Exploring The Planets

Discovering New Planets

Only six planets, including the Earth, were known until the 18th Century.

Discovery of Uranus

Sir William Herschel

21k GIF

Photo from Zeitlin & VerBrugge,
Los Angeles, CA

The planet Uranus was discovered by the noted British astronomer, Sir William Herschel, on March 13, 1781. Actually, the planet had been observed numerous times by other astronomers as early as 1690, but it was thought to be another star.
The planet was discovered accidentally while Herschel was surveying all stars down to magnitude eight -- those that are about ten times dimmer than can be seen by the naked eye. One "star" seemed different and within a year was shown to have a planetary orbit 18 times farther from the Sun than Earth. The new planet was named Uranus after the father of Saturn in Roman mythology. HERSCHEL'S TELESCOPE
142k GIF

Uranus was discovered using this 15-centimeter (6-inch) telescope designed and built by Herschel.

71k GIF - 12k JPEG
NASA Image #P29313

Voyager image of Uranus and 3 of its moons.

William Herschel built his own telescopes , including one which had a focal length of 6 meters (20 feet) and a larger telescope of the same design with a 12 meter (40 foot) focal length.

See Uranus page for more.


Discovery of Ceres

In the course of preparing a star catalog in 1801, the Sicilian astronomer Guiseppi Piazzi accidentally discovered a small planet between the orbits of Mars and Jupiter. He named the planet Ceres after the Sicilian god of the harvest.

Ceres is now known to be the largest of thousands of minor planets, or asteroids, most of which have orbits in the region of the Solar System between Mars and Jupiter.


iscovery of Neptune

150K GIF
NASA image
Unlike Uranus and Ceres, Neptune was not discovered by accident. It was proposed that a planet beyond Uranus could account for irregularities in Uranus' orbit. Independently, two astronomers, John Couch Adams in England and Urbain-Jean-Joseph Le Verrier in France, calculated the position of this yet unknown planet.

The search began. The British astronomer James Challis, using Adams' predictions, observed the planet on the night of August 4, 1846, but failed to compare his observations with those of the previous night and did not recognize the planet. On September 23, 1846, the planet was finally found on the first try by the German astronomer Johann Galle using Le Verrier's predictions.

Who Discovered Neptune?

Who should receive credit for discovering Neptune? Adams or Le Verrier who predicted its position? Challis who saw it but did not know it? Galle who found it? Controversy raged.

It was fortunate that Neptune was discovered in the mid-19th century. Just 30 years later the planet was far from the positions predicted by both Adams and Le Verrier.

Go to Neptune page for more information.

iscovery of Pluto

Several astronomers interpreted irregularities in the orbits of Uranus and Neptune as being caused by a more distant planet. Among these astronomers was the American Percival Lowell who is credited with the successful prediction of the planet's orbit. Lowell also started the search for the planet which was ultimately found in 1930 by his successors at the Lowell Observatory in Arizona.

Pluto Discovery Plate

plutplate1.T.jpg (8539 bytes)

Pluto was discovered by Clyde Tombaugh of the Lowell Observatory by comparing this photographic plate taken on the night of January 23, 1930 with two others taken in the same month. Because Pluto is so small and so distant from the Sun, the image of the planet is extremely faint and difficult to see. The original plate is shown at left on display in the Exploring The Planets gallery in the National Air and Space Museum.  It is unlighted to protect the photographic emulsion.

Discovery Plate Image ©2002 Smithsonian Institution #W1998EP0002

See the Pluto page for more information.



Ancient Times & The Greeks || The Renaissance || Age of The Telescope || Galileo
Discovering Planets || Planetary Satellites


©2002 National Air and Space Museum