120px|Photograph of the planet Neptune (large, background) and its moon
Triton (small, foreground), taken by [[Voyager 2 as it entered the
outer Solar System]]
The definition of planet has comprised many different things. Over
the millennia, use of the term was never strict and its meaning has
blurred to include or exclude a variety of objects. By the end of the
19th century, the word planet had settled into a comfortable working
term. It applied only to objects in the Solar System; a number small
enough that any differences could be dealt with on an individual basis.
After 1992 however, astronomers began to discover many additional
objects beyond the orbit of Neptune, as well as hundreds of objects
orbiting other stars. These discoveries not only increased the number
of potential planets, but expanded their variety and peculiarity. The
issue of a clear definition for planet came to a head in 2005 with the
discovery of the trans-Neptunian object Eris, a body larger than the
smallest then-accepted planet, Pluto. In its 2006 response, the
International Astronomical Union released its decision on the matter.
This definition, which applies only to the Solar System, states that a
planet is a body that orbits the Sun, is massive enough for its own
gravity to make it round, and has "cleared its neighbourhood" of
smaller objects around its orbit. Under this new definition, Pluto,
along with the other trans-Neptunian objects, does not qualify as a
Recently featured: Horses in World War I – Christopher Smart's
asylum confinement – Jack Warner
Archive – By email – More featured articles...
Read the rest of this article:
Today's selected anniversaries:
John Komnenos the Fat briefly seized the throne of the Byzantine Empire
from Alexios III Angelos, but he was soon caught and executed.
World War I: The Battle of Passchendaele began near Ypres in West
Flanders, Belgium, with the Allied Powers aiming to force German troops
to withdraw from the Channel Ports.
The Holocaust: Under instructions from Adolf Hitler, Hermann Göring
ordered SS General Reinhard Heydrich to settle "the final solution of
the Jewish question".
The Soviet Union and United States signed the bilteral START I treaty,
the largest and most complex arms control treaty in history, which
eventually removed 80% of all strategic nuclear weapons then in
Following intestinal surgery, Fidel Castro provisionally transferred
the duties of the Cuban presidency to his brother Raúl .
Wiktionary's word of the day:
The scientific study of mountains, or the mountainous features of a
Wikiquote quote of the day:
Only a crisis — actual or perceived — produces real change. When that
crisis occurs, the actions that are taken depend on the ideas that are
lying around. That, I believe, is our basic function: to develop
alternatives to existing policies, to keep them alive and available
until the politically impossible becomes politically inevitable.
Show replies by date