Earth and the other three inner planets of our solar system (Mercury, Venus and Mars) are made of rock, containing common minerals like feldspars and metals like magnesium and aluminum. So is Pluto. The other planets are not solid. Jupiter, for instance, is made up mostly of trapped helium, hydrogen, and water.

Planets form from particles in a disk of gas and dust, colliding and sticking together as they orbit the star. The planets nearest to the star tend to be rockier because the star’s wind blows away their gases and because they are made of heavier materials attracted by the star’s gravity.

A star is born when atoms of light elements are squeezed under enough pressure for their nuclei to undergo fusion. All stars are the result of a balance of forces: the force of gravity compresses atoms in interstellar gas until the fusion reactions begin.

A planet is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighbourhood around its orbit.

The main difference between stars and planets is that stars have high temperatures compared to planets. Stars undergo nuclear reactions—usually they burn hydrogen in their cores, releasing massive amounts of energy. Planets do not generate their own energy through nuclear reactions.

Mercury

Unlike the Earth’s atmosphere, which is mainly composed of nitrogen (78%) and oxygen (21%), Venus’ atmosphere contains about 96% carbon dioxide, 3% nitrogen, some argon and traces of water vapour (varying from 0.1 to 0.4%), oxygen, hydrogen chloride, hydrogen fluoride, hydrogen sulphide, sulphur dioxide, and carbon …

So there’s no water on the surface of Venus today. This planet – orbiting next-inward from Earth around the sun – is one of the most inhospitable places in our solar system. But scientists think that, a few billion years ago, Venus might have had oceans, perhaps much like those on Earth.