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.
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Today's selected anniversaries:
The Union of Utrecht was signed, unifying the provinces in northern
Holy Roman Emperor Charles VI created Liechtenstein, the only
principality in the Holy Roman Empire still remaining today.
The bathyscaphe Trieste reached the record depth of 10,916 m (35,813
feet) in Challenger Deep of the Mariana Trench.
USS Pueblo was seized by North Korean forces, who claimed that it had
violated their territorial waters while spying.
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.
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.