Dwarf Planets: history,Exploration,mass moons,Name of dwarf Planets

Dwarf Planets 

(Dwarf Planets: history,Exploration,mass moons,Name of dwarf Planets)

A dwarf planet is a planetary-mass object that is neither a planet nor a natural satellite. That is, it is in direct orbit of a star, and is massive enough for its gravity to crush it into a hydrostatically equilibrious shape (usually a spheroid), but has not cleared the neighborhood of other material around its orbit.

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 objects farther away from the Sun than Neptune that rivaled Pluto in size, and finally precipitated by the discovery of an even more massive object, Eris. The exclusion of dwarf planets from the roster of planets by the IAU has been both praised and criticized; it was said to be the "right decision" by astronomer Mike Brown, who discovered Eris and other new dwarf planets, but has been rejected by Alan Stern, who had coined the term dwarf planet in April 1991.

The International Astronomical Union (IAU) currently recognizes five dwarf planets: Ceres; Pluto; Haumea; Makemake; and Eris. Brown criticizes this official recognition: "A reasonable person might think that this means that there are five known objects in the solar system which fit the IAU definition of dwarf planet, but this reasonable person would be nowhere close to correct."

Another hundred or so known objects in the Solar System are suspected to be dwarf planets. Estimates are that up to 200 dwarf planets will be identified when the entire region known as the Kuiper belt is explored, and that the number may exceed 10,000 when objects scattered outside the Kuiper belt are considered.[dead link] Individual astronomers recognize several of these, and in August 2011 Mike Brown published a list of 390 candidate objects, ranging from "nearly certain" to "possible" dwarf planets. Brown currently identifies ten known trans-Neptunian objects—the four accepted by the IAU plus 2007 OR10, Quaoar, Sedna, Orcus, (307261) 2002 MS4 and Salacia—as "virtually certain", with another twenty highly likely. Stern states that there are more than a dozen known dwarf planets.

Only two of these bodies, Ceres and Pluto, have been observed in enough detail to demonstrate that they actually fit the IAU's definition. The IAU accepted Eris as a dwarf planet because it is more massive than Pluto. They subsequently decided that unnamed trans-Neptunian objects with an absolute magnitude brighter than +1 (and hence a diameter of ≥838 km assuming a geometric albedo of ≤1) are to be named under the assumption that they are dwarf planets.

The classification of bodies in other planetary systems with the characteristics of dwarf planets has not been addressed.

History of the concept

Starting in 1801, astronomers discovered Ceres and other bodies between Mars and Jupiter which were for decades considered to be planets. Between then and around 1851, when the number of planets had reached 23, astronomers started using the word asteroid for the smaller bodies and then stopped naming or classifying them as planets.

With the discovery of Pluto in 1930, most astronomers considered the Solar System to have nine planets, along with thousands of significantly smaller bodies (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 Pluto's mass accurately and to determine that it was much smaller than initial estimates. 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 had one-fifth the mass 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 different kind of 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 several of Pluto's key orbital characteristics, 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 subplanet and planetoid, started to be used for the bodies now known as dwarf planets. By 2005, three trans-Neptunian objects comparable in size to Pluto (Quaoar, Sedna, and Eris) had been reported. It became clear that either they would 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.

Eris (then known as 2003 UB313) was discovered in January 2005; it was thought to be slightly larger than Pluto, and some reports informally 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. The 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: he proposed an intermediate category for objects large enough to be round but which had not cleared their orbits of planetesimals. Dropping Charon from the list, the new proposal also removed Pluto, Ceres, and Eris, because they have not cleared their orbits.

The IAU's final Resolution 5A preserved this three-category system for the celestial bodies orbiting the Sun. It reads:

The IAU ... resolves that planets and other bodies, except satellites, in our Solar System be defined into three distinct categories in the following way:

(1) A planet1 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 hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighbourhood around its orbit.

(2) A "dwarf planet" 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 hydrostatic equilibrium (nearly round) shape,2 (c) has not cleared the neighbourhood around its orbit, and (d) is not a satellite.

(3) All other objects,3 except satellites, orbiting the Sun shall be referred to collectively as "Small Solar System Bodies."

Recognized

The IAU has recognized five bodies as dwarf planets since 2008: Ceres, Pluto, Eris, Haumea, and Makemake. Ceres and Pluto are known to be dwarf planets through direct observation. Eris is recognized as a dwarf planet because it is more massive than Pluto (measurements by New Horizons indicate that Pluto's diameter is larger than that of Eris), whereas Haumea and Makemake qualify based on their absolute magnitudes. In relative distance from the Sun, the five are:

1.            Ceres  – discovered on January 1, 1801, 45 years before Neptune. Considered a planet for half a century before reclassification as an asteroid. Accepted as a dwarf planet by the IAU on September 13, 2006.

2.            Pluto ♇ – discovered on February 18, 1930. Classified as a planet for 76 years. Reclassified as a dwarf planet by the IAU on August 24, 2006.

3.            Haumea – discovered on December 28, 2004. Accepted by the IAU as a dwarf planet on September 17, 2008.

4.            Makemake – discovered on March 31, 2005. Accepted by the IAU as a dwarf planet on July 11, 2008.

5.            Eris – discovered on January 5, 2005. Called the "tenth planet" in media reports. Accepted by the IAU as a dwarf planet on September 13, 2006.

Exploration

On March 6, 2015, the Dawn spacecraft began to orbit Ceres, becoming the first spacecraft to orbit a dwarf planet. On July 14, 2015, the New Horizons space probe flew by Pluto and its five moons. Dawn has also explored the former dwarf planet Vesta. Phoebe has been explored by Cassini (most recently) and Voyager 2, which also explored Triton. These three are thought to be former dwarf planets and therefore their exploration helps in the study of the evolution of dwarf planets.

Planetary-mass moons

Nineteen moons are known to be massive enough to have relaxed into a rounded shape under their own gravity, and seven of them are more massive than either Eris or Pluto. They are not physically distinct from the dwarf planets, but are not dwarf planets because they do not directly orbit the Sun. The seven that are more massive than Eris are the Moon, the four Galilean moons of Jupiter (Io, Europa, Ganymede, and Callisto), one moon of Saturn (Titan), and one moon of Neptune (Triton). The others are six moons of Saturn (Mimas, Enceladus, Tethys, Dione, Rhea, and Iapetus), five moons of Uranus (Miranda, Ariel, Umbriel, Titania, and Oberon), and one moon of Pluto (Charon). There are additional possibilities among TNOs, including Dysnomia orbiting Eris. Alan Stern calls these moons "satellite planets", one of three categories of planet together with dwarf planets and classical planets. The term planemo ("planetary-mass object") covers all three.

In a draft resolution for the IAU definition of planet, both Pluto and Charon would have been considered dwarf planets in a binary system, given that they both satisfied the mass and shape requirements for dwarf planets and revolved around a common center of mass located between the two bodies (rather than within one of the bodies).[note 1] The IAU currently states that Charon is not considered to be a dwarf planet and is just a satellite of Pluto, although the idea that Charon might qualify to be a dwarf planet in its own right may be considered at a later date. The location of the barycenter depends not only on the relative masses of the bodies, but also on the distance between them; the barycenter of the Sun–Jupiter orbit, for example, lies outside the Sun.

Dwarf planets and possible dwarf planets

Many trans-Neptunian objects (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. As of January 2015, about 150 known TNOs are considered potential dwarf planets, although only rough estimates of the diameters of most of these objects are available. A team is investigating thirty of these, and think that the number will eventually prove to be around 200 in the Kuiper belt, with thousands more beyond.