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 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. 

© 2017 USPS
Pop Culture
Speedy Delivery: Mister Rogers Will Get His Own Stamp in 2018
© 2017 USPS
© 2017 USPS

USPS 2018 Mister Rogers stamp
© 2017 USPS

After weeks of mailing out this year’s holiday cards, postage might be the last thing you want to think about. But the U.S. Postal Service has just given us a sneak peek at the many iconic people, places, and things that will be commemorated with their own stamps in 2018, and one in particular has us excited to send out a few birthday cards: Mister Rogers.

In celebration of the 50th anniversary of Mister Rogers’ Neighborhood, Fred Rogers’s groundbreaking PBS series that the USPS says “inspired and educated young viewers with warmth, sensitivity, and honesty,” the mail service shared a mockup of what the final stamp may look like. On it, Rogers—decked out in one of his trademark colorful cardigans (all of which were hand-knitted by his mom, by the way)—smiles for the camera alongside King Friday XIII, ruler of the Neighborhood of Make-Believe.

Though no official release date for Fred’s forever stamp has been given, Mister Rogers is just one of many legendary figures whose visages will grace a piece of postage in 2018. Singer/activist Lena Horne will be the 41st figure to appear as part of the USPS’s Black Heritage series, while former Beatle John Lennon will be the face of the newest Music Icons collection. Sally Ride, the first American woman in space, will also be honored.

Can You Spot the Christmas Pudding?

Whether it’s a sheep hanging out with Santa Claus or a panda bear hiding among some snowmen, regular Mental Floss readers know that hidden picture brainteasers are one of our favorite things. And the optical experts at have released a delicious one, just in time for Christmas. Somewhere in the midst of all these holiday-themed goodies above, there’s a holiday pudding just waiting to be discovered. Can you spot it? Your time starts … now.

If you give up, or are the kind of person who reads the last page of a book before the first one and just wants to know the answer, scroll down to see where it’s hiding.



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