine networks of subterranean tunnels. One important commonality between these worlds, however, is their mere existence: six of the eight major planets in our solar system have moons. Logic would suggest that the same should be true elsewhere. “Moons are common,” says Jessie Christiansen of the California Institute of Technology. “In our solar system, almost everything has a moon. I’m very confident that moons are all over the galaxy.
The only problem is finding them. We can search for exoplanets in a number of ways, from spying on the dip in light they produce as they move past their star, an event known as a transit, to getting a telltale glimpse of their gravitational tug on their host star. Finding exomoons, which are by nature significantly smaller than the planets they orbit, is much more difficult, however. “They are so small,” says Christiansen. To date, only one truly plausible candidate has been found: Kepler-1625 bi, a supposed Neptune-sized world orbiting a Jupiter-sized exoplanet about 8,000 light-years from Earth that has was reported in October 2018. But even the existence of this more behemothic world has been questioned by later analyses.
The existence of Kepler-1708 bi was first hinted at in 2018, during a review of archival data by David Kipping of Columbia University, one of the discoverers of Kepler-1625 bi, and these partner’work. The team analyzed transit data from NASA’s Kepler Space Telescope of 70 so-called cold giants – gas giants, such as Jupiter and Saturn, that orbit relatively far from their stars, with years consisting of more than 400 Earth days . The team searched for signs of transiting exomoons orbiting these worlds, looking for additional dips in the light of any shadowy lunar companions. Then the researchers spent the next few years killing their darlings, examining one potential exomoon candidate after another, and finding each one best explained by other phenomena, with one exception: Kepler-1708 bi. “He’s a lunar candidate we can’t kill,” Kipping says. “For four years we tried to prove this thing wrong. It has passed every test we can imagine.
The magnitude of the further smaller and relevant dip in light points to the existence of a moon about 2.6 times the size of Earth. The nature of the transit method means that only the radius of worlds can be directly gleaned, not their mass. But the size of this one suggests some sort of gas giant. “It’s probably in the ‘mini Neptune’ category,” Kipping says, referring to a type of world that, although it doesn’t exist in our solar system, is present in abundance around other stars. The planet around which this putative Neptune mini-moon orbits, Jupiter-sized Kepler-1708 b, orbits its star every 737 days at a distance 1.6 times that between Earth and the sun. . Assuming the candidate is truly a moon, it would orbit the planet once every 4.6 Earth days, at a distance of more than 740,000 kilometers, nearly twice the distance of our own orbit. moon around the Earth. The fact that only this single candidate emerged from the analysis of 70 cold giants could suggest that large gas moons are “not super common” in the cosmos, Christiansen says.
The apparently large size of this exomoon, compared to its host planet, is “surprising”, says Kipping, but not entirely unexpected: Kepler-1625 b, the planet on which the previous exomoon candidate Kepler-1625 bi is allegedly in orbit, appears to have a similar configuration, although slightly larger. If these two moons really exist, it could tell us something very interesting about possible planet-moon configurations in the galaxy, namely that giant worlds could host equally giant moons. This in itself raises questions about the genesis of such worlds. It’s unlikely that such a large moon could form directly orbiting a planet, with the planet more likely to sweep away any potential satellite birth material, suggesting that another origin story is more likely. likely.
“One scenario is that this moon was captured by the planet as the planetary system was forming,” says Christiansen. “Early planetary systems are quite violent and chaotic places. We see examples of capture in our own solar system: for example, Triton, one of Neptune’s moons. We believe this was captured. So we know it can happen, we just hadn’t scaled it up to the idea that a Jupiter-sized planet could capture a Neptune-sized moon.
However, not everyone is convinced of the supposed existence of this moon. René Heller of the Max Planck Institute for Solar System Research in Göttingen, Germany, says he’s not sure if the transit signal the team saw was the result of a moon. “That doesn’t convince me,” he said. Instead, Heller adds, the dip in light could simply be the result of natural variations on the star, such as the sunspots we see on our own sun, crossing its surface along with the planetary transit. Kipping and his team, for their part, say they ruled out such a possibility because the supposed plunge caused by the moon began before the planet began to pass in front of the star.
Laura Kreidberg of the Max Planck Institute for Astronomy in Heidelberg, Germany, says she “wouldn’t call it a slam dunk just yet”, but the result is “absolutely worth following” to try and see another transit from the alleged moon. However, we will not be able to do this immediately. Given the planet’s long orbit, it and its eventual moon won’t transit again until 2023, Kipping says, which means we’ll have to wait until then to try to spy on the exomoon again. If the exomoon is really there, the recently deployed James Webb Space Telescope (JWST) would be able to almost instantly confirm or disprove its existence. “It would be a breeze for Webb,” Kipping says. “It could find moons smaller than Europa around Jupiter. It’s a ridiculously powerful telescope.
That in itself raises an exciting possibility: JWST could be used to perform some kind of survey to search for exomoons. In the same way that its predecessor, the Hubble Space Telescope, made huge strides in exoplanet science, JWST may turn out to be defined by its contribution to exomoons. “My team is currently planning what an exomoon investigation would look like strategically for Webb,” Kipping says. “It will be the first time in human history that this will be possible. I’m really excited for the future.
The reasons for doing so are many. Once we start finding exomoons in abundance, we will begin to fully understand their variability and importance. The tides of our own moon, for example, may have played a role in Earth’s habitability, leading to the evolution of life in tidal pools. The study of exomoons can also tell us more about the process of planet formation. “If we want to have a comprehensive understanding of how planet formation works, we need to understand moons,” Kreidberg says. And there’s another, more simplistic reason to study them: “moons are cool.”
Exomoons themselves can also be prime targets in the hunt for life. Given that they can apparently range in size from small to Earth-sized and beyond, it is reasonable to assume that some rocky exomoons can orbit gas giant planets in the habitable zone of their stars, where l liquid water can exist. “This is one of those cases where science fiction might precede science fact,” Christiansen says. “You have the example of the film Avatar of a habitable moon around a gas giant. In star wars, you have habitable moons around the gas giants. You can technically create a rock around a gas giant that has the average radiation from the sun such that it could have liquid water on the surface.
There are, however, complications. A moon around a giant planet would experience considerable gravitational pull and pull from that larger world, which in extreme circumstances, such as Jupiter’s moon Io, can lead to intense volcanic activity. Radiation from gas giants such as Jupiter can also be deadly. And such systems can have special characteristics. “If you’re well aligned, you would have your day and night from your rotation, but an extra day-night cycle to go behind the planet,” Christiansen explains. “There are almost certainly rocks at the right temperature around the gas giants. Whether they are habitable or not is an open question and something that many people are excited about.
Kepler-1708 bi is not such a world, but it is another exciting early precursor to what could become an eventual era of exomoon science. “We’re on this long-term goal to try to understand the frequency and nature of extrasolar moons in the universe,” Kipping says. “The future is very bright.”