If you spend time observing the solar system, you will notice that some planets — especially Venus and Mercury — seem to move backward in the sky. These planets don't really move backward, but they appear to move backward because their position in relation to Earth is changing. It's the same thing that happens when your car passes another car on the highway. The car you are passing seems to be going backward, but it's only because your car has gone past it. This odd backward motion is called "retrograde motion." A planet can also have "retrograde rotation," which means it rotates in the opposite direction of its orbit. Most planets in our solar system have "prograde rotation," which means they rotate in the same direction as their orbits.
Our planet Earth is part of a solar system that consists of nine (and possibly ten) planets orbiting a giant, fiery star we call the sun. For thousands of years, astronomers studying the solar system have noticed that these planets march across the sky in a predictable way. They've also noticed that some move faster than others . . . and some seem to be moving backward.
The Sun: The Center of Our Solar System
The sun (which, incidentally, is only a medium-size star) is larger than any of the planets in our solar system. Its diameter is 1,392,000 kilometers (864,949 miles). Earth's diameter is only 12,756 kilometers (7,926 miles). More than one million Earths could fit inside the sun. The large mass of the sun produces an enormous gravitational pull that keeps all the planets of the solar system in their orbits. Even Pluto, which is six billion kilometers (3,728,227,153 miles) away, is kept in orbit by the sun.Planets in Our Solar System
Each planet in our solar system is unique, but they all have a few things in common, too. For example, every planet has a north and a south pole. These points are in the center of the planet at its ends. A planet's axis is an imaginary line that runs through the center of the planet and connects the north and south poles. The imaginary line that runs around the planet at its middle (like your waist) is called its equator. While every planet rotates on its axis, some planets rotate quickly and some rotate slowly. The time that it takes for a planet to rotate once on its axis is its rotation period. For most planets in our solar system, the rotation period is close to the length of its day. (A day length is the time between sunrises at the same point on the planet.) Mercury and Venus are exceptions.
As each planet in our solar system rotates on its axis, it also revolves around the sun. The time that it takes for a planet to make a complete revolution around the sun is the planet's year. The path that the planet follows around the sun is called its orbit. Different planets have different orbits — and orbits can take different shapes. Some orbits are nearly circular and some are more elliptical (egg-shaped).What Else Is in Our Solar System?
Although we tend to think only about the sun and the planets when we consider our solar system, there are many other types of bodies that huddle around the sun along with Earth and its planetary brothers and sisters. The solar system includes moons (and some of those moons have moons), supernova explosions, comets, meteors, asteroids, and plain old space dust. To be sure, there are more objects in our solar system, some of which have yet to be discovered.What's This About a Tenth Planet in Our Solar System?
A name like 2003 UB313 doesn't sound very exciting, but this far-flung body of rock and ice has rocked the world of astronomy. At about 3,000 kilometers (1,864 miles) across, it is a bit larger than Pluto, and it looks to be a lot farther away — about three times as far. But it does travel in the same circles (orbitpaths) as the other nine planets in the solar system. There isn't a question about its presence. It has been sighted from quite a few spots (the Palomar Observatory and the Gemini North telescope on MaunaKea to name two). The real question is: What constitutes a planet? And does 2003 UB313 meet the criteria? It has been 75 years since the last planet was discovered in our solar system and the International Astronomical Union is still debating whether Pluto qualifies as a planet. On the other hand, NASA has referred to 2003 UB313 as our tenth planet, a pretty hefty endorsement by any standard. Maybe 75 years from now they'll still be debating over its status.