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    Home / Science lessons / Learn About the Planets
    • Learn About the Planets

      Can you name the eight main planets orbiting our sun? To keep them all straight, your children might find it helpful to make a planet chart. Write down each one's size, distance from the sun, and other interesting facts about each one. Be sure to include pictures! (You can find this kind of information in an encyclopedia, the internet, or a good science dictionary. You can find pictures at the Exploring the Planets site.)

      Mercury is only visible at twilight (in the west) and sunrise (in the east) because its orbit is so close to the sun. Just how close is that, though? From Mercury, the sun is 'only' 36 million miles away. Mercury's temperatures change radically during the day: 800° to -200°F between day and night. Although small, Mercury is the densest planet after Earth.

      Venus, the second-closest planet to the sun, has the highest planetary temperature: 900°F/475°C. It also has the longest rotational period of any of the planets, the equivalent of 243 Earth days. You would need more than an umbrella if you got caught in a storm on Venus: it rains sulfuric acid there! Venus is the third brightest object in the sky, followed by Jupiter.

      Mars has a day length just slightly longer than Earth's, but its average surface temperature is a chilly -80°F. Do this experiment to demonstrate one of theories about why Mars has such a red surface. Put a layer of sand in the bottom of a ceramic baking dish. Cut some steel wool into 2 cm (1 inch) pieces and mix them with the sand, then cover the mixture with water. (You could also sprinkle iron filings on top of the sand instead of using steel wool.) You might need to add more water each day as some evaporates. Every day, check on the experiment and record how the surface is changing. How long did it take before the sand turned red like the surface of Mars? Scientists believe Mars' color occurs because of iron oxide (rust) in the soil.

      Jupiter, named after the king of the Roman gods, has a diameter more than 11 times the size of Earth's. One day on Jupiter is only 10 hours, though, since the planet rotates very quickly. One of Jupiter's moons, Io, has the most violent volcanic eruptions known in our solar system. You can actually see Io and three of Jupiter's other moons (Europa, Ganymede, and Callisto) with just binoculars. Galileo was the first to see these moons, using a telescope in the 1600s.

      Saturn is one of the 'gas planets,' along with Jupiter, Uranus, and Neptune. Like Jupiter, Saturn probably has a rocky core and outer envelope composed of liquid metallic hydrogen. However, Saturn is notable as the planet with the lowest density: it's less dense than water. Although all of the gas planets have rings around them, Saturn's are the brightest and most famous. In the mid-1800s, Scottish physicist James Clerk Maxwell correctly hypothesized that Saturn's rings must be made up of solid particles (such as ice) or else the rings could not maintain their stability.

      Uranus and Neptune are mostly rock and ice, with around 15% hydrogen. Uranus's satellites (moons or huge rocks orbiting it) are named after characters from Shakespeare's plays and from an Alexander Pope poem. Neptune has a really, really long sidereal period, or year: 165 Earth years would equal just one year on Neptune! Since Neptune wasn't discovered until 1846, the planet has yet to make a full orbit around the sun while we've been aware of it.

      Pluto was classified as one of the 'nine planets' for most of the 21st century, until a ruling by the International Astronomical Union in 2006 declared it a dwarf planet instead. Debate continues even among scientists whether this is an accurate definition, since it orbits a star (our sun) but is not a star or a moon itself, just like the other planets. However, Pluto has a somewhat wacky orbit and sometimes nips in closer to the sun and leaves Neptune behind as the farthest planet. It is also surrounded by objects in the Kuiper Belt, which it orbits through. The other planets have cleared the area they orbit through. Pluto has colder temperatures than the eight bigger planets: -400°F/-250°C. In fact, it's so cold there that it 'snows' methane gas crystals.

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