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On foot
I had to cross the solar system
before I found the first thread of my red dress.
— Edith Södergran, "On Foot I Had to Cross the Solar System", 1919
[translation by Stina Katchadourian (1937– )]

What is a Planet?

The solar system boasts a population of eight planets, five dwarf planets, dozens of satellites and countless small solar-system bodies. The charts below list a few properties of the planets and their named satellites.

Although the innermost six planets of the solar system were known from antiquity, Uranus was discovered in 1781 by W. Herschel. Irregularities in the orbit of Uranus led J. Adams and U. Le Verrier to predict the existance of an eighth planet and in 1846, J. Galle discovered Neptune. Interestingly, Galileo was the first astronomer to observe Neptune in 1612 and 1613 but he recorded it as a fixed star!

The official definition (adopted by the International Astronomical Union on 24 August 2006) of a planet is a celestial body that

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

Thus Pluto was demoted from planet to dwarf planet status and the solar system shrank from nine to eight planets.

Physical Characteristics

The rotational period is the length of a day on a planet. Obliquity measures the angle of inclination of the equator to the orbital plane. Notice that Venus is nearly "upside-down" as a result of a large axial tilt. Thus, it appears to rotate retrograde or backwards from our point of view. Uranus is practically laying on its side, 'rolling' in its orbit around the Sun. It is the axial tilt on the Earth that is largely responsible for the seasons.

Orbital Characteristics

The semi-major axis defines the size of the orbit. The sidereal period is the length of time it takes for a planet to complete one orbit around the Sun. The synodic period, however, is the time it takes a planet to return to the same configuration with respect to the Earth. For instance, it takes Jupiter nearly 12 years to orbit the Sun but the time between successive oppositions, say, is around 400 days. Inclination measures the angle of inclination of the orbit to the ecliptic (which is defined by the orbit of the Earth around the Sun). Most of the planets are found close to the ecliptic but minor planets and comets often have much higher inclinations. Finally, eccentricity indicates the shape of the orbit: An eccentricity of zero denotes a circular orbit but if the eccentricity is between zero and one, the orbit is elliptical. The closer the eccentricity is to one, the more stretched-out the orbit becomes. Many comets have highly elliptical orbits. An eccentricity of exactly one means that the orbit is parabolic and an eccentricity higher than one gives a hyperbolic orbit.

Satellites

The next table lists alphabetically the named satellites of the major planets. Many tiny new satellites have been discovered since space probes have been sent to explore the solar system. The Voyager and Cassini projects were particularly successful. The Galilean satellites of Jupiter are marked with an asterisk * and are easily seen through a small telescope.