chap 7

• The solar system consists of the Sun, and many smaller objects: the planets, their moon and rings, and debris such as asteroids, comets, and dust.

  • PLANETARY ASTRONOMY only branch of science in which we can, at least vicariously, travel to the objects we want to study.
• The Sun, a star that is brighter than about 80% of the stars in the galaxy, is by far the most massive member of the solar system.

  Beside Earth, five other planets were know to the ancients - Mercury, Venus, Mars, Jupiter, and Saturn -- and two were discovered after the invention of the telescope: Uranus and Neptune.
• • TRANS-NEPTUNIAN OBJECTS (TNOs) smaller worlds beyond Neptune and the first to be found in 1930, was Pluto.

  Pluto has five known moons.
• The largest TNOs are classed as dwarf planets, as is the largest asteroid, ceres.

  More than 2600 of these TNOs have been discovered, and one called Arrokoth, was explored by the New Horizons spacecraft.