Science Web Corner I.S.S.N.: 1579-1149
El rincón de la Ciencia nº 37 (septiembre-2006)
How many planets does the Solar System have? (RC-92)
M. A. Gómez
English Translation: J. Cantos
This is a English version of the ¿Cuántos planetas tiene el Sistema Solar?
Traditionally, since Pluto was discovered, we have thought that the Solar System was formed by 9 planets. But, it seems that this is not so clear and the definition of what it is a planet has been recently questioned. During August 2006, new definitions have been proposed, which have made the number of planets range from 8 up to 12.
Finally, on August 24th, 2006, in the XXVIth Assembly of the International Astronomical Union (IAU) a new definition of planet was voted, with three categories established for the objects that orbit around the Sun.
The First Category. "A planet is a celestial body that is in orbit around the Sun, which has enough mass to have its own gravity to overcome the rigid forces of a body so that it assumes a hydrostatic balance, that is to say, round, and with cleared surrounded areas out of its orbit ".
In this category, Mercury, Venus, Earth, Mars, Júpiter, Saturn, Uranus and Neptune are included.
The Second category. "A dwarf planet is a celestial body that is in orbit around the Sun, which has enough mass to have its own gravity to overcome the rigid forces of a body so that it assumes a hydrostatic balance, that is to say, round, and without cleared surrounded areas out of its orbit and which is considered not to be a satellite ".
In this category, Pluto, Xena, Ceres are included.
The Third category. "All the rest of the objects that orbit around the Sun are considered to be ‘small bodies of the Solar System’ ".
In this category asteroids and comets are included.
The result is that the Solar System comes to have only 8 main planets and Pluto remains degraded to the second category.