Planet Facts



  • Planets are globe-shaped space objects that orbit a star.
  • Planets begin life at the same time as their star, from the left over clouds of gas and dust.
  • Planets are never more than 20 percent of the size of their star. If they were bigger, they would have become stars.
  • Some planets, called terrestrial planets, have a surface of solid rock. Others, called gas planets, have a surface of liquid or airy gas.
  • The solar system has nine planets including Pluto. But Pluto may be an escaped moon or an asteroid, not a planet.
  • Giant planets have now been detected orbiting stars other than the Sun. These are called extra-solar planets.
  • Extra-solar planets are too far away to see, but can be detected because they make their star wobble.
  • One extra-solar planet has now been photographed.
  • Among the nine stars so far known to have planets are 47 Ursae Majoris, 51 Pegasi, and 70 Virginis.
  • Four of the new planets – called 51 Peg planets, after the planet that circles 51 Pegasi – seem to orbit their stars in less than 15 days. The planet orbiting Tau Bootis gets around in just 3.3 days!