1. Pluto and the Kuiper Belt Objects (KBOs)

2. The 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.”

3. RESOLUTIONS 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.

4. About g/cm3

5. Discovered 1930 by C. Tombaugh.
Pluto
Discovered 1930 by C. Tombaugh.
Existence predicted from orbital disturbances of Neptune, but Pluto is actually too small to cause those disturbances.

6. Pluto and its moon, Charon, may be typical of a thousand icy objects that orbit far from the Sun
Pluto was discovered after a long search
Pluto and its moon, Charon, move together in a highly elliptical orbit steeply inclined to the plane of the ecliptic
They are the only worlds in the solar system not yet visited by spacecraft

7. The Discovery of Pluto
Pluto’s orbit is eccentric and inclined to the plane of the ecliptic; it also crosses the orbit of Neptune:

8. The Physical Properties of Pluto
Its orbit is at a large angle to the plane of Pluto’s orbit:

9. Pluto
Virtually 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.

10. Pluto 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.

12. The 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.

13. 2 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).

14. Pluto and its moons?

15. Pluto and its moons

16. The 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.

17. Several hundred small, icy worlds have been discovered beyond Neptune
Pluto and Charon are part of this population

18. Kuiper Belt Objects
Several hundred small, icy worlds have been discovered beyond Neptune
Classical Kuiper Belt Objects (CKBO)
Orbits far from Neptune
Scattered Kuiper Belt Objects (SKBO)
Come close enough to Neptune to be influenced (at least once every billion years)
Plutinos
In a 3:2 resonant orbit with Neptune. Just like Pluto! (more than 15 at present)
For more info on KBOs try:

19. Quaoar CKBO

20. Sedna An Oddity! Beyond even the SKBO!

22. ERIS

24. Ixion
Chaos

25. Semimajor axis Pluto 1930 C. Tombaugh occultation 2005 FY9 2005
Permanent
Provisional
Absolute
magnitude
Albedo
Equatorial
diameter
Semimajor axis
Date
found
Discoverer
Diameter method
Designation
(km)
(AU)
Pluto
−1.0
0.6
2320
39.4
1930
C. Tombaugh
occultation
2005 FY9
−0.2
(assumed)
~1900
45.7
2005
M. Brown, C. Trujillo & D. Rabinowitz
assumed albedo
2003 EL61
0.1
0.6 (assumed)
~1600
43.3
J.L. Ortiz et al.
Charon 1
0.4
1205
1978
J. Christy
(90482) Orcus
2004 DW
2.3
0.1 (assumed)
~1500
2004
(50000) Quaoar
2002 LM60
2.6
0.10 ± 0.03
1260 ± 190
43.5
2002
C. Trujillo & M. Brown
disk resolved
(28978) Ixion
2001 KX76
3.2
0.25 – 0.50
400 – 550
39.6
2001
Deep Ecliptic Survey
thermal
55636
2002 TX300
3.3
> 0.19
< 709
43.1
NEAT
55565
2002 AW197
0.14 – 0.20
650 – 750
47.4
C. Trujillo, M. Brown, E. Helin, S. Pravdo,
K. Lawrence & M. Hicks / Palomar Observatory
55637
2002 UX25
3.6
0.08?
~910
42.5
A. Descour / Spacewatch
(20000) Varuna
2000 WR106
3.7
0.12 – 0.30
450 – 750 43
2000
R. McMillan
2002 MS4
3.8
730?
41.8
2003 AZ84
3.9
700?