Pluto and Xena (Dwarf Planets)

Pluto and Xena (Dwarf Planets)

Pluto, is the second-largest known dwarf planet in the Solar System (after Eris) and the tenth-largest body observed directly orbiting the Sun. Originally classified as a planet, Pluto is now considered the largest member of a distinct population called the Kuiper belt.

Like other members of the Kuiper belt, Pluto is composed primarily of rock and ice and is relatively small: approximately a fifth the mass of the Earth's Moon and a third its volume. It has a highly eccentric and highly inclined orbit.

Pluto's eccentricity takes it from 30 to 49 AU (4.4-7.4 billion km) from the Sun, causing Pluto to occasionally come closer to the Sun than Neptune. Pluto and its largest moon, Charon, are often treated together as a binary system because the barycentre of their orbits does not lie within either body.

The International Astronomical Union (IAU) has yet to formalise a definition for binary dwarf planets, and until it passes such a ruling, Charon is classified as a moon of Pluto. Pluto has two known smaller moons, Nix and Hydra, discovered in 2005.

From its discovery in 1930 until 2006, Pluto was considered the Solar System's ninth planet. In the late 20th and early 21st centuries, however, many objects similar to Pluto were discovered in the outer solar system, notably the scattered disc object Eris, which is 27% more massive than Pluto.

On August 24, 2006, the IAU defined the term "planet" for the first time. This definition excluded Pluto, which the IAU reclassified as a member of the new category of dwarf planets along with Eris and Ceres: Just as Ceres is part of the asteroid belt without being dominant within it, so Pluto is part of the Kuiper belt without being dominant within it.

After the reclassification, Pluto was added to the list of minor planets and given the number 134340. A number of scientists continue to hold that Pluto should be classified as a planet. This is still being debated.

`Xena is out of this world! --NASA/JPL-Caltech This artist's concept is of the world currently cataloged as 2003 UB313. The "10th planet" was discovered with the Samuel Oschin Telescope at the Palomar Observatory near San Diego. For now, they're calling it Xena. Astronomers have discovered what many are saying is our Solar System's 10th planet. It's reportedly to be named Xena, and its discovery is the first of a planet since Pluto was found 75 years ago. Xena is an object whose size hasn't been precisely determined, but it could be about 1.5 times the size of Pluto. That makes it a frozen world perhaps a little smaller than our Moon, but one much farther away. For now, the new world is officially dubbed 2003 UB313. The International Astronomical Union will have to rule on any less-esoteric designation. By that or any other name, however, it's a truly significant find. It's the largest nonstellar object now known to exist beyond the orbit of Neptune. And, at roughly 97 AU, or Astronomical Units (one AU equals the Earth's distance from the Sun, about 93 million miles) it's also the most distant. First imaged by Caltech astronomers in 2003 - hence the 2003 in its temporary moniker - it wasn't recognized as part of the Solar System till later. Then scientists re-examined images taken of the region over time and noticed an object that appeared to move against the stellar field. That motion indicated that UB313 was not a distant star but an object orbiting the Sun. And the investigation began in earnest. Official announcement of "Xena" was made July 29 by discoverers Michael E. Brown of Caltech, Chad Trujillo of Gemini Observatory and David Rabinowitz of Yale University. It was somewhat premature, prompted leaks of the data and images of the new planet. The disclosure on UB313 came just hours after an announcement that another team had discovered another world beyond Neptune that was somewhat smaller than Pluto. UB313 is in a highly elliptical orbit inclined about 45 degrees from the main plane of our Solar System. Currently it's near aphelion (farthest distance from the Sun) at some 9 billion miles from the center of the Solar System. It gets as close to the Sun as 38 A.U., or some 3.5 billion miles, during a 557-year orbit. By contrast, Pluto's mean distance from the Sun is just 39 A.U., or 3.6 billion miles, and it orbits in just 248.5 years. The new world is thought to be about 2,100 miles in diameter, roughly 1.5 times as large as Pluto. Near-infra-red spectrometry indicates that, like Pluto, it has methane ice on its surface. "Xena" has not been discovered until now, because no one had thought to look in an orbit so far removed from those of the other planets in our Solar System.

There are other Dwarf Planets in the Solar System, but Pluto and Xena are the most well-known.

Xena was named after the protagonist of the TV series "Xena, Warrior Princess"