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By now, planets around other stars are old news.
In the last couple of decades, astronomers have found thousands of these exoplanets,
and now, it’s time to start learning more about them.
And one of the things researchers are looking for are rings.
After all, our solar system isn’t just a star and eight blobs.
Most of our neighborhood planets have moons, and half of them have rings.
So far, exoplanet rings, or exorings, have been pretty elusive.
But we’ve already found what might be the first set of exorings, and if we find more,
we’ll have a treasure trove of new information.
In our solar system, astronomers hunt for planetary rings using a technique called stellar
occultation, where something passes in front of a background star,
blocking out some or all of its light.
Since we can measure changes in brightness very accurately, astronomers can use occultations
to spot things too faint or far away to be picked up otherwise.
For example, we knew that Pluto had an atmosphere
decades before New Horizons arrived because of occultations.
They were used to discover the rings of Uranus, Neptune, and the minor planet Chariklo.
Outside of our solar system, stellar occultations are so useful that they’ve been the most
successful technique for discovering exoplanets.
It’s usually called the transit method when used in planet hunting,
but the basic principle is the same: as a planet passes in front of its star as seen from Earth,
we detect a small dip in the star’s light.
That dip is called the lightcurve, and all transiting planets have lightcurves with basically the same shape.
So if astronomers see an unusual pattern, it might mean there’s something
extra blocking the star’s light, like a ring!
Finding a ring would be awesome, but what it could tell us might be even cooler.
During its 13-year mission, NASA’s Cassini spacecraft revealed that Saturn’s ring system
is intricately related to both the planet and its moons.
A gap, for example, could mean an unseen moon is embedded within the ring.
Occultations of Saturn’s rings revealed the narrow Keeler gap
well before Cassini discovered a tiny moon named Daphnis orbiting inside it.
On the other hand, a moving pattern of higher density, called a density wave,
could reveal the presence of a moon orbiting outside of the rings.
Nearly all the fine detail in Saturn’s rings is due to the gravitational sway of its many moons.
With an exoplanet, if astronomers knew the mass of that moon, something that’s possible
with the transit method, the wave’s shape could even reveal the density of the ring.
But wait! There’s more.
Cassini found that special density waves in Saturn’s inner rings
actually revealed the motion of material within the planet.
Now, researchers are probably a long way from repeating those observations in another star system.
We’re still working on spotting whether rings exist at all;
it’s a lot harder to see what’s happening in their internal structure.
But it’s a tantalizing hint of what the future might hold.
And the path to that future is already in motion,
because astronomers might have found the first set of exorings back in 2012.
The rings were found orbiting a planet with a name so bad
I’m not even going to try to say it, so let’s call it J1407 b for short.
In the world of exoplanets, everything is over the top, and this system is no exception.
At dozens of times the mass of Jupiter, J1407 b is so large that it’s tough to even be
sure whether it’s a planet or a failed star called a brown dwarf.
The rings are no slouch, either.
Based on their data, the researchers think that a total of 30 rings stretch about
120 million kilometers, a span more than 200 times larger than the rings of Saturn.
One study indicates that a gap between two of the rings
could be caused by a moon up to 80% the mass of the Earth.
With their gigantic size and high density,
some scientists wonder if it’s even right to call them rings at all.
Instead, we might be seeing the remnants of a circumplanetary disk,
the disk of material around a planet that gives rise to moons early in its life.
Our solar system went through that process billions of years ago,
but the J1407 system is only about 16 million years old.
It’s probably not worth worrying about whether to call this material a ring or not
until we have other examples to compare it to.
Which begs the question: if planets are common in the galaxy and rings are common around
planets in our solar system, where are all the exorings?
Why have we found only this one example?
Part of the answer is that we need a new generation of technology to see them.
Just finding an exoplanet is still a pretty tricky task and rings are even harder.
But geometry is also working against us.
The transit method, which is our best, and possibly only, way to find exorings,
only works when we’re looking at a star system edge on.
But models of planet formation suggest that planetary rings should also be edge-on
in that scenario, making them nearly impossible to detect.
After all, Saturn’s rings become almost invisible to a spacecraft
orbiting the planet itself when viewed edge-on.
From hundreds of lightyears away, we’ll need to be looking at really big rings in a
really obvious configuration to have a chance of seeing them anytime soon.
That would be a pretty unusual situation, so it makes sense that we’d be having
so much trouble finding exorings even if they’re all over the place.
But, hey, if there’s one thing exoplanets are awesome at, it’s being unusual.
So the hunt is on!
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