Jupiter

Jupiter is the largest of our Sun’s planets.  It has over twice as much matter in it as all the other planets put together.  It has been known since ancient times.  This is not surprising because itis the fourth brightest regular object in our sky.  The brightest is the Sun, followed by the Moon and Venus.  It was recognized a long time ago that the planets were different from the stars because of their quite different apparent motion in the sky.

Galileo

In 1610, Galileo pointed a telescope at Jupiter and discovered four of its Moons.  (Io, Europa, Ganymede and Callisto)  This was the first observation of astronomical motion not apparently centred on the Earth, and gave some indirect support to the wild idea that the Earth is not the centre of the universe.

Size

Jupiter is about143,000 kilometres (nearly 90,000 miles) in diameter at the equator.  This is probably close to the maximum size a planet can get.  Planets can be more massive than Jupiter, but as more matter is added, the gravity increases and the matter gets more compressed.  If enough extra matter in added, nuclear fusion starts and the object is no longer classed as a planet, but is called a star. The mass at which this will happen is argued about, but one upper limit used for planetary mass is 13 times that of Jupiter.  As mentioned in Extra Solar Planets, more massive planets than Jupiter have been discovered.

Gas Giant

Jupiter is one of the planets classed as a “Gas Giant”.  It probably has a rocky core.  Above the core, scientists guess that there is a layer of liquid metallic Hydrogen.  This form of Hydrogen is quite different from what we are used to, and only exists under a pressure of about 4,000,000 times that in the Earth’s atmosphere.

Gas Giants are believed to be composed mainly of Hydrogen and Helium, and their density tends to be less than that of the rocky planets.  However, a gas giant planet much more massive than Jupiter would have a fairly high density.

The average density of Jupiter is much less than that of the Earth, and is only about one and a third times that of water.  The gravity of Jupiter is about two and a half times that of the Earth.

Sources

 I am indebted to many sources for information about Jupiter, including: http://www.nineplanets.org/jupiter.html,, Science News and Science Daily.