The five naked eye planets - Mercury, Venus, Mars, Jupiter and Saturn - have been known for millennia. Therefore, no one person can be said to have “discovered” these bodies. In a sense Earth was “discovered” to be a planet with the acceptance of the heliocentric theory in the sixteenth century, but no single individual could reasonably claim credit for this. However, the case of the three outer planets - Uranus, Neptune, and Pluto - is different. These planets were discovered in the telescope by specific individuals, and the first discovery, that of Uranus, caused a sensation throughout the world.
q (1781)
Uranus William Herschel, at that time an amateur astronomer in
Bath, England, was making a systematic survey of the sky, noting nebulae,
double stars, and the occasional comet.
In 1781 he discovered a small green object that he assumed was a
comet. However, after following it for
a few days, it was clear that it was orbiting the sun as a planet. He had discovered the planet we now know as Uranus,
although his original designation was “Georgium Sidus” or George’s
Star, after England’s reigning monarch, George III. The effect on Herschel’s career was
dramatic. George III awarded Herschel a
lifetime stipend so that he (Herschel) could devote full time to astronomy.
q (1845)
Neptune While the discovery of Uranus was an accident, Neptune was
actively searched for. A detailed study
of Uranus’ orbit indicated that an unseen object was making it deviate from the
expected path. Two astronomers, John
Adams in England and Urbain Le Verrier in France, made detailed mathematical
calculations that predicted where the planet would be found. Both men gave the calculations to
observational astronomers, and observers in several countries found the planet
very close to the predicted location in 1845.
However, Le Verrier’s observational team was the first to report the
discovery and is usually given the credit.
The planet was christened Neptune, in keeping with the
classical theme of planetary nomenclature.
q (1930)
Pluto The fact that a planet had been discovered “mathematically”
caused as great a sensation among scientists as the original discovery of
Uranus had a century earlier. Thus the
orbit of Neptune was carefully scrutinized to see if still other planets lurked
in the darkness beyond. By the turn of
the twentieth century most planetary astronomers had convinced themselves that
Neptune was indeed showing the signs of being tugged by an as yet undiscovered
ninth planet. This effect was much smaller
than the corresponding effect on Uranus, and, therefore, the search for Planet
X (as the unseen planet came to be called) was much more difficult.
Two American observatories, Harvard Observatory in Cambridge, Massachusetts and Lowell Observatory in Flagstaff, Arizona devoted a good deal of resources in the early decades of the 20th century to finding Planet X. Success finally came in 1930 when Clyde Tombaugh at Lowell found a dim star-like point moving through a set of photographic plates. This planet was called Pluto, but it was immediately obvious that it was not the Planet X everyone had been looking for. Even cursory examination of the new planet and its orbit indicated that it was very small, possibly (and later confirmed to be) the least massive planet in the solar system. Today there is controversy as to whether Pluto should be called a planet at all. Its physical characteristics are more akin to comets – mostly icy objects that normally orbit in the outer reaches of the solar system.
q (1995)
51 Pegasi Ever since it was established that stars were essentially
other “suns” many have speculated whether other solar systems exist. In the first half of the twentieth century
the general opinion among astronomers was that the likelihood was low, because
the prevailing theories of solar system formation required an extremely unusual
set of circumstances – for example, a passing star pulling off material to form
planets. Toward the end of the century,
however, new theories of solar system formation, basically stellar formation
from rotating nebulae, implied that some matter at the edge of the cloud would
not be captured by the star, but, rather, orbit around it, eventually
coalescing into planets. From this
point of view the universe should be teeming with planets.
The fact that no planets had ever
been detected was easily explained.
Planets, even large ones, would be so close to the star and so much
dimmer that no telescope on Earth could possibly see them. The only hope was to try to detect tiny
wobbles in the position of the star produced by very large planetary
objects. The first successful detection
of just such a wobble was announced in 1995 by Michel Mayer and Didier Queloz
of the Geneva (Switzerland) observatory. This team used newly improved
spectroscopic techniques in their detection system. The first star found to have a planet was the star numbered 51 in
the constellation of Pegasus, 51 Pegasi. With this breakthrough, teams in the United
States and elsewhere began finding planetary systems around many nearby
stars. At present (October, 2001) there
are over 50 known planets outside our solar system.