Planets

1. What is a Planet?
FAR-OUTFACTS
• Our solar system’s eight planets come in two flavours: smaller rocky planets with solid ground
(Mercury, Venus, Earth and Mars) and larger gas giants (Jupiter, Saturn, Uranus, and Neptune).
• You can spot six of the solar system’s planets with the naked eye from Earth—and all eight if you
have binoculars or a telescope.
• Astronomers have discovered hundreds of worlds beyond our solar system and are spotting
more every day. Scientists believe some of these “exoplanets” may support alien life. Who
knows? Maybe an alien kidelsewhere inthe galaxy is wondering ifyou exist.
Thousands of years before the invention of the Internet or Super Mario Kart, people entertained
themselves by peering into the night sky and connecting the stars to create pictures—called
constellations—of animals, objects, and legendary heroes. But these ancient stargazers noticed that
some stars didn’t cooperate with the constellations. These oddball balls of light followed their own
paths across the night sky, so the ancient Greeks called them “planetes,” meaning wanderers. By 1930,
nine planets had been discovered: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and
Pluto.
But what exactly is a planet? It seems like an easy question. After all, you’re standing on one right now!
But the scientific definition was fairly loose until recently. The discovery of other solar-system
“wanderers” rivaling Pluto in size suddenly had scientists asking what wasn’t a planet. They put their
heads together in 2006 and came up with three conditions for planethood: A planet must orbit the sun,
be large enough so that its own gravity molds it into a spherical shape, and it must have an orbit free of
other small objects. Unfortunately for Pluto, our one-time “ninth planet” failed to meet the third
condition. It was downgraded toa “dwarf planet”.