Explained: What is Planet Nine?

What is Planet Nine

News: A strange exoplanet named HD106906 b orbiting a double-star 336 light years away has caught the interest of astronomers as it can provide clues about our own mysterious Planet Nine — if it exists.


  • Planet Nine: It is a hypothetical planet in the outer region of the Solar System.It is said to be 10 times the size of the Earth and orbits far beyond Neptune in a highly eccentric orbit around the Sun.
  • When was it proposed? It was first proposed in 2012 to explain perturbations in the orbits of dwarf planets beyond Neptune’s orbit which are called Kuiper Belt objects.

Additional Facts:

What is the criteria for planet?

INTERNATIONAL ASTRANAUMICAL UNION determines the criteria and vote on the decisions.

3 conditions for a celestial object to be called as a planet

  1. It must orbit the Sun.
  2. It should be massive enough to acquire an approximately spherical shape.
  3. It has to clear its orbit (being the object that exerts maximum gravitational pull within its orbit).

If it satisfies any 2 then it is called as dwarf planet.

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Ex. Ceres (in asteroid belt), Xena (in Kuiper belt)

 Dwarf Planet: According to the International Astronomical Union, a dwarf planet is a celestial body that- orbits the sun, has enough mass to assume a nearly round shape, has not cleared the neighborhood around its orbit and is not a moon.

Kuiper belt (Edgeworth–Kuiper belt): This is an Icy ring of frozen objects in the shape of circumstellar disc present in the outer Solar System, extending from the orbit of Neptune (at 30 AU) to approximately 50 AU from the Sun.

Exoplanet: It is a planet outside the Solar System.The first confirmation of detection of exoplanet occurred in 1992.

Oort cloud – a shell of objects that surrounds the entire solar system far beyond the Kuiper belt

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