International Space Agencies – Missions and Discoveries

Ring around a dwarf planet lies in Roche Limit: What it means, why it matters


From UPSC perspective, the following things are important :

Prelims level: Roche Limit, Quaoar, Dwarf Planets

Mains level: Not Much

roche limit

Central idea: A new study shows that a dwarf planet, named Quaoar, has a ring system that exists within its Roche limit.

What is the news?

  • Astronomers have found a ring around a dwarf planet, located in the Kuiper Belt at the solar system’s edge, called Quaoar, according to a new study.
  • The ring, however, is positioned much further away from the planet than is usual and defies theoretical explanations.

About Quaoar

  • With an estimated radius of 555 km, Quaoar is roughly half the size of Pluto and orbits beyond Neptune.
  • It also has a moon of its own, which is known as Weywot.
  • As the dwarf planet is too small and too distant to be observed directly, the researchers detected the ring with the help of a phenomenon called stellar occultation.

How was the ring discovered?

  • A stellar occultation occurs when, as seen from Earth, a bright star passes behind a planet.
  • This allows astronomers or anybody on Earth to observe the sharp silhouette of the planet for a brief period of time.
  • The phenomenon, which rarely occurs, is used by researchers to analyze a planet’s atmosphere and determine if it has a ring around it — in 1977, scientists discovered the Uranian ring system with the help of stellar occultation.

What is the Roche limit?

  • The most intriguing part of the findings is the distance between Quaoar and its ring.
  • Located 2,500 miles away from the dwarf planet, the ring is around 1,400 miles further away from the Roche limit, as per the calculations of the scientists.
  • It suggests that at such a distance, the particles of the ring should have come together to form a moon.
  • For a further understanding of the Roche limit, let’s look at the Earth and the moon. The Earth’s gravity pulls on the moon.
  • However, one side of the moon is closer to the planet and hence, the pull is stronger on the side facing the Earth.
  • The result is the so-called tidal force, which either stretches or compresses the moon from all sides.

What is the reason behind Quaoar’s far-out ring?

  • As of now, nobody exactly knows how Quaoar’s ring has managed to remain stable at such a distance from the Roche limit.
  • The researchers said that there can be a variety of possible explanations but they aren’t sure about any one of them.
  • It might be possible that Quaoar’s moon, Weywot, or some other unseen moon contributes gravity that somehow holds the ring stable.
  • Another potential explanation can be that the particles of the ring are colliding with each other in such a way that they are avoiding to coalesce into a moon.


Try this MCQ:

Q.What is the Roche limit?

A) The distance from a planet where its gravity is balanced by the gravitational force of another celestial body

B) The minimum distance from a planet that a moon can orbit without being pulled apart by tidal forces

C) The distance from the sun at which a planet can have a stable orbit

D) The distance from the earth where meteoroids burn up upon entering the atmosphere


Post your answers here.
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