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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 planet. (more...)

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• Ollanta Humala (pictured) is inaugurated as President of Peru.
• Asteroid 2010 TK₇ is confirmed as the first Earth Trojan asteroid discovered.
• Truong Tan Sang becomes the new President of Vietnam and nominates Nguyen Tan Dung to another term as Prime Minister.
• In cycling, Cadel Evans wins the 98th edition of the Tour de France, becoming the first Australian to do so.
• Protests against rising house prices in Israel continue, with tens of thousands gathering in Tel Aviv and Jerusalem.
• Thousands of protesters encounter violence while marching toward the Egyptian Ministry of Defense in Cairo.
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July 31: Ka Hae Hawai'i Day (Flag Day) in Hawaii; Feast day of Saint Ignatius of Loyola

• 1201 – John Komnenos the Fat briefly seized the throne of the Byzantine Empire from Alexios III Angelos, but he was soon caught and executed.
• 1917 – 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.
• 1941 – The Holocaust: Under instructions from Adolf Hitler, Hermann Göring ordered SS General Reinhard Heydrich to settle "the final solution of the Jewish question".
• 1991 – The Soviet Union and the United States signed the bilateral START I treaty, the largest and most complex arms control treaty in history, which eventually removed 80% of all strategic nuclear weapons then in existence.
• 2006 – Following intestinal surgery, Fidel Castro provisionally transferred the duties of the Cuban presidency to his brother Raúl (pictured).

More anniversaries: July 30 – July 31 – August 1