Why doesn't the Earth have rings?

Ever wondered why planets like Saturn have beautiful rings, while the Earth only has a moon?


The answer lies in the concept of the Roche Limit. This is an imaginary circle around every planet which determines if the planet will have rings. Within this circle, the forces from the planet’s gravity (tidal forces) are strong enough to break a moon into pieces and form a ring.



The Earth had rings 4.5 billion years ago when another planet crashed into it and ejected a ring of debris into orbit. However, this ring was outside the Earth’s Roche Limit, which meant that the Earth’s gravity was not strong enough to preserve the ring. Hence, the pieces in the rings started combining and formed a ball which we now know as our Moon.



A more scientific explanation

An examination of the inverse square law of gravitation will show that the rate at which the gravitational field strength changes increases as you get closer to the planet (the gradient is steeper, as seen on the graph to the right). This means that if a moon is very close to a planet, the gravitational field strength will be a lot stronger on the near side as compared to the far side. This will cause the moon to get stretched and break into pieces to form a ring.



The Roche limit for the Earth and Moon

The Roche Limit for the Earth is about 10 000 km above the surface. The Moon orbits the Earth at a distance of about 384 000 km, which is almost 40 times further away than the Roche Limit. If the Moon were to come closer than 10 000 km, it would be torn apart by the Earth and form a ring around the planet.


Jupiter shatters a comet

In July 1992, a comet called 'Shoemaker–Levy 9' came within the Roche limit of Jupiter. The gravitational forces of the giant planet caused the comet to break apart into 21 fragments (as seen in the image below). These pieces orbited Jupiter temporarily for 2 years, before crashing into the planet in July 1994.



Black holes and spaghetti

This phenomenon is more significant for black holes as they have a very strong gravitational field. Any object falling into a black hole will feel a much greater difference in gravity between its near and far side. This will stretch the object into a long stream of particles, which looks like spaghetti (as seen in the image below). Astronomers call this effect as spaghettification or the ‘noodle effect’.




Image credits:

  1. Space FM. (2020). Roche Limit [Image]. Retrieved from https://www.space.fm/astronomy/planetarysystems/rochelimit.html
  2. Universe Today. (2016). A Cataclysmic Collision Formed the Moon, but Killed Theia [Image]. Retrieved from https://www.universetoday.com/127139/127139/
  3. Hyperphysics. (2020). Inverse Square Law Plot [Image]. Retrieved from http://hyperphysics.phy-astr.gsu.edu/hbase/Acoustic/invsqs.html
  4. NASA. (2016). Hubble Memorable Moments: Comet Impact [Image]. Retrieved from https://www.nasa.gov/feature/goddard/2016/hubble-memorable-moments-comet-impact
  5. Interesting Engineering. (2020). Researchers Capture Rare ‘Spaghettification’ Moment of Star Around Black Hole [Image]. Retrieved from https://interestingengineering.com/researchers-capture-rare-spaghettification-moment-of-star-around-black-hole

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