Why Is Pluto Not a Planet?

Pluto was first discovered by Clyde W. Tombaugh in 1930. The name Pluto originated when an 11 year old girl suggested that the solar system’s smallest planet should be named after the Roman God of the underworld, Pluto. Since its discovery, the former planet was the subject of constant study and photographic plate comparisons performed for almost a year, leading to the official designation of Pluto as a planet. However, Pluto's planet status has since been revoked.

Pluto Is Not a Planet

So, what changed and why is Pluto not considered a planet any more? Over the years, with continued rigorous studies, scientists have discovered more and more about the solar system and its components, including Pluto. With such findings, some scientists began to confirm what they had been suspicious about for quite a while - that Pluto was not really a planet after all.

Pluto is only 0.07 times the mass of the other celestial objects found in its orbit, so it does not meet the IAU's third criteria that an object "should be the dominant gravitational body in its orbit."

What Is a Planet?

The International Astronomical Union (IAU) states that a celestial or astronomical body must meet the following requirements before being officially called a planet:

• The object must orbit the sun. The object cannot orbit another planet or another celestial object. The object can only be a satellite of the sun.
• The object must be a sphere. The self-gravity of the object must be strong enough to smooth out any ridges or bumps and become spherical in structure.
• The object must be cleared of its orbital neighborhood. There should be no other bodies in its orbit. During its formation, it must have already cleaned or absorbed any object or debris in its orbit (except moons). In simpler terms, a planet should be the dominant gravitational body in its orbit. When a planet comes across other smaller objects, the planet should either sling them away or consume them.

IAU's Designation of Pluto

Although for many years, scientists had been uncertain of the mass of Pluto, there was no suspicion whatsoever that Pluto was not a planet until 1978. During that year, James Christy discovered that Pluto has a moon (named Charon) with a diameter that is estimated to be half of Pluto.

In August of 2006, the IAU officially made a textbook altering decision. This decision provides the answer to the question of why is Pluto not a planet. The IAU is considered the official naming and definition-making organization of the entire astronomical community.

After many discussions and debates, the scientists at the IAU meeting decided to downgrade Pluto and to take away Pluto's planetary title and status. Instead, the decision was made to label Pluto as a dwarf planet since it fails to meet their specific criteria of being a planet. A dwarf planet is classified as any spherical object that fails to clear its neighborhood around its orbit.

Some Astronomers Still Think Pluto Is a Planet

Despite its demotion to dwarf planet by the IAU, Pluto remains a fascinating topic of study and research. In fact, NASA has sent their "New Horizons" spacecraft to visit Pluto. New Horizons will touchdown in Pluto in July 2015.

The decision by the IAU was opposed by many professional astronomers including Dr. Alan Stern, the Principal Investigator of NASA's "New Horizons" mission to Pluto. These astronomers believe that the definition of a planet should be broader than the definition proposed by the IAU. They believe a planet to be any non-self-luminous spheroid body which is orbiting around a star.

Using this broader definition, there are 13 planets in our solar system:

• Mercury
• Venus
• Earth
• Mars
• Ceres
• Jupiter
• Saturn
• Uranus
• Neptune
• Pluto
• Haumea
• Makemake
• Eris

Because the IAU has concurred that Pluto does not meet the criteria. However, other planetary scientists still consider Pluto to be a planet in our solar system.