Dwarf planet Totally Explained

Everything about Dwarf Planet totally explained

A dwarf planet, as defined by the International Astronomical Union (IAU), is a celestial body orbiting the Sun that's massive enough to be rounded by its own gravity but which hasn't cleared its neighbouring region of planetesimals and isn't a satellite. More explicitly, it has to have sufficient mass for its self-gravity to overcome rigid body forces in order to assume a hydrostatic equilibrium and acquire a near-spherical shape.
The term dwarf planet was adopted in 2006 as part of a three-way classification of bodies orbiting the Sun.

History of the name

Before the discoveries of the early 21st century, astronomers had no strong need for a formal definition of a planet. With the discovery of Pluto in 1930, astronomers considered the Solar System to have nine planets, along with thousands of significantly smaller bodies such as asteroids and comets. For almost 50 years Pluto was thought to be larger than Mercury, but with the discovery in 1978 of Pluto's moon Charon, it became possible to measure the mass of Pluto accurately and it was noticed that actual mass was much smaller than the initial estimations. It was roughly one-twentieth the mass of Mercury, which made Pluto by far the smallest planet. Although it was still more than ten times as massive as the largest object in the asteroid belt, Ceres, it was one-fifth that of Earth's Moon. Furthermore, having some unusual characteristics such as large orbital eccentricity and a high orbital inclination, it became evident that it was a completely different body from any of the other planets.
   In the 1990s, astronomers began to find objects in the same region of space as Pluto (now known as the Kuiper belt), and some even farther away. Many of these shared some of the key orbital characteristics of Pluto, and Pluto started being seen as the largest member of a new class of objects, plutinos. This led some astronomers to stop referring to Pluto as a planet. Several terms including minor planet, subplanet, and planetoid started to be used for the bodies now known as a dwarf planets. By 2005, three other bodies (Quaoar, Sedna, and Eris) comparable to Pluto in terms of size and orbit had been reported in the scientific literature. It became clear that either they'd also have to be classified as planets, or Pluto would have to be reclassified. Astronomers were also confident that more objects as large as Pluto would be discovered, and the number of planets would start growing quickly if Pluto were to remain a planet.
   In 2006, Eris (known at that time as ) was determined to be slightly larger than Pluto, and some reports unofficially referred to it as the tenth planet. As a consequence, the issue became a matter of intense debate during the IAU General Assembly in August 2006. IAU's initial draft proposal included Charon, Eris, and Ceres in the list of planets. After many astronomers objected to this proposal, an alternative was drawn up by Uruguayan astronomer Julio Ángel Fernández, in which he created a median classification for objects large enough to be round but that hadn't cleared their orbits of planetesimals. Dropping Charon from the list, the new proposal also removed Pluto, Ceres, and Eris, since they've not cleared their orbits.
   The IAU's final resolution preserved this three-category system for the celestial bodies orbiting the Sun. Fernández suggested calling these median objects planetoids, but the IAU's division III plenary session voted unanimously to call them dwarf planets. The resolution read, in full:
Although there were concerns about the classification of planets in other solar systems, In other words, Soter went on to propose a parameter he called the planetary discriminant, designated with the symbol µ (mu), that represents an experimental measure of the actual degree of cleanliness of the orbital zone (where µ is calculated by dividing the mass of the candidate body by the total mass of the other objects that share its orbital zone). There are several other schemes that try to differentiate between planets and dwarf planets, The lower limit is determined by the concept of hydrostatic equilibrium shape, but the size or mass at which an object attains this shape hasn't been defined. The original draft of the 2006 IAU resolution defined hydrostatic equilibrium shape as applying "to objects with mass above 5 kg 1.1 lb and diameter greater than 800 km [500 mi]", but this wasn't retained in the final draft.

– discovered on January 1, 1801 (16 years before Neptune), considered a planet for half a century before reclassification as an asteroid;
– discovered on February 18, 1930, classified as a planet for 76 years;
– discovered on October 21, 2003, once referred to as the "tenth planet" in the media.

Here is a comparison of the main characteristics of these three bodies:

Orbital attributes of dwarf planets												0.074	974.6±3.2	0.0002	0.095	2.08	0.27	0.51	~3°	0.38	0	167	none
Pluto	0.19	2306±30	0.0021	1.305	2.0	0.58	1.2	119.59°	-6.39	3	44	temporary
Eris	0.19	2400±100	0.0025	1.67	2.3	~0.8	1.3		~0.3	1	42	temporary
ť Measured relative to the Earth.

No human space probes have yet visited any of the dwarf planets and therefore these objects are not as studied as any of the other larger bodies in the Solar System. This will presumably change if NASA's spacecrafts Dawn and New Horizons will reach Ceres and Pluto, respectively, in 2015.

Candidates

As with Ceres, the next three largest objects in the main asteroid belt – Vesta, Pallas, and Hygiea – could also eventually be classified as dwarf planets if it's shown that their shape is determined by hydrostatic equilibrium. While uncertain, the present data suggests that it's unlikely for Pallas and Hygiea. Vesta however appears to deviate from hydrostatic equilibrium only because of a large impact that occurred after it solidified; the definition of dwarf planet doesn't specifically address this issue. The Dawn probe scheduled to enter orbit around Vesta in 2011 may help clarify matters. This definition, however, wasn't preserved in the IAU's final resolution and it's unknown if it'll be included in future debates.

TNO candidates

Trans-Neptunian objects (or simply TNOs) are thought to have icy cores and therefore would require a diameter of perhaps 400 km (250 mi) – only about 3% of that of Earth – to relax into gravitational equilibrium. A team is investigating another 30 such objects, and believe that the total number will eventually prove to be about 200 in the Kuiper belt, and many more beyond it.
Although the status of many of these objects may be debatable, TNOs with estimated diameters approaching that of Ceres are likely to be dwarf planets regardless of measurement errors. These include cubewanos, plutinos, and scattered disc objects (SDOs):
Pluto prototype The 2006 IAU's Resolution 6a recognizes Pluto as "the prototype of a new category of trans-Neptunian objects". The name and precise nature of this category are not specified, but in the debate leading up to the resolution, the members of the category were variously referred to as plutons and plutonian objects. The former name was generally deprecated and was abandoned in the final draft resolution (6b); eventually even the latter name failed to win majority approval on a 183–186 vote in the IAU General Assembly on August 24 2006. This category of Pluto-like objects now only applies to dwarf planets that meet the conditions of being trans-Neptunian and "like Pluto" in terms of period, inclination, and eccentricity. A dwarf planet may or may not be a member of this category, but all members of the category must be dwarf planets. it's unclear whether or not it would qualify as a member of this category.

Contention

In the immediate aftermath of the IAU definition of dwarf planet, a number of scientists expressed their disagreement with the IAU resolution. Campaigns included car bumper stickers and T-shirts. Mike Brown (the discoverer of the would-have-been-the-10th-planet Eris) agrees with the reduction of the number of planets to eight.
NASA has announced that it'll use the new guidelines established by the IAU. However, Alan Stern, the director of the NASA's mission to Pluto, rejects the current IAU definition of planet, both in terms of defining dwarf planets as something other than a type of planet, and in using orbital characteristics (rather than intrinsic characteristics) of objects to define them as dwarf planets. Thus, he and his team still refer to Pluto as the ninth planet, while accepting the characterisation of dwarf planet for Ceres and Eris (dwarf planet in this case meaning just a small planet).

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