For a long time, the only moon human beings knew of was our own. In the early-17th century, the invention of the telescope extended our vision into the cosmos and allowed Galileo to discover four new moons, in orbit around Jupiter. Five moons around Saturn were found in that century, and two more in the next. By the 19th century, astronomers had detected moons around Mars, Neptune, and Uranus. Today, we know the solar system is flush with them. There are nearly 200 known moons, dutifully circling their planets and dwarf planets.
Now, after more than 400 years of studying these planetary companions, the search has reached beyond our home in the cosmos, to a different sun thousands of light-years away.
A pair of astronomers announced Wednesday that they have detected a distant object they believe to be a moon. If confirmed, the finding would mark the first discovery of a moon orbiting a planet outside of our solar system, known as an exomoon.
The potential exomoon orbits a planet about 8,000 light-years away from Earth. It’s unlike any moon astronomers have ever observed: It is not rocky or icy like the moons we know, but gaseous. And it’s about the size of Neptune, which makes it 10 times bigger than the largest moon in our solar system.
“The closest analog would be picking up Neptune and putting it around Jupiter,” says David Kipping, the astronomer who discovered the object with Alex Teachey, his colleague at Columbia University. Their findings were published Wednesday in Science Advances.
Kipping and Teachey first came across the object in data from the Kepler space telescope, a NASA mission that has discovered more than 5,000 exoplanets and confirmed about half of them since it launched in 2009. The telescope detects planets through a technique called the transit method. When a planet passes, or transits, across the face of its star, it blocks a tiny fraction of the star’s light. Kepler stares at stars for years and watches for this dimming in action.
Kepler recently surveyed 284 exoplanets in its repertoire that astronomers suspected would make good hosts for moons. They’re large, and “the bigger the planet, the more leftover material there is to form moons,” Kipping explained. (The biggest planet in our solar system, Jupiter, also has the most moons and may add more; just this summer, astronomers discovered 10 new ones around the gas giant.)
The 284 planets also have wide orbits around their star, which means they may not have experienced a migration that can be risky for moons. Scientists believe planets across the universe form farther out from their star, as ours did, and then move closer. Planets can survive this journey, but their moons might not. “It really can strip off all of the moons and eject them, or lead them to collide with the planets,” Kipping says.
Of the candidate planets, one called Kepler-1625b stood out. Kipping and Teachey detected some strange features in the light coming from its direction.
The astronomers decided they needed to get a better look, so they applied for observation time on NASA’s Hubble Space Telescope, which is four times more precise than Kepler. They monitored 1625b as it made its 19-hour trek across its star, dimming its light. About three and a half hours after 1625b finished its transit and disappeared from view, the astronomers detected a second, much smaller dimming in the star’s brightness. They believe the signal indicates the presence of a moon trailing behind its planet.
Kipping and Teachey also detected a shift in 1625b’s orbit. During the Hubble observations, the planet began its transit about an hour earlier than expected, which suggests that an interaction between the planet and a nearby moon caused the planet to wobble slightly from its usual course. An alien astronomer watching the Earth and moon move around the sun would see similar effects, Kipping says.
But when the target of observations is thousands of light-years away, how can you be sure you’re looking at a moon and not, say, another planet? The explanation lies in the transit signals of the two objects, Kipping says. “An exoplanet is normally pretty easy [to detect], to be honest,” he says. “It repeats like clockwork, or very close to clockwork. Every orbital period, you should see the planet in the same place.”
The movements of a moon are less predictable.“You’d expect to see one large transit due to the planet, and then buried within that, a smaller, second transit, either before or after, depending on the phase of the moon,” he says. “And that’s going to be different every time you look.”
Kipping and Teachey are not describing their detection as the first-ever discovery of an exomoon, in part because it is so unlike the known architecture of our solar system.“It looks very convincing on this one detection, but it’s so strange compared to what moons are like in our own solar system that it’s kind of hard to believe it,” Kipping says. “I don’t think any of us can really have 100 percent faith in it until we’ve seen multiple examples of systems like this.”
Kepler-1625b and its hypothesized moon orbit within their star’s habitable zone, the region where temperatures are just right for liquid water. But both objects are gaseous, which means they can’t sustain life as we know it on Earth.
The gaseous nature of the potential exomoon is especially interesting. Our own rocky moon formed out of the debris from a collision between Earth and a Mars-sized object about 4.5 billion years ago, according to the leading theory. A similar scenario is unlikely to produce a massive moon made of gas. Kipping says he’s leaving the question of the object’s creation up to theoretical physicists rather than observational astronomers.
“I don’t think theorists are too on board with the idea of making a moon this big. I’m also convinced they could easily explain it if necessary, because that’s the talent of theorists,” he says. “We’re really not trying to worry about whether this is theoretically allowed or not, we’re just trying to follow the data and do the best we can with interpreting our observations.”
The detection of a potential exomoon is reminiscent of the discovery of a planet orbiting another star in 1995. Astronomers had long suspected planets existed around other stars, and they expected the first one they found to look like something in our own solar system. It did not: 51 Pegasi b, discovered by a pair of Swiss astronomers, had about half the mass of Jupiter and orbited extremely close to its star, completing one loop in a head-spinning four days. The announcement was met with considerable skepticism. More objects like 51 Pegasi b were eventually found, and astronomers came to accept the prevalence of these strange planets they called “hot Jupiters.” The first known exoplanet eventually turned out to be representative of a rich class of planets in the universe.
The maybe-moon around Kepler-1625b may be the first of many detections that will, once again, upend our understanding of planetary systems. Perhaps 20 years from now, the object will not be remembered as a fluke, but as the first in a line of thousands of other moons scattered across the cosmos.