February 18, 1930: Clyde Tombaugh Discovers the Planet Pluto

February 18, 1930: Astronomer Clyde Tombaugh discovers the planet Pluto, raising the number of known planets in Earth's solar system to 9.

The 6 planets closest to the Sun had all been known since antiquity. In ascending order of distance from the Sun, they are: Mercury, Venus, Earth, Mars, Jupiter and Saturn. From the naked eye on Earth, each of the others can be seen.

When 2 or more of them are close together in the sky, it is known as a "conjunction." It has been speculated that a conjunction of of 3 of them -- perhaps Venus, the closest to Earth; Mars, the next-closest; and Jupiter, the largest in the system -- formed the Star of Bethlehem, which led the the Wise Men to the birth of Christ. (The Bible never calls them "kings," and it has been credibly suggested that they were astronomers, or perhaps astrologers; either way, they would have studied the planets known at the time.)

In 1781, British astronomer William Herschel discovered a new planet. He named it "Georgium Sides," meaning "Georgian Star," after the King at the time, George III. Eventually, like the other planets, it was given a name from mythology: Uranus.

But this 7th planet didn't move the way astronomers had predicted. Using Isaac Newton's Laws of Motion, they predicted that there must be an 8th planet out there. In 1846, German astronomer Johann Galle discovered Neptune. But it still didn't affect Uranus enough to explain all of its odd movements, and Neptune didn't always move as expected, either. American astronomer Percival Lowell suggested a 9th planet was out there, but died in 1916, before it could be found.

Clyde William Tombaugh was born on February 4, 1906 outside Chicago in Streator, Illinois, and grew up in Burdett, Kansas. As a boy, he built his own telescopes, and sent his findings to the Lowell Observatory in Flagstaff, Arizona. Impressed, they offered him a job in 1926.

Eventually, his job became to take the data already available, and use it to find Planet 9. Using Lowell's predictions, he examined photographs he took from the Observatory's 13-inch astrograph in January 1930, and, on February 11, wrote his paper confirming that Planet 9 had been found. On May 1, 1930, the American Astronomical Society and Britain's Royal Astronomical Society jointly announced the discovery, and the official naming, of the planet Pluto. Not only was it a name from mythology, but its 1st 2 letters were Percival Lowell's initials.

In 1978, it was discovered that Pluto was actually the smallest planet in the solar system, taking that presumed status away from Mercury. So speculation began that there had to be a 10th planet out there. But when Voyager 2 sent data back from Neptune in 1989, it was discovered that Neptune's mass had been overestimated, and that explained any remaining questions about Uranus' orbit. Later, more powerful telescopes proved beyond any doubt that there is no Planet 10 in our solar system. They were also, by 1992, able to discover planets orbiting distant stars.

On August 24, 2006, the size of Pluto, and the relatively large size of its moon Charon, suggesting a "double planet," led to a reconsideration of Pluto's definition. But instead of promoting Charon and ruling that there were actually 10 planets, the International Astronomical Union demoted Pluto, reclassifying it as a "dwarf planet," leaving 8 planets in the solar system, and Neptune as the furthest from the Sun.

Tombaugh went on to discover 15 asteroids, several variable stars, star clusters, and one known comet. He served in World War II, and taught at New Mexico State University.

He was not available for reaction to the denigration of his discovery: He died on January 17, 1997, at 90. Through his brother, Robert, he is the great uncle of Los Angeles Dodgers pitching star Clayton Kershaw.

Nor did Tombaugh live to see the first close-up pictures of his discovery, taken by the unmanned New Horizons spacecraft in 2015.

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February 18, 1930 was a Tuesday. Baseball and football were out of season. The NBA had not yet been founded. But the entire NHL, 10 teams, were in action:

* The New York Rangers lost to the Toronto Maple Leafs, 5-1 at the old Madison Square Garden.

* The New York Americans beat the Chicago Black Hawks, 6-4 at the Chicago Stadium.

* The Boston Bruins beat the Montreal Maroons, 3-2 at the Boston Garden. The Maroons folded after the 1938 season.

* The Ottawa Senators beat the Pittsburgh Pirates, 3-2 at the Ottawa Auditorium. The Pirates moved after the season, becoming the Philadelphia Quakers. The Senators moved in 1934, becoming the St. Louis Eagles. Each team lasted only one more year after its respective move, and folded. 

* And the Montreal Canadiens beat the Detroit Cougars, 2-0 at the Olympia Stadium in Detroit. In 1932, the Cougars were renamed the Detroit Red Wings.