It turns out there are at least 100 dwarf planets the size of Pluto in our solar system. That’s why astronomers have decided that Pluto can’t be an official planet. Admitting Pluto to the planetary brotherhood would open up the floodgates to scores of new smaller planetary bodies being named official planets, and that unfortunate situation must be avoided.
“There are over 100 objects like Pluto [in the solar system], so we’re not going to have the schoolchildren of the world memorize over 100 planets,” says Jay Pasachoff, director of Hopkins Observatory at Williams College.
The International Astronomical Union held a controversial vote back in 2006 that established criteria for planets. The new definition of planet meant that Pluto was no longer a planet. The IAU decided to call planetary bodies like Pluto, Ceres, Makemake and Haumea dwarf planets rather than planets.
The IAU definition said that a planet must orbit a star, be fairly round and must clear the orbital path around its star by ejecting or accumulating most debris.
Pluto does not meet the new definition of planet
It was the discovery of a rocky body larger than Pluto, called Eris today, in the Kuiper asteroid belt (a zone of icy bodies located beyond the orbit of Neptune) that led to the official definition of a planet. The controversy grew around the discovery of Eris, and it was now evident there could be many new potential additions to the official list of planets.
This sticky situation led UCLA planetary scientist Jean-Luc Margot to develop a new definition of planet.
“How round is round?” Margot asked during a press conference yesterday at the American Astronomical Society’s Division of Planetary Sciences meeting.
This ambiguity in the current definition led Margot to come up with a test to clearly separate planets from other orbital objects. His test requires estimating the star’s mass, the planet’s mass and its orbital period, all of which can be easily measured from ground or space telescopes.
With these values, Margot’s equation can determine whether an orbiting body is capable of clearing its path of debris without actually seeing the debris being cleared.
Margot tested his equation on the eight “official” planets of our solar system and Eris, Pluto and its moon Charon, and the results were unequivocal. The data showed that Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune are all “planets”, while the other bodies are not.
“Make no mistake about it, we have two very clear types of bodies in our solar system,” commented Margot, whose paper has been accepted for publication in the Astronomical Journal.
Margot’s ground-breaking equation even works for exoplanets (planets that orbit stars other than the sun). Margot claims his test can classify 99% of the thousands of known exoplanets. The 1% that can’t be categorized are planetary bodies without accurate estimates for mass or orbital period.