Dwarf planet

Geography glossary

Topic: Origin and Evolution of the Earth and the Solar System · NCERT: Beyond NCERT

Meaning

A dwarf planet is a body that orbits the Sun and is round because of its own gravity, but has not cleared its orbit of other objects, and is not a moon. The definition comes from the IAU's Resolution B5, adopted on 24 August 2006. That same decision moved Pluto from "planet" to "dwarf planet", which is why it is a common Prelims trap.

Explanation

The IAU's three-part test

  • The IAU (International Astronomical Union) is the world body that names and classifies objects in space.
  • Under Resolution B5 (24 August 2006), every body that orbits the Sun is checked against three conditions:
Condition Planet Dwarf planet
1. Orbits the Sun Yes Yes
2. Is round because of its own gravity Yes Yes
3. Has cleared its orbit (its neighbourhood) of other objects Yes No
  • A dwarf planet passes conditions 1 and 2 but fails condition 3.
  • A moon is never a dwarf planet, even if it is round. A moon orbits a planet, not the Sun directly.

What "clearing the orbit" means

  • Clearing the orbit: the body is the main gravitational force along its path. Over time it has swept up most of the smaller objects in its path or pushed them away.
  • Why some bodies fail this test:
  • They are too small to control their path.
  • So they share their zone with many other bodies of similar kind.
  • Example: Pluto shares its zone with many other Kuiper belt bodies, so it has not cleared its orbit.

Why dwarf planets stayed small

  • The usual way planets grow:

  • Planets grew by accretion. Small bodies (planetesimals) collided and stuck together under gravity, and a few large bodies formed.

  • Dwarf planets grew enough to become round, but stopped there:

  • They had enough mass for their own gravity to pull them into a round shape.
  • They did not grow big enough to take over their zone.
  • So they remain among crowds of smaller bodies: rocky asteroids (Ceres) or icy bodies beyond Neptune (the others).

The five recognised dwarf planets

Dwarf planet Location
Ceres Asteroid belt (between Mars and Jupiter)
Pluto Kuiper belt
Haumea Kuiper belt (named in September 2008) [1]
Makemake Kuiper belt (recognised in July 2008) [1]
Eris Beyond Neptune, in the scattered outer region

Where it occurs

  • Inner solar system, in the asteroid belt:
  • Asteroid belt: a ring of rocky and metal bodies between Mars and Jupiter. It separates the terrestrial (Earth-like) planets from the Jovian (Jupiter-like) planets.

  • Ceres is the only dwarf planet in the inner solar system.

  • Kuiper belt, beyond Neptune:

  • Kuiper belt: a doughnut-shaped zone of icy bodies about 30–50 AU from the Sun. AU (astronomical unit) is the average distance between the Earth and the Sun.
  • Pluto, Haumea and Makemake are found here.

  • The first Kuiper belt object (KBO) after Pluto was found in 1992. More than 1,300 KBOs had been identified by 2013 [1].

  • The scattered outer region beyond Neptune:

  • Eris lies here.

  • India:

  • No Indian location or mission is tied to this concept. It is a solar system classification that India's syllabus tests under the origin and evolution of the Earth and the solar system.

Don't confuse with

  • Planet: a planet passes all three IAU tests. A dwarf planet fails only one, the "cleared its orbit" test. Both orbit the Sun and both are round.
  • Asteroid: asteroids are small rocky or metal bodies, most of them in the asteroid belt. Ceres lies in the same belt, but it is round under its own gravity, so it counts as a dwarf planet.
  • Moon: a moon may be round, but it orbits a planet. A dwarf planet orbits the Sun directly.
  • Oort cloud object: these icy bodies lie in a spherical shell about 5,000–100,000 AU away and have never been directly observed [1]. The Kuiper belt dwarf planets (Pluto, Haumea, Makemake) have been observed.

Prelims Hooks

  • IAU Resolution B5, 24 August 2006: a dwarf planet fails only the "cleared its orbit" test. It still orbits the Sun and is round.
  • Five dwarf planets: Ceres, Pluto, Eris, Haumea, Makemake.
  • Location trap: Ceres is the only one in the inner solar system (asteroid belt, between Mars and Jupiter). All the others lie beyond Neptune.
  • Pluto was moved from planet to dwarf planet in 2006. It lives in the Kuiper belt.
  • Haumea was named in September 2008. Makemake was recognised in July 2008 [1].
  • "Which is NOT a dwarf planet?" Watch for round moons and for asteroids placed among the options. Only the five named above are recognised.

Mains Points

  • Classification in science rests on evidence and can change (GS-I):
  • Pluto was a planet for decades but lost that status in 2006 under the IAU's three-part test.
  • New finds in the Kuiper belt, which has held more than 1,300 known objects since 2013 [1], showed that Pluto is one of many similar bodies in its zone.

  • Lesson: a scientific category changes when new evidence shows the old rule no longer fits.

  • Dwarf planets show how the solar system was built:

  • They are bodies whose growth by accretion stopped part-way: they became round but never took control of their zones.
  • Their location, mostly in the icy Kuiper belt and beyond, shows that the solar system does not end at Neptune.
  • These icy bodies formed early in the solar system's history [1]. This makes them useful records of the material left over from when the planets formed.

Related concepts

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Sources

  1. 1NASA — Kuiper Belt (Solar System Exploration resource, 2013 revision)solarsystem.nasa.gov · tier 2