Descriptive Astronomy 101
Planetary Geology - Kuiper Belt and Oort Cloud

Kuiper Belt

A region of the outer Solar System populated by an estimated ten billion to one trillion rock-ice bodies known as Kuiper Belt Objects (KBO's).
It stretches from 30 AU to at least 50 AU
The source region for short-period comets and centaurs.
Short-period comets: icy objects with orbital periods of less than 200 years.
Centaurs: minor planets that orbit around the Sun between the orbits of Jupiter and Neptune.
Estimates suggest that as many as 70,000 Kuiper Belt Objects with a diameter of more than 100 km may exist in this belt.

Eris & Sedna

2003 UB313 - nicknamed "Xena"
The largest known Kuiper Belt Object.
Approx. 2,400 km diameter
Eccentric orbit from 38 AU to 97.6 AU
Eccentric orbit from 76 AU to 928 AU


Pluto is the best known of the dwarf planets, but very little is known about it.
No past missions to Pluto
Other probes to the other outer planets did not come close to Pluto
New Horizons Probe (formerly known as the "Pluto Express")
Planned arrival at Pluto:

Pluto Statistics

Mean distance from Sun: 5,910 million km
Mean distance from Sun: 39.4 AU
Diameter: 2,320 km
Density: 2.02 g/cm3
Axial period: 6.39 days
Orbital period: 247.7 years
Orbital inclination: 17.2°
Orbital eccentricity: 0.248
Number of moons: 3


Axial inclination = 122°
Pluto spins on its side - like Uranus


Very thin atmosphere with a surface pressure 100,000 times less than that on Earth.
Composed of 98% nitrogen and small amounts of methane and carbon monoxide.
Pluto's atmosphere gradually freezes and collects on the surface as the planet moves away from the Sun.

Interior Structure

Not very well understood at present.
Surface is covered with:
Surface temperature is about -230°C

Moons of Pluto

Pluto has three known satellites.
Two small outer satellites were discovered in 2005
Charon, the largest, is more than half its diameter
The Pluto-Charon system is often referred to as a double planet.
Both Pluto and Charon are tidally locked to each other
Albedo and density data suggest that the two bodies formed separately.
Charon seems to be mostly ice with very little rock.

Pluto: Dwarf Planet or KBO?

Very eccentric and inclined orbit.
Pluto is small when compared to other planets. Diameter = 2300 km.
All of the Galilean moons of Jupiter are larger than Pluto
Largest known asteroid, Ceres, is 457 km diameter

Planetary Statistics

Earth 6370 km
Venus 6051 km
Mars 3394 km
Ganymede 2631 km satellite of Jupiter
Titan 2575 km satellite of Saturn
Mercury 2439 km
Callisto 2400 km satellite of Jupiter
Io 1815 km satellite of Jupiter
Moon 1738 km satellite of Earth
Europa 1569 km satellite of Jupiter
Triton 1350 km satellite of Neptune
Eris 1200 km Largest known KBO
Pluto 1190 km
Titania 790 km satellite of Uranus
Ceres 457 km asteroid


There is now a commonly agreed upon definition of a planet.
Pluto is a dwarf planet, but also is the second largest known Kuiper Belt object.

Oort Cloud

Beyond the Kuiper Belt
A vast reservoir of ice dwarfs, or deep-frozen cometary nuclei.
Marks the material outer limit of the solar system.
The maximum extent of the Sun's gravitational influence.
50 to 50,000 AU
Source of long-period comets - A comet with a period of more than 200 years to as much as several million years.
Formed from icy bodies (comets) coalesced in the vicinity of Jupiter, Saturn, Uranus, and Neptune, and were subsequently hurled into the Oort Cloud by gravity-assists from the giant planets.