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Should Pluto be a Dwarf Planet?


Gordon Rowlinson

Should Pluto be a Dwarf Planet?

As NASA's New Horizons spacecraft continues to send back photographs of unprecedented detail of distant Pluto, the mini-debate on whether Pluto should be considered a dwarf planet or a planet has reignited.

To give a quick recap: Pluto was discovered in 1930 by Clyde Tombaugh, an American astronomer at Lowell Observatory in Flagstaff, AZ. Even though Pluto was only two thirds the size of the Earth's moon, Pluto was named the 9th and the final planet in our solar system. In 2000, discovery of smaller planets beyond Pluto and, in 2005, the discovery of Eris, a body the size of Pluto, led researchers to consider a distinction other than planet for these smaller bodies. In 2006 after several days of debate, the IAU (International Astronomical Union), came up with several rules to define what is a planet and a dwarf planet.

By the new rules, Pluto was downgraded to a dwarf planet. The new rules stated that a planet is a celestial body that is: a) in orbit around the sun; (b) and had sufficient mass for its self-gravity so that it assumes a hydostatis equilibrium (round) shape; (c) and has cleared the neighborhood around its orbit. A dwarf planet is defined as a celestial body that has the first two qualifications, but has not cleared the neighborhood around its orbit.

Harvard science historian Owen Gingrich is one of the crowd who believes that Pluto should be considered a planet. He maintains that achieving 2 out of the 3 qualifications should be enough to reach planet status. Pluto is in orbit around the sun and is round. However it doesn't have the mass and gravity to clear its neighborhood.

  Tim DeBenedicits, an astronomy software writer and MIT graduate, believes that the IAU planet qualifications are too objective. He raises several questions. How round does a planet need to be? How cleared does a celestial body's neighborhood need to be? Since Pluto's odd orbit crosses the orbit of Neptune, has giant Neptune cleared its neighborhood?

The International Astronomical Union general assembly meets in Honolulu in August. It is inevitable that the Pluto/planet controversy will be debated again then.

At any rate, it is clear that the small bodies: Pluto; Eris; Ceres; Makemake; and Hamea are much different than Earth. These small bodies are smaller than our moon and probably don't have enough gravity to contain a significant atmosphere. Perhaps the current IAU designation is accurate enough.

It should be noted that the planets Uranus and Neptune were once considered to be gas giants along with Jupiter and Saturn. However during the 1990s, scientific discoveries showed that that Uranus and Neptune were composed mainly of heaver volatile substances and thus were much different than Jupiter and Saturn. Both Uranus and Neptune are now referred to as ice giants.

Science is always changing. Descriptions should also change to match the current scientific understanding. Your comments below are welcome.

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