Extra-Solar Planets

An extra-solar planet, or exoplanet, orbits around a star other than our Sun i.e. beyond our Solar System.

The first exoplanet was discovered in 1995, orbiting close to the sun-like star 51 Pegasus. Since then more than 190 other planets have been found by astronomers and most appear similar is mass to Jupiter.

Around 20% of the planets have very short orbital periods, taking less than 10 days to go around their parent stars. Physics tells us that planets with shorter orbital periods are closer to the central star, whereas those with longer periods are further out. In the Solar System, Mercury, the closest planet, has an 88 day orbital period whilst Pluto, the furthest, takes an incredible 248 years.

Now surface temperatures on Mercury are hot enough at around 400°C, but planets with less than 10 day orbits are expected to be well above 1000°C. It is fair to say that we do not expect to find life on these "Hot Jupiters".

Click here to find out how astronomers detect these exoplanets.

When a star is born it is surrounded by a nebula of gas and dust, which is spread into a thin disk because the nebula was spinning slowly. Atoms and molecules within this dusty disk combined to form larger particles, and over millions of years gravitational interactions slowly brought these particles together in "sticky" collisions that gradually formed asteroids and then planets.

The stars determine what kinds of particles can exist. Close to the stars, solar heat vaporizes ices and prevents lightweight elements, like hydrogen and helium, from condensing out of the nebula. This means that the hot Jupiters we find, which are mainly made of hydrogen and helium, must have formed further away from the star where it is cooler and then migrated inward.

Whilst the exoplanets detected so far have been large gas-giants, our technology is not yet able to discover smaller terrestrial planets like Earth and Mars, which no doubt exist in much larger numbers.