The majestic rings of Saturn are one of the most beautiful sights in our Solar System and a favorite target of stargazers. But all of the gas giant planets in our Solar System have rings, at least very faint ones. Even Earth has a ring, albeit one that we put there.
But do planets in other solar systems have rings? Probably. The same gravitational forces should cause them to form. Have we seen them? Well…maybe.
There is a star that is known only as 1SWASP J140747.93-394542.6, which lies 420 light-years away. Attention was drawn to this star by the SuperWASP planet survey. SuperWASP uses eight small telescopes to watch a big patch of the sky to see if any planets pass in front of their stars. It’s like the Kepler mission, but it covers an area 5 times as large (though less accurately).
Anyway, 1SWASP J140747.93-394542.6 had something pass in front of it, and that something was big. A normal planetary transit lasts only a few hours. Whatever passed in front of this star took almost two months! The star dimmed and brightened four times over the course of the first month, then the pattern repeated in reverse order in the second month. To astronomers, that says one very important thing: rings. First the rings on one side of the planet block the starlight with a certain pattern, then the rings on the other side do it in reverse. And those rings have to be huge: 60 million kilometers (37 million miles) wide. That’s 40% of the distance from Earth to the Sun and almost 200 times as wide as the main part of Saturn’s rings.
So have we found a planet with giant rings? Well…maybe. You see, this star is only 16 million years old. That means any planets it has are still forming. These rings might just be a debris disk from the planet’s formation that it hasn’t finished sweeping up yet. Also, the planet might not be a planet at all. It could be something bigger like a brown dwarf, or even a small star that’s too dim to see directly.
Now, remember that the star dimmed four times on each side of the alleged planet. That means there are four separate rings. Because of how the gravity works out, a big gap in a big debris disk like that suggests that there are moons going around the planet in those gaps. Moons outside of our Solar System are called exomoons, and they are an active field of study, even though they’ve never actually been seen. So have we found a planet with exomoons here? Maybe not. After all, these moons would be millions of kilometers away from the planet, which is awfully far to form naturally. We need better observations to tell if there’s really anything there.