Pluto’s Moons and Planetary Status

Wednesday, June 28th, 2006 at 01:26 PM by Dave

(Click on image for full-size version.) The Plutonian planetary system on February 15, 2006 as imaged by the HST. (Courtesy: Hubblesite.org)

For years people have debated the planetary status of Pluto. Some have said it is a planet whereas others have said it doesn’t fit the characteristics of a solar system planet. Word is expected soon to clarify this question.
The International Astronomical Union (IAU) is the internationally recognized authority for assigning designations to celestial bodies. Recently, the IAU named the 2005 discovery of two additional moons of Pluto, Hydra and Nix.

Pluto was discovered in 1930 by Clyde Tombaugh. 48 years later, Charon (Pluto’s first moon) was discovered. 27 years later, Nix and Hydra were observed. Nix and Hydra are about 5,000 times fainter than Pluto, thus the reason for the long stretch in time for their discovery. They are not easily seen.

In mythology, Nix is the goddess of darkness, a good description of such a faint object. Hydra is the terrifying monster with the body of a serpent and nine heads, befitting the outermost moon of Pluto.

Now, do you think Pluto is a planet? Historically, Pluto has been called a planet since its discovery. So history is on its side. However, astronomers have observed that Pluto does not fit the same orbital characteristics of the other planets. It has a highly elliptical orbit, whereas the other planet’s orbits are nearly circular. All of the 8 other planets fall on a narrow plane called the ecliptic. The accepted solar hypothesis theory, the theory which describes the formation of the solar system, suggests that planets will form along a disk or plane around the Sun. Pluto’s orbit is inclined to this plane by 17 degrees. Pluto is made of frozen gases and rock. The other planets are either gaseous or rocky. The composition of Pluto is similar to those of comets, which lie outside of Neptune’s orbit.

The object 2003 UB313, or also known as Xena, will also be looked at closely to be determined if it is a planet or not. 2003 UB 313 is slightly larger than Pluto, is further out than Pluto, and has a greater inclination to the ecliptic (44 degrees).

In August this summer, the IAU will give a definition of a planet and will decide if Pluto or Xena fit the definition. If they don’t we’ll have only 8 planets. If they do, we’ll have 10 planets.

What do you think?

Posted in Astronomy News

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