In understanding exoplanets and their properties, we are forced initially to consider a very small sample of one; our own Solar System. It's comprised of a star, eight planets (let's not go there !), five dwarf planets and countless moons, asteroids, comets, etc. Broadly speaking, we can divide the planets into two groups of four; the inner planets (Mercury, Venus, Earth, Mars) have rocky surfaces whereas the outer four (Jupiter, Saturn, Uranus, Neptune) are either gas or ice giants (see Figure 1).

Within our Solar System, our Sun has a mass of around 330,000 Earths; in fact, the Sun contains 99.8% of the mass of the entire Solar System. Its radius is around 100 times that of the Earth, meaning that it has a volume which is 100³ (a million) times that of the Earth (see Figure 2). You can learn more about our Solar System using the Electric Orrery.

Within our Solar System, we can define a region known as the 'Habitable Zone' where we believe conditions are right for life to perhaps both develop and survive. You can read more about the habitable zone, but broadly speaking, it's based on a region or volume of space at just the right distance from its host star for water to be found in liquid form.

In the last 50 years or so, we have been able to explore our own Solar System in many ways. We can point telescopes at objects such as the Sun and the outer planets, we've sent manned missions to the Moon and unmanned spacecraft have sampled the atmosphere, orbited, imaged or crashed on most of the other worlds. In terms of life in the Solar System, we continue to base our findings and understanding on the "less than satisfactory" sample of one (!) but there are of course several other bodies that continue to capture our attention. These include the discoveries of dried riverbeds on Mars, the possibilities of a sub-surface ocean on Jupiter's moon, Europa, and the water jets detected (by the Cassini-Huygens mission) on Saturn's moon, Enceladus.

Find out more about the habitable zone.