Planet

Modified from Image Source: www.businessinsider.com

The term “planet” originally comes from the Greek word for “wanderer” since these objects were seen to move in the sky independently from the background of fixed stars that moved together through the seasons. The IAU last defined the term planet in 2006, however the new definition has remained controversial. The IAU’s definition applies only to our Solar System and so the definition below has been edited for clarity and generalized to apply to extrasolar planets (exoplanets) as well.

  1. A celestial body that orbits a star or stellar remnant.
  2. Has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, but not massive enough to become a star.
  3. Has cleared the gravitational neighborhood around its orbit.

For an object to assume a round or nearly round shape, the object should generally have a mass above 5 x 1020 kg and diameter greater than 800 km. However, borderline cases would have to be established by observation.

The original draft of the 2006 IAU resolution redefined hydrostatic equilibrium shape as applying “to objects with mass above 5 × 1020 kg and diameter greater than 800 km”, but this was not retained in the final draft since the composition and mass of an object determine the minimum diameter to achieve a rounded shape. Even though bodies made of denser materials become spherical at smaller radii, an objects composition and associated compressibility tend to drive the minimum diameter. Therefore, for bodies made mainly of rock, the minimum diameter to assume a round or nearly round shape is about 600 km diameter. For bodies made mainly of ice, the minimum diameter drops to about 400 km diameter1.

The IAU accepted planets of our Solar System closest to farthest from the Sun are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. A stellar memory device (mnemonic) can be seen on the page Remember the Planets!.

The Case For and Against Pluto

Since, according to the IAU’s new definition, Pluto has not cleared its orbital space and shares it with many other objects in the Kuiper Belt, it was reclassified as a dwarf planet. However, not all scientists agree. Dr. Alan Stern argues that the requirement for clearing the neighborhood or zone is “sloppy” based on the following reasons summarized from his interview Fighting for Pluto’s Planet Title and from this author’s additional research:

  1. No planets in our Solar System orbit in a fully cleared zone. All have some form of asteroids, comets or Kuiper Belt objects passing through their orbital zone. These objects typically have orbits that intersect Pluto’s orbit, but do not necessarily share its same orbit. However, even if we assume a stricter definition of clearing the neighborhood of objects that share the same orbit, even this requirement cannot be met by other planets in our Solar System. For example, the Trojan asteroids are comprised of two large groups of asteroids that orbit ahead and behind Jupiter in the same orbit around the Sun as Jupiter.
  2. At increasingly farther distances from the sun, an object’s orbit increases and it’s orbital speed decreases, therefore zone clearing takes longer and is more difficult. This situation imposes a distance dependence where identical objects don’t classify identically at different distances from the Sun. As Alan Stern puts it, “when people say Pluto can’t clear its orbital zone, they should be fair and also point out that planet Earth also couldn’t clear a zone this far out, so the IAU definition would exclude an Earth – and a Mercury,a Venus or a Mars – at Pluto’s distance.”

Geophysical Definition of a Planet

This continued controversy led to the following geophysical definition for planet being proposed by Runyon et al., #1448 (2017), and emphasizes a body’s intrinsic physical properties over its extrinsic orbital properties. This definition currently describes ~110 known planets in the solar system.

Image Credit: NASA New Horizons / LORRI / Johns Hopkins University Applied Physics Laboratory / Southwest Research Institute, July 11, 2015 Color Processing: Elisabetta Bonora & Marco Faccin /Alive Universe Images

“A planet is a sub-stellar mass body that has never undergone nuclear fusion and that has sufficient self-gravitation to assume a spheroidal shape adequately described by a triaxial ellipsoid regardless of its orbital parameters.”