Little Bits of History

Pluto v. Neptune

Posted in History by patriciahysell on February 7, 2010

Hubble computer generated map of Pluto, false color and among the highest resolutions possible at the time

February 7, 1979:  Neptune becomes the furthest planet from the sun as Pluto moves inside Neptune’s orbit for the first time since either planet was known to science. Since the writing of this article, Pluto has been downgraded and is no longer considered a planet. So Neptune is always the furthest planet from the sun.

Pluto was the ninth and smallest of the planets of the solar system. Today it is called a dwarf planet and is the second-largest entity so designated in the solar system. Eris is about 27% larger than Pluto and was discovered in January 2005. Eris is three times farther out from the sun than Pluto. Pluto is about 18% of Earth’s size, measuring about 1423 miles. Pluto has its own moon, Charon, which is about half the size of the planet and two smaller moons were discovered in 2005.

Pluto has an eccentric orbit that has caused some scientists to claim that it is not a true planet. The small planet is the largest body in the Kuiper belt. This belt is similar to the asteroid belt found between Mars and Jupiter. Unlike that smaller area, the Kuiper region is much larger, about 20 times wider with about 20-200 times more mass. This second belt of debris was also left over from the solar system’s formation and the bodies are mostly rock and metal although there are some frozen volatiles as well. This belt is not only home to Pluto, but two other bodies designated as dwarf planet, Haumea and Makemake.

Pluto’s orbit is not on the same plane as the rest of the Solar system, but inclined by more than 17º as well as being eccentric by ~0.25, meaning it’s oval pattern is different as well. This means that the two planets’ paths don’t actually cross in the three dimensions. Pluto again became the farthest former planet on February 11, 1999.

“It used to be that Pluto was a misfit. Now it turns out that Earth is the misfit. Most planets in the solar system look like Pluto, and not like the terrestrial planets.” – Alan Stern

“It may very well be that solar systems like our own are probably not rare in the galaxy. They may actually be a very common case.” – Alan Boss

“This comet formed at very edge of the solar system … out by Pluto … and spent all its lifetime out there until recently it came into the inner part of the solar system, where we could sample it.” – Don Brownlee

“The solar system is not a stable and quiet place.” – Jack Lissauer

Also on this day, in 1971 the women of Switzerland were finally given the vote.

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3 Responses

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  1. Laurel Kornfeld said, on February 7, 2010 at 3:04 pm

    Both Pluto and Eris are “true planets,” and Dr. Stern is the first one who will tell you so. An eccentric orbit does not preclude an object from being a planet. If it did, a large number of the exoplanets discovered, many of which have orbits far more elliptical and eccentric than Pluto, would not be considered planets either.

    Please do not blindly accept the controversial demotion of Pluto, which was done by only four percent of the International Astronomical Union, most of whom are not planetary scientists. Their decision was immediately opposed in a formal petition by hundreds of professional astronomers led by Stern, Principal Investigator of NASA’s New Horizons mission to Pluto. Stern and like-minded scientists favor a broader planet definition that includes any non-self-luminous spheroidal body in orbit around a star. The spherical part is important because objects become spherical when they attain a state known as hydrostatic equilibrium, meaning they are large enough for their own gravity to pull them into a round shape. This is a characteristic of planets and not of shapeless asteroids and Kuiper Belt Objects. Pluto meets this criterion and is therefore a planet. At the very least, you should note that there is an ongoing debate rather than portraying one side as fact when it is only one interpretation of fact.

  2. patriciahysell said, on February 7, 2010 at 3:08 pm

    Thanks, I believe you just have given the other side of the argument, that in favor of keeping Pluto’s planet status.

  3. […] on this day: Pluto v. Neptune – In 1979, Pluto moved inside Neptune’s orbit. Suffragettes – In 1971, the women […]


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