planets, orbits

Planets 

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. Some planets, called terrestrial planets, have a surface of solid rock. 

planets in orbits

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. A Most of the nine planets in our Solar System have been known since ancient times, but in the last few years planets have been found orbiting other, far away stars.

 The planets of our Solar System: 

from the front, Neptune, Uranus, Saturn, Jupiter, Mars, Earth and its moon, Venus and Mercury. Pluto (not shown) is the furthest out from the Sun. These all planets move in orbits around the sun.

 Orbits

 Orbit means ‘travel round’, and a moon, planet or other space object may be held within a larger space object’s gravitational field and orbit it.
 Space stations are artificial satellites that orbit the Earth. The Moon is the Earth’s natural satellite. Orbits may be circular, elliptical (oval) or parabolic (conical). The orbits of the planets are elliptical. An orbiting space object is called a satellite. 
 The biggest-known orbits are those of the stars in the Milky Way, which can take 200 million or more years. Momentum is what keeps a satellite moving in space. How much momentum a satellite has depends on its mass and its speed. 
A satellite orbits at the height where its momentum exactly balances the pull of the larger object’s gravity. If the gravitational pull is greater than a satellite’s momentum, it falls in towards the larger space object. If a satellite’s momentum is greater than the pull of the larger object’s gravity, it flies off into space. 
The lower a satellite orbits, the faster it must travel to stop it falling in towards the larger space object. Geostationary orbit for one of Earth’s artificial satellites is 35,786 km over the Equator. At this height, it must travel around 11,000 km/h to complete its orbit in 24 hours. 
Since Earth also takes 24 hours to rotate, the satellite spins with it and so stays in the same place over the Equator. Earth is about 150 million km from the Sun. However, this distance varies as our planet's orbit is slightly oval. The time it takes for Earth to orbit the Sun once is 365 days. In comparison, Pluto takes 248 years to orbit the Sun.