MISSION PLANETARY EXPLORATION Have you ever looked up at the night sky On a clear night you can see thousands of lights Most of these lights are stars But if you're lucky you might see a planet Planets are often bigger and brighter than stars Our Home Planet If you wanted to visit a planet you wouldn't need to travel very far In fact you wouldn't need to travel at all Earth is the planet we live on It is also the only known planet that can support life Visiting Venus Other planets are far away Venus is usually the closest planet to Earth In 2005 it took the Venus Express spacecraft five months to reach Venus from Earth
In this photograph the two brightest lights are the planets Jupiter left and Venus right They are much brighter than the stars around them A Long Long Trip It would take a long time to visit a planet outside our solar system There our nearest known planet orbits the star Alpha Centauri B It would take more than 60 000 years to reach this planet in our fastest spacecraft
CHAPTER 1 WHAT ARE PLANETS Planets are celestial bodies that orbit a star An official planet must meet three requirements First it must be spherical in shape Second it must be large enough to have its own gravitational pull And finally it can't have any larger planets or objects in its orbit ALL ABOUT GRAVITY Everything with mass has a gravitational pull even you The more mass an object has the stronger its gravitational pull The massive sun has a very powerful pull It is strong SUN MERCURY VENUS EARTH MARS JUPITER
DID YOU KNOW If Jupiter were hollow all the other planets in our solar system would fit inside it enough to attract all the planets in our solar system Planets have less mass than the stars they orbit A planet's gravitational pull isn't strong enough to attract objects more massive than itself But planets may attract moons ice dust or other less massive objects These become satellites They orbit the planet Planets come in many sizes Some are very big Jupiter is the largest planet in our solar system It is more than 1 000 times bigger than Earth Mercury is the smallest planet in our solar system It is about three times smaller than Earth SATURN URANUS NEPTUNE
