Eccentricity. The eccentricity of an orbit is a measure of how elliptical (elongated) it is. All the planets of the Solar System except for Mercury have near-circular orbits (e<0.1). Most exoplanets with orbital periods of 20 days or less have near-circular orbits, i.e. very low eccentricity.

Imagine a gaseous exoplanet five times the size of Jupiter but much closer to its star as Mercury is to our Sun. This planet orbits its star in just a couple of days and always shows the same face towards it.

Mercury has a more eccentric orbit than any other planet, taking it to 0.467 AU from the Sun at aphelion but only 0.307 AU at perihelion (where AU, astronomical unit, is the average Earth–Sun distance).

Planets that orbit around other stars are called exoplanets. All of the planets in our solar system orbit around the Sun. Planets that orbit around other stars are called exoplanets.