Download Some additional information on Dwarf Planets

Some additional information on Dwarf Planets:
On August 24, 2006, the International Astronomical Union voted at the end of its 26th
General Assembly to establish definitions for three classes of substellar objects in the
Solar System: planets, dwarf planets, and smaller Solar System bodies. These classes
were defined as follows:
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Planet - This is a celestial body that: (a) is in orbit around the Sun; (b) has
sufficient mass for its self-gravity to overcome rigid body forces so that it
assumes a nearly round shape (due to a hydrostatic equilibrium); and (c) has
cleared the neighborhood around its orbit. As now defined, the Solar System has
eight planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.
Dwarf Planet - This is a celestial body that: (a) is in orbit around the Sun; and (b)
has sufficient mass for its self-gravity to overcome rigid body forces so that it
assumes a nearly round shape (due to a hydrostatic equilibrium); but (c) has not
cleared the neighborhood around its orbit; and (d) is not a satellite of another
planet. As defined, the Solar System has probably has at least one dwarf planet in
the Main Asteroid Belt (Ceres), Edgeworth-Kuiper Belt (Pluto and Eris); and Oort
Cloud (possibly Sedna). In addition, the "dwarf planet" Pluto is recognized as a
prototype of a new class of trans-Neptunian objects.
Smaller Solar System Bodies - This category collectively refers to all other
celestial bodies orbiting our Sun, Sol (including asteroids, comets, and satellites).
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,[7][8] 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.[9] 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.[10] 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.[11]
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 further away.[12] 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.[13][14] 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.[15] It became clear that either they would also have to
be classified as planets, or Pluto would have to be reclassified.[16] 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.[17]
The IAU currently recognizes only three dwarf planets – Ceres, Pluto, and Eris – but it is
suspected that at least another 42 discovered objects in the Solar System might belong in
this category.[4] There are estimates that the actual number of dwarf planets might
increase to 200 when the entire region known as Kuiper belt will be fully explored, and
that the number might increase to around 2000 when objects outside this region will also
be accounted for.[4]
1. Ceres – discovered on January 1, 1801 (16 years before Neptune), considered a
planet for half a century before reclassification as an asteroid;
2. Pluto – discovered on February 18, 1930, classified as a planet for 76 years;
3. Eris – discovered on October 21, 2003, once referred to as the "tenth planet" in
the media.