How many planets?

by John Swanson

Copyright ©2016 Puzzles with a Purpose. All rights reserved.

The currently accepted definition of a planet according to the International Astronomical Union (IAU): A planet is an astronomical object that is orbiting a star or stellar remnant and… massive enough to be rounded by its own gravity, not massive enough to cause thermonuclear fusion , and

...has cleared its neighboring region of planetesimals .


To the Greeks and Romans there were seven known planets, each presumed to be circling Earth according to the complex laws laid out by Ptolemy. They were, in increasing order from Earth (in Ptolemy's order): the Moon, Mercury, Venus, the Sun, Mars, Jupiter, and Saturn




With the advent of the Scientific Revolution (ca. 1543), the use of the term "planet" changed from something that moved across the sky, in relation to the star field, to a body that orbited Earth (or that was believed to do so at the time). By the 18th century the heliocentric model of Copernicus, Galileo and Kepler gained sway. Even the Catholic Church capitulated when in 1758 the general prohibition of books advocating heliocentrism was removed from the Index of Forbidden Books.

Thus, Earth became included in the list of planets, whereas the Sun and Moon were excluded. We were reduced to six planets.


Sir William Herschel observed Uranus on March 13, 1781 from the garden of his house in Bath, England but initially reported it as a comet . The discovery was serendipitous. Herschel had been "engaged in a series of observations on the parallax of the fixed stars". Uranus has been observed before but was assumed to be a star. Observations and orbit calculations by other astronomers convinced Herschel his comet was actually a newly discovered planet. We were back up to seven planets.

In recognition of his achievement, King George III gave Herschel an annual stipend of £200 on condition that he move to Windsor so that the Royal Family could look through his telescopes. 




Until the mid-19th century, the number of "planets" rose rapidly because any newly discovered object directly orbiting the Sun was listed as a planet by the scientific community. First came Ceres (1801), then Pallas (1802), Juno (1804), Vesta (1807), then eleven more between 1845 and 1851: Astrea, Hebe , Iris , Flora , Metis , Hygeia , Parthenope , Victoria , Egeria , Irene , Eunomia .

Neptune, whose existence had been deduced from variations in the orbit of Uranus, was located with certainty in 1846. Primary credit of discovery has been given to Urbain Le Verrier, although many contributed. We were up to an astounding 23 planets.




The rapidly expanding list of bodies between Mars and Jupiter prompted their reclassification as asteroids, which was widely accepted by 1854. The number of planets was reduced to eight.




Variations in the orbit of Uranus were not fully explained by the discovery of Neptune. The search for another planet was begun by Percival Lowell at his observatory in 1906. The search was put on hold when Lowell passed away. It wasn't until 1930 that Clyde Tombaugh, a young astronomer, identified "Planet X". He had ben summarily assigned the job of locating the planet by the observatory director.

The object was officially named Pluto, for the god of the underworld, after a vote by the staff of the Lowell Observatory. The name was proposed by Venetia Burney, an eleven-year-old schoolgirl in Oxford , England, who was interested in classical mythology. Pluto received every vote, the other candidate names of Minerva and Cronus losing out. The choice of name was helped in part by the fact that the first two letters of Pluto are the initials of Percival Lowell.

Did you know that elements are named after planets?  That is how Uranium, Neptunium and Plutonium got their names. We may be running out of both new elements and new planets.

Pluto was accepted as the ninth planet after initial observations led to the belief that the object was larger than Earth. Further monitoring found the body was actually much smaller. It was later suggested that Pluto may be an escaped satellite of Neptune or could be a comet. As it was still larger than all known asteroids and seemingly did not exist within a larger population, it retained its status until 2006. If only Pluto had cleared its neighborhood.




A growing number of astronomers argued for Pluto to be declassified as a planet, because many similar objects approaching its size had been found in the same region of the Solar System (the Kuiper belt ) during the 1990s and early 2000s. Pluto was found to be just one small body in a population of thousands. It was the asteroid problem all over again.

Some of them, such as Quaoar , Sedna , and Eris , were initially heralded in the popular press as the tenth planet , but failed to receive widespread scientific recognition as such. The announcement of Eris in 2005, an object 27% more massive than Pluto, created the necessity in the astronomical community for an official definition of a planet.

Acknowledging the problem, the IAU set about creating the definition of planet , and produced one in August 2006 (given above). The number of planets was reduced to the eight significantly larger bodies that had cleared their orbit (Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune), and a new class of dwarf planets was created, initially containing three objects ( Ceres, Pluto and Eris ). [Observation: it does seem strange that a "dwarf planet" is not a "planet". There is a terminology issue.]




Caltech researchers have found evidence of a giant planet tracing a bizarre, highly elongated orbit in the outer solar system. The object, which the researchers have nicknamed Planet Nine, has a mass about 10 times that of Earth and orbits about 20 times farther from the sun on average than does Neptune (which orbits the sun at an average distance of 2.8 billion miles). In fact, it would take this new planet between 10,000 and 20,000 years to make just one full orbit around the sun. Could we be back up to nine?


Maximum is 105.  It would be astounding if someone scored 90, but even Caltech astronomers might be pressed to score higher. A reasonable guess is that 70 is well above average.


John Swanson is known worldwide as a winner of contract bridge championships.  In his professional life in computer technology, he was responsible for many accomplishments in software engineering, which is how we began our friendship in 1982.  John has also made contributions to several entries in Puzzles with a Purpose, including Challengee vs Challenger,
Ellipse Illusion, and Prime Numbers Are Odd.  
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