On April 17

NASA announced the discovery of Kepler-186f (pictured), the first exoplanet with a radius similar to Earth's discovered in the habitable zone of another star.

Having supposedly been at war for 335 years without a single shot having been fired and no casualties incurred, the Netherlands and the Isles of Scilly declared peace.

Metropolitan Police officer Yvonne Fletcher was shot and killed while on duty during a protest outside the Libyan embassy in London, resulting in an 11-day police siege of the building and a breakdown of Libya–United Kingdom relations.

The Khmer Rouge captured Phnom Penh, the capital of the Khmer Republic, ending the Cambodian Civil War and establishing the socialist state of Democratic Kampuchea.

George Lucas began writing a 13-page film treatment that later formed the basis of Star Wars.

Ustaše-affiliated Croatian separatists attacked the Yugoslav embassy in Stockholm, fatally shooting the ambassador, Vladimir Rolović.

Sirhan Sirhan was convicted of the assassination of United States Senator Robert F. Kennedy.

The Peak District was designated the first national park in the United Kingdom.

Minas Geraes (pictured), the first of three Brazilian dreadnought battleships, was laid down, sparking a vastly expensive South American naval arms race with Argentina and Chile.

Napoleonic Wars: After a three-day chase, the French ship D'Hautpoul was captured off Puerto Rico by a British squadron under Alexander Cochrane.

French Revolutionary Wars: British forces commanded by Lieutenant-General Ralph Abercromby invaded the Spanish colonial port of San Juan, Puerto Rico.

The Mechanical Turk, a fraudulent chess-playing "machine" by Wolfgang von Kempelen that was secretly controlled by a hidden human, began a tour of Europe.

Lithuanian Crusade: After a month-long siege, forces of the Teutonic Order captured and destroyed Kaunas Castle (reconstruction pictured), which was defended by troops of the Grand Duchy of Lithuania.

Canute IV became King of Denmark upon the death of his brother Harald III.