15. Beyond Neptune
- The discovery of Pluto was
serendipitous, the result of a long careful search based on incorrect
- Pluto moves between 29.7 and 49.3 AU from
the Sun, and just crosses within the orbit of Neptune, at 30 AU without ever
colliding with it. One AU is the mean distance between the Earth and the Sun.
- Pluto has a thin atmosphere of
nitrogen, methane and carbon dioxide, and large variable dark and light surface
- Pluto has three companion moons,
named Charon, Nix and Hydra. Charon is an oversized satellite, about half the
size of Pluto.
- The New Horizons spacecraft will encounter Pluto in 2015, to study its
atmosphere, surface and interior, then traveling into the Kuiper belt.
- Centaurs are small bodies that
orbit the Sun between Jupiter and Neptune and cross the orbit of one or more
giant planets. They are thought to originate further away from the Sun in the
Kuiper belt. Some centaurs display comet comas, and most of them are expected
to become comets.
- Several trans-Neptunian objects
orbit the Sun beyond the orbit of Neptune, including Eris, Makemake and Haumea,
which are either larger than Pluto or comparable to it in size.
- Pluto has been demoted from the
ninth major planet to a dwarf planet, and then reclassified as a plutoid.
- A dwarf planet orbits the Sun and
has a rounded shape, but it does not clear out the orbit around it. The
asteroid 1 Ceres has been designated a dwarf planet.
- Plutoids are trans-Neptunian
dwarf planets, and they include Eris, Haumea, Makemake and Pluto.
- The Voyager 1 and 2 spacecraft have crossed the termination shock of the solar wind at 94 AU and 84
- The outer Oort cloud lies between
20,000 and 200,000 AU, far beyond the termination of the solar wind, but still
within the Sunís gravitational control.
Copyright 2010, Professor Kenneth R. Lang, Tufts University