Pluto is different
TNOIt is smaller than they thought at first, even smaller than Mercury. Yet, yet it has a sizeable moon, Charon
Its path around the Sun is more elongated than the other planets. It is so elongated that it is closer to the Sun than Neptune for about 20 years of its 248-year orbital period.
Another oddity about Pluto is the inclination of its orbit. All the other planets follow paths in a narrow band within the boundaries of the constellations we call the zodiac, but Pluto's orbit takes it well above and below that band.
In 1992, astronomers began finding smaller Pluto-like bodies in the distant reaches of the Solar System called the Kuiper (KI-per) Belt. In 2002, when nearly 100 Kuiper Belt Objects (KBOs) were already known, astronomers found a particularly large one, about half the size of Pluto, that they named Quaoar (kwa-oh-wahr) after a creation god of the Native American Tongva tribe. Its discovery was especially big news since it orbits the sun once in 288 years and has a more planet-like orbit than Pluto-Charon (more circular and less inclined).
Scientists who study the way planetary systems formed have been speaking for years about a region far beyond the Kuiper Belt where many comets are born. Called the Oort Cloud, it is full of icy bodies that orbit the sun at about thirty times the distance of Pluto. In November 2003, astronomers observed a large icy body far beyond Pluto. By March 2004, they recognized that it was the first known Oort Cloud object. They named it Sedna for the Inuit goddess of the ocean, and reported a number of details about it and its orbit.
Sedna is nearly as red as Mars, but too far away and dim for scientists to determine what is is made of (besides ice). Because it is nearing its closest point to the Sun, Sedna is experiencing a warm spell. Its temperature is probably a balmy 240 degrees below zero Celsius (400 degrees below zero Fahrenheit). At its most distant point, it will be many degrees colder. The best estimate of its size is 1700 kilometers (1000 miles), halfway between Pluto and Quaoar. That's large enough for the NASA news release to call it a "planet-like body."
On July 29, 2005, Dr. Mike Brown of the California Institute of Technology announced that he and fellow astronomers Chad Trujillo (of the Gemini Observatory in Hawaii) and David Rabinowitz (of Yale University) had found a world bigger than Pluto. On October 31, 2003, they first photographed it using the Samuel Oschin Telescope at Palomar Observatory near San Diego. But it is so far from the Sun that it moves very slowly across the sky. (Its orbital period is 560 years.) The astronomers only detected that motion when they took a second look at their images in January, 2005. By early 2006, they had determined that it is approximately 3000 kilometers (nearly 1900 miles) in diameter, or almost as large as Earth's Moon.
Finally, Why isn't Pluto a Planet?
It is not a planet because it did not fit the definition of a planet per the following IAU's Resolutions 5 and 6 adopted in August 2006 in Prague.
RESOLUTION 5: Definition of a Planet in the Solar System
Contemporary observations are changing our understanding of planetary systems, and it is important that our nomenclature for objects reflect our current understanding. This applies, in particular, to the designation "planets". The word "planet" originally described "wanderers" that were known only as moving lights in the sky. Recent discoveries lead us to create a new definition, which we can make using currently available scientific information.
The IAU therefore resolves that planets and other bodies, except satellites, in our Solar System be defined into three distinct categories in the following way:
(1) A 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, 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,(c) has not cleared the neighbourhood around its orbit, and (d) is not a satellite. (3) All other objects,except satellites, orbiting the Sun shall be referred to collectively as "Small Solar System Bodies".
RESOLUTION 6: Pluto
The IAU further resolves: Pluto is a "dwarf planet" by the above definition and is recognized as the prototype of a new category of Trans-Neptunian Objects.
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