15. Beyond Neptune

    • The discovery of Pluto was serendipitous, the result of a long careful search based on incorrect predictions.

    • 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 makings.

    • 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 AU respectively.

    • 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