What are they?

A dwarf planet, as defined by the International Astronomical Union (IAU), is a celestial body orbiting the Sun that is massive enough to be rounded by its own gravity but has not cleared its neighbouring region of planetesimals and is not a satellite. More explicitly, it has to have sufficient mass to overcome its compressive strength and achieve hydrostatic equilibrium. It should not be confused with a minor planet.

The term dwarf planet was adopted in 2006 as part of a three-way categorization of bodies orbiting the Sun, brought about by an increase in discoveries of trans-Neptunian objects that rivalled Pluto in size, and finally precipitated by the discovery of an even larger object, Eris. This classification states that bodies large enough to have cleared the neighbourhood of their orbit are defined as planets, while those that are not massive enough to be rounded by their own gravity are defined as small solar system bodies. Dwarf planets come in between. The definition officially adopted by the IAU in 2006 has been both praised and criticized, and has been disputed by scientists such as Alan Stern, who worked at the Southwest Research Institute at the time.


Meet Sedna:

90377 Sedna is a large minor planet in the outer reaches of the Solar System that was, as of 2015, at a distance of about 86 astronomical units (AU) from the Sun, about three times as far as Neptune. Spectroscopy has revealed that Sedna’s surface composition is similar to that of some other trans-Neptunian objects, being largely a mixture of water, methane, and nitrogen ices with tholins. Its surface is one of the reddest among Solar System objects. It is most likely a dwarf planet.

For most of its orbit, it is even farther from the Sun than at present, with its aphelion estimated at 937 AU (31 times Neptune’s distance), making it one of the most distant-known objects in the Solar System other than long-period comets.

Sedna has an exceptionally long and elongated orbit, taking approximately 11,400 years to complete and a distant point of closest approach to the Sun at 76 AU. These facts have led to much speculation about its origin. The Minor Planet Center currently places Sedna in the scattered disc, a group of objects sent into highly elongated orbits by the gravitational influence of Neptune. This classification has been contested because Sedna never comes close enough to Neptune to have been scattered by it, leading some astronomers to informally refer to it as the first known member of the inner Oort cloud. Others speculate that it might have been tugged into its current orbit by a passing star, perhaps one within the Sun’s birth cluster (an open cluster), or even that it was captured from another star system. Another hypothesis suggests that its orbit may be evidence for a large planet beyond the orbit of Neptune.

Astronomer Michael E. Brown, co-discoverer of Sedna and the dwarf planets Eris, Haumea, and Makemake, thinks that it is the most scientifically important trans-Neptunian object found to date, because understanding its unusual orbit is likely to yield valuable information about the origin and early evolution of the Solar System.

Dwarf Planet Facts:

  • The International Astronomical Union officially recognizes five dwarf planets in our Solar System. They are Ceres, Haumea, Makemake, Eris and Pluto. Ceres is found in the outer Solar System and the other four are found in the asteroid belt. The largest is Pluto and the smallest is Ceres.
  • The order of the dwarf planets from closest to the Sun outwards is Ceres, Pluto, Haumea, Makemake and then Eres, which is the furthest from the Sun.
  • Ceres is the earliest known and the smallest of the dwarf planets. It was discovered in 1801 by Sicilian astronomer Giuseppe Piazzi. It is only 590 miles in diameter and has a mass of just 0.015 percent of the Earth. It is classified as both a dwarf planet and an asteroid. Ceres is the largest resident of the main asteroid belt between Jupiter and Mars. It is also the first dwarf planet to be visited by a spacecraft.
  • There have only been two visits by space probes to dwarf planets. In 2015 NASA’s Dawn and New Horizons missions reached Ceres and Pluto.
  • Pluto, Haumea, Eris and Makemake are also known as plutoids, unlike the asteroidal dwarf planetoid Ceres. A plutoid is a dwarf planet with an orbit outside that of Neptune.
  • Plutoids are sometimes also referred to as ice dwarfs due to their cold surface temperatures and diminutive size.
  • A Pluto-sized world, astronomers discovered Eris in 2003. It takes icy Eris 557 Earth years to complete a single orbit around the sun. All the asteroids in the asteroid belt would fit inside Eris. However, like Pluto, Eris is still smaller than the Earth’s moon.
  • Scientists believe there may be dozens or possibly even more than one hundred dwarf planets awaiting discovery in the Solar System. Estimates are even up to 200 dwarf planets that could be found when the entire region known as the Kuiper belt is explored and that number may exceed 10,000 when objects scattered outside the Kuiper belt are considered.
  • The most well-known dwarf planet Pluto has been somewhat controversial. Pluto was classified as the ninth planet of the Sun for 76 years. It was discovered on February 18, 1930 by Clyde W. Tombaugh.

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