Kepler May Uncover Numerous Ring Worlds
In Star Wars Episode II "Attack of the Clones," a deadly cat-and-mouse chase between Obi-Wan Kenobi and Boba Fett takes place in a rocky ring encircling the desert planet Geonosis.
We don't have such a planet in our solar system, but rings made of rock or silicates might be common in the Milky Way galaxy. And, there is a chance the planet-hunting Kepler space observatory might find them says Hilke Schlichting of UCLA, in a recently posted paper.
Kepler can't photograph exoplanets, much less rings. But it is building an unprecedented planet survey by measuring the telltale dimming of a star as a planet passes across the face of it. (This only works if the planet's orbit is tilted edge-on to our view from Earth). The signature of the planet with rings would have a different-shaped shadow transit footprint from that of a planet without rings (as shown in the chart here for various transit paths.)
No rings have yet been found around the extrasolar planets that have been discovered to date. But the majority of the Kepler planets are so close to their stars that gravitational forces would keep the planets spinning perpendicular to their orbits. This means a ring system would be tilted edge-on to our view too, and be virtually invisible.
The planet would need to be tilted relative to its orbital plane, like Saturn is. And this would more likely be the case for planets farther from their stars. These less frequently transiting farther out planets, with periods approaching one year, are only beginning to be identified in the Kepler observations.
The rings would need to be gravel-like to survive, says Schlichting, containing BB-sized dust particles. Smaller particles would be eroded away by the Poynting-Robertson effect, where stellar radiation would cause dust grains to slowly spiral inward toward the parent star.
A ring might form if a moon got too close to its planet and was ripped apart by tidal forces. This is called the Roche limit where the gravitational stretching of a body rips it apart.
Earth may have had a temporary ring billions of years ago following a collision between our planet and a Mars-sized body. The material eventually coagulated to form the moon.
Weather would be complicated on a terrestrial planet with rings. If it were inclined like Earth shadows would engulf the northern and southern hemispheres with seasonal cycles. Winters would be bitterly cold with the disappearance of the star.
There would be no such thing as dark nights either because a brilliantly illuminated ring arc would span horizon to horizon.
Life would evolve markedly differently than on Earth on a world without a nocturnal cycle, and possessing extreme seasons.
It's possible that Mars might develop a ring in the future. Its largest moon, Phobos, is spiraling close enough to the planet that it will eventually break apart and perhaps form a ring.
If Schlichting's prediction is right, then within a few years the Kepler catalog will include ringed planets. This will add yet another dimension to our growing intimacy with these far-flung worlds.
Image credit: NASA, diagram: Barnes & Fortney, 2004