Planetary habitability is the measure of an astronomical body's potential for developing and sustaining life. It may be applied both to planets and to the natural satellites of planets. The only absolute requirement for life is an energy source (usually but not necessarily solar energy), but the notion of planetary habitability implies that many other geophysical, geochemical, and astrophysical criteria must be met before an astronomical body is able to support life. The idea that planets beyond Earth might host life is an ancient one, though historically it was framed by philosophy as much as physical science. The late 20th century saw two breakthroughs in the field. To begin with, the observation and robotic exploration of other planets and moons within the solar system has provided critical information on defining habitability criteria and allowed for substantial geophysical comparisons between the Earth and other bodies. The discovery of extra-solar planets—beginning in 1995 and accelerating thereafter—was the second milestone. It confirmed that the Sun is not unique in hosting planets and expanded the habitability research horizon beyond our own solar system.
Read the rest of this article: http://en.wikipedia.org/wiki/Planetary_habitability
_______________________________ Today's selected anniversaries:
1579: The Union of Utrecht was signed, unifying the provinces in northern Netherlands. (http://en.wikipedia.org/wiki/Union_of_Utrecht)
1719: Holy Roman Emperor Charles VI created Liechtenstein, the only principality in the Holy Roman Empire still remaining today. (http://en.wikipedia.org/wiki/Liechtenstein)
1960: The bathyscaphe Trieste reached the record depth of 10,916 m (35,813 feet) in Challenger Deep of the Mariana Trench. (http://en.wikipedia.org/wiki/Bathyscaphe_Trieste)
1968: USS Pueblo was seized by North Korean forces, who claimed that it had violated their territorial waters while spying. (http://en.wikipedia.org/wiki/USS_Pueblo_%28AGER-2%29)
1986: Chuck Berry, James Brown, Ray Charles, Fats Domino, the Everly Brothers, Buddy Holly, Jerry Lee Lewis and Elvis Presley became the first inductees into the Rock and Roll Hall of Fame. (http://en.wikipedia.org/wiki/Rock_and_Roll_Hall_of_Fame)
_____________________ Wikiquote of the day:
"To minimize suffering and to maximize security were natural and proper ends of society and Caesar. But then they became the only ends, somehow, and the only basis of law — a perversion. Inevitably, then, in seeking only them, we found only their opposites: maximum suffering and minimum security." -- Walter M. Miller, Jr. (http://en.wikiquote.org/wiki/Walter_M._Miller%2C_Jr.)
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