2The IAU draft definition of "planet" and "plutons" Q: What is a “double planet”? A: A pair of objects, which each independently satisfy the definition of “planet” are considered a “double planet” if they orbit each other around a common point in space that is technically known as the “barycentre”. In addition, the definition of “double planet” requires that this “barycentre” point must not be located within the interior of either body. Q: What is a “satellite” of a planet? A: For a body that is large enough (massive enough) to satisfy the definition of “planet”, an object in orbit around the planet is called a “satellite” of the planet if the point that represents their common centre of gravity (called the “barycentre”) is located inside the surface of the planet. Q: The Earth’s moon is spherical. Is the Moon now eligible to be called a “planet”? A: No. The Moon is a satellite of the Earth. The reason the Moon is called a “satellite” instead of a “planet” is because the common centre of gravity between the Earth and Moon (called the “barycentre”) resides below the surface of the Earth. Q: Why is Pluto-Charon a “double dwarf planet” and not a “dwarf planet with a satellite”? A: Both Pluto and Charon each are large enough (massive enough) to be spherical. Both bodies independently satisfy the definition of “dwarf planet”. The reason they are called a “double dwarf planet” is that their common centre of gravity is a point that is located in free space outside the surface of Pluto. Because both conditions are met: each body is “planet-like” and each body orbits around a point in free space that is not inside one of them, the system qualifies to be called a “double dwarf planet.”
3RESOLUTIONS 5 and 6 Definition of a Planet in the Solar System and Pluto 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 neighborhood around its orbit.(2) A "dwarf planet" is a celestial body that(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 neighborhood 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".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.1. The eight planets are: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.2. An IAU process will be established to assign borderline objects into either dwarf planet and other categories.3. These currently include most of the Solar System asteroids, most Trans-Neptunian Objects (TNOs), comets, and other small bodies.
5Discovered 1930 by C. Tombaugh. PlutoDiscovered 1930 by C. Tombaugh.Existence predicted from orbital disturbances of Neptune, but Pluto is actually too small to cause those disturbances.
6Pluto and its moon, Charon, may be typical of a thousand icy objects that orbit far from the Sun Pluto was discovered after a long searchPluto and its moon, Charon, move together in a highly elliptical orbit steeply inclined to the plane of the eclipticThey are the only worlds in the solar system not yet visited by spacecraft
7The Discovery of PlutoPluto’s orbit is eccentric and inclined to the plane of the ecliptic; it also crosses the orbit of Neptune:
8The Physical Properties of Pluto Its orbit is at a large angle to the plane of Pluto’s orbit:
9PlutoVirtually no surface features visible from Earth.~ 65 % of size of Earth’s Moon.Highly elliptical orbit; coming occasionally closer to the sun than Neptune.Orbit highly inclined (17o) against other planets’ orbits Neptune and Pluto will never collide.Surface covered with nitrogen ice; traces of frozen methane and carbon monoxide.Daytime temperature (50 K) enough to vaporize some N and CO to form a very tenuous atmosphere.
10Pluto and Charon From separation and orbital period: Mpluto ~ 0.2 Earth masses.Density ≈ 2 g/cm3(both Pluto and Charon) ~ 35 % ice and 65 % rock.Large orbital inclinations Large seasonal changes on Pluto and Charon.
12The Physical Properties of Pluto Pluto’s moon, Charon, was discovered in 1978.It is tidally locked to Pluto, about half the size, and 1/12 the mass of Pluto itself.
132 New Moons for Pluto!The candidate moons are approximately 27,000 miles (44,000 kilometers) away from Pluto--in other words, two to three times as far from Pluto as Charon.These are tiny moons. Their estimated diameters lie between 40 and 125 miles (64 and 200 kilometers). Charon, for comparison, is about 730 miles (1170 km) wide, while Pluto itself has a diameter of about 1410 miles (2270 km).
16The Origin of Pluto and Charon Probably very different history than neighboring Jovian planets.Older theory:Pluto and Charon formed as moons of Neptune, ejected by interaction with massive planetesimal.Mostly abandoned today since such interactions are unlikely.Modern theory: Pluto and Charon members of Kuiper belt of small, icy objects.Collision between Pluto and Charon may have caused the peculiar orbital patterns and large inclination of Pluto’s rotation axis.
17Several hundred small, icy worlds have been discovered beyond Neptune Pluto and Charon are part of this population
18Kuiper Belt ObjectsSeveral hundred small, icy worlds have been discovered beyond NeptuneClassical Kuiper Belt Objects (CKBO)Orbits far from NeptuneScattered Kuiper Belt Objects (SKBO)Come close enough to Neptune to be influenced (at least once every billion years)PlutinosIn a 3:2 resonant orbit with Neptune. Just like Pluto! (more than 15 at present)For more info on KBOs try: