Our Solar System Once Had 21 Planets

| April 25, 2018 @ 6:37 am

While it has not really changed physically, the number of planets in our solar system has grown and shrunk with changing definitions of the word “planet” itself.

Before recorded history. seven classical planets were known to the Babylonians.  The word comes from Greek for wanders since they moved across the sky against the fixed background of stars.  These 7 wanders were the Sun, Mercury, Venus, our Moon, Mars, Jupiter, and Saturn, also the source of the days of the week.

Galileo found IoEuropaGanymede, and Callisto in 1610, initially calling them planets. Over the next hundred years, the planet family grew to 16 as Saturn’s larger moons were added to the list.

As those moons were found to be in orbit around their planets, not just the Sun, our cosmological model shifted from an Earth-centered to a Sun-centered one, shrinking the planet count down to 6.

In 1781 Uranus was discovered, several asteroids were found and considered planets until Neptune brought the list to 13 in 1846.    More asteroids were added until the solar system grew to its largest in 1850 at 21.

All asteroids were demoted soon there after and the planetary club remained at 7 members until Clyde Tombaugh found a faint dot of light moving between images taken a few days apart.  He liked the name Pluto suggested by an 11 year form Oxford, England and the solar system of 8 + earth that most of us learned of in school was set.

But throughout this, the world planet was the most poorly defined word we were sure we all know the definition of.

By 2005 technology advanced to the point where these rocks past Neptune were getting easier to find  .  The International Astronomical Union, the AMS and NWA of astronomy, recogznied something had to give. If the word planet didn’t get amore clear definition, membership in the solar system was about to explode.

The rules, as approved by IAU members at are: a Planet is (a) in orbit around the Sun, (b) has sufficient mass to form a nearly round shape through its own gravity  and    (c) has cleared the neighbourhood around its orbit. 

Get centermtal aout Pluto’s demotion if you must but the clarification of what is and isn’t a planet was very necessary. Had that consensus among not been reached at the 2006 IAU General Assembly in Prague   today we could have move 200 planets.

There’s not a good nmumonic for remembering all those, my very easy memory jingle seems useless now

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About the Author ()

Tony Rice is a Solar System Ambassador for NASA/JPL and the voice and brains behinds the weekly Astronomy Report on the WeatherBrains podcast. He grew up in Southern California with Space Shuttle landings and was hooked. He brings weather and space together to communicate the excitement of space exploration and promote a greater appreciation for Earth sciences.

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