Why Doesn't Earth Have Rings?
Saturn, Jupiter, Neptune, and Uranus all have rings, so why not Earth? Turns out, it once did.
Planetary rings are made of a combination of ice, rock, and dust particles. They can form in a number of ways: as the result of a collision that kicks up debris; when a planetary satellite gets too close and is pulled apart by the planet's gravity; or simply from debris left behind during the planet's formation.
In the case of Earth, the space debris went on to serve another purpose. As Julia Wilde of D News explains in the video above: "The Earth had a ring too once, it just coalesced into the Moon."
Not all rings become moons, thanks to the Roche limit. Named after the 19th-century astronomer who first described it, the Roche limit is "the minimum distance that a moon or other large object can be from a planet without being torn to bits," as NASA describes. That distance is 2.5 times the radius of the planet if the orbiting object and the planet have the same density. Because the Moon is outside the Earth-Moon Roche limit of 11,470 miles, it stays intact.
But it may not always. There are theories that say that the Moon will one day become space debris and potentially form a ring around Earth, thanks to the Sun's inevitable red giant phase. David Powell writes on Space.com that billions of years from now, "as the Earth and Moon near this blistering hot region, the drag caused by the Sun's extended atmosphere will cause the Moon's orbit to decay. The Moon will swing ever closer to Earth until it reaches a point 11,470 miles above our planet." So it's goodbye Moon and hello ring—at least, until chunks of rock from the short-lived ring "rain down onto Earth's surface," Powell writes.