What is a Planet

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What is a Planet?

• The Controversy

51.106 The Planets
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Pre 2006

• The planets were major objects in orbit around
  the Sun.
• Mercury
• Venus
• Earth
• Mars
• Jupiter
• Saturn
• Uranus (Discovered in 1786)
• Neptune (Discovered in 1842)
• Pluto (Discovered in 1930)

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Pre 2006

• Asteroid Ceres was discovered between the orbits
  of Mars and Jupiter in 1801. At the time, it was
  called a planet.
• Once many more were discovered, it was realized
  that this was more of a debris field and it was
  renamed a minor planet or asteroid (from
  aster Greek for star).

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What was wrong with the old definitions?

• Many astronomers werent happy with Pluto being a
  planet. Why?
• 1. Its the smallest planet and only 2300 km in
  diameter, 2/3 the size of our own Moon!
• 2. Its orbit is highly elliptical and inclined,
  unlike every other planet. In fact, it crosses
  Neptunes orbit.
• 3. The outer 4 planets are Gas Giants Pluto is
  small and icy.
• 4. In recent years, dozens of other objects in
  similar orbits around the sun were discovered,
  one of which 2003 UB313 (nicknamed Xena), is
  larger than Pluto. Do we make these planets?

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2006

• To account for the slew of new objects being
  discovered, the International Astronomical Union
  (IAU), attempted to formalize the definition of
  Planet at its meeting in Prague in Aug 2006.
  The executive committee recommended the following
  definition of Planet for a vote
• "A planet is a celestial body that (a) has
  sufficient mass for its self-gravity to overcome
  rigid body forces so that it assumes a
  hydrostatic equilibrium (nearly round) shape, and
  (b) is in orbit around a star, and is neither a
  star nor a satellite of a planet."

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The Result.

• Under this definition, Pluto would stay a planet.
  In addition, its moon Charon would become a
  planet, as would 2003 UB 313 (Xena) and the
  asteroid 1 Ceres, bringing the total number of
  planets to 12, with dozens more likely.

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The Problem

• Many astronomers hated this definition how can
  you compare the dinky object Ceres (1,000 km)
  with the larger planets like Earth (13,000 km) or
  Jupiter (143,000 km)?
• They offered an alternative

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Alternative definition

• RESOLUTION 5AThe IAU therefore resolves that
  "planets" and other bodies in our Solar System be
  defined into three distinct categories in the
  following way(1) A "planet" is a celestial
  body that (a) is in orbit around the Sun, (b) has
  sufficient mass for its self-gravity to overcome
  rigid body forces so that it assumes a
  hydrostatic equilibrium (nearly round) shape, and
  (c) has cleared the neighborhood around its
  orbit.(1)(2) A "dwarf planet" is a celestial
  body that (a) is in orbit around the Sun, (b) has
  sufficient mass for its self-gravity to overcome
  rigid body forces so that it assumes a
  hydrostatic equilibrium (nearly round) shape, (c)
  has not cleared the neighborhood around its
  orbit, and (d) is not a satellite.(2)(3) All
  other objects except satellites orbiting the Sun
  shall be referred to collectively as "Small
  Solar-System Bodies".(3)
• 1The eight planets are Mercury, Venus, Earth,
  Mars, Jupiter, Saturn, Uranus, and Neptune.
• 2An IAU process will be established to assign
  borderline objects into either dwarf planet and
  other categories.
• 3These currently include most of the Solar System
  asteroids, most Trans-Neptunian Objects (TNOs),
  comets, and other small bodies.

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Continuing.

• RESOLUTION 6AThe IAU further resolvesPluto is
  a "dwarf planet" by the above definition and is
  recognized as the prototype of a new category of
  trans-Neptunian objects.

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A proposed addendum failed

• The proposed resolution 5B attempted to insert
  the work Classical in front of planet in 5A, so
  that
• A Classical planet" is a celestial body that (a)
  is in orbit around the Sun, (b) has sufficient
  mass for its self-gravity to overcome rigid body
  forces so that it assumes a hydrostatic
  equilibrium (nearly round) shape, and (c) has
  cleared the neighborhood around its orbit.
• This would make two definitions of planet
  classical and dwarf. Under this addendum, Pluto
  would still be a planet.

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How is this different?

• In 5A, we have 3 groups
• Planets Dwarf Planets
  Small solar system bodies
• Mercury, Venus Pluto, Ceres, Xena
  other asteroids, comets, moons
• Earth, Mars, etc.
• In 5B, we have 2 groups
• Planets
  Small solar system bodies
• ? ?
• Classical Dwarf
• Mercury, Venus Pluto, Ceres, Xena
  other asteroids, comets, moons
• Earth, Mars, etc

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As of today

• The IAU adopted both 5A and 6A so that
• Planets Dwarf Planets
  Small solar system bodies
• Mercury, Venus Pluto, Ceres, Xena
  other asteroids, comets, moons
• Earth, Mars, etc.
• Pluto is a dwarf planet and the prototype of
  Trans-Neptunian Objects (TNOs)

http//astronomy2006.com/media-stream-archive.php
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It aint over til its over

• About 95 of astronomers eligible to vote on the
  definition of planet were absent from the
  meeting in Czechoslovakia and therefore were not
  able to vote. Many of those are unhappy with the
  undemocratic decision.
• A number, including some with great respect
  throughout the community, have formed a statement
  and petition which says they have no intention of
  using the adopted terminology and are calling for
  a longer discussion and more representative
  voting method.