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Ring Around a Dwarf Planet


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.


GS III: Science and Technology

Dimensions of the Article:

  1. Details
  2. How was the ring discovered?
  3. What is the Roche limit?


  • A new study published in the journal Nature reports the discovery of a dense ring around the trans-Neptunian object Quaoar.
  • The ring lies beyond the Roche limit, the mathematically determined distance beyond which planetary rings are not supposed to exist.
  • Quaoar is a dwarf planet about half the size of Pluto, with a moon of its own.
  • The researchers detected the ring using a phenomenon called stellar occultation, as the planet is too small and distant to be observed directly.
  • The findings are considered “very strange” and may require a rethinking of the laws governing planetary rings.

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 analyse 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.
  • The team involved in the latest study examined Quaoar for around three years, between 2018 and 2021, through Earth-based and space-based telescopes.
  • During these years, the dwarf planet passed in front of four stars, helping researchers observe the shadow of the eclipses.

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.
  • They suggest that at such a distance, the particles of the ring should have come together to form a moon.
Understanding of the Roche limit
  • 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 helps the moon keep it together is its own gravity. It essentially counteracts the effect of the tidal force.
  • But if you bring the moon closer to the Earth, the tidal force will overcome the satellite’s gravity and then disintegrate it, turning the moon into a ring.
  • The minimum distance at which this happens is known as the Roche limit. It is named after the French astronomer Édouard Roche, who discovered the limit in 1848.
  • The Roche limit doesn’t just exist between just the Earth and the moon. It is applicable to any planet and the celestial bodies around it. For instance, Saturn. The beautiful rings that you see around the planet are within the Roche limit and therefore, there are no moons in that area.
  • In 1992, comet Shoemaker–Levy 9 got too close to Jupiter, breaching the Roche limit, and was broken apart by the tidal force. Two years later, parts of it collided with the planet, providing the first direct observation of an extraterrestrial collision of solar system objects.

-Source: Indian Express

December 2023