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Defining 'planet' - my suggestion
Jul. 16th, 2015 10:23 amI've been interested in space and astronomy all my life, and I'm a lawyer. As such I've been thinking about the question - back in the public eye again with the Pluto encounter - of what the rules should be for defining a planet.
Here is my suggestion on the 'what is a planet' question in light of what we saw at Pluto.
A planet is a celestial body that:
1) Orbits the sun, not any other body.
2) Has achieved hydrostatic equilibrium (i.e. its own gravity has squashed it to be round).
3) Has one of the following properties:
3a) Is a gas giant*; or
3b) Has a surface modified by self-generated geological processes.
(*A more formal definition might be along the lines of 'more than half its mass is not in solid phase')
Point (1) excludes geologically active moons such as Io, Triton or Enceladus.
Point (2) excludes comets.
Point (3a) is probably obvious but ensures that Jupiter etc count despite not having a 'surface' for the purposes of point (3b).
Point (3b) is what distinguishes a planet from a large asteroid and also excludes Io etc as their geological activity is generated externally (tidal forces from their primary body).
On this definition, Jupiter, Saturn, Uranus and Nepture are all planets by virtue of point (3a). Mercury, Venus, Earth, Mars and Pluto (as seen in the last couple of days) are all planets by virtue of point (3b).
Pluto and Charon probably count as a 'double planet' given that Charon seems to meet (3b) too.
Ceres is not a planet because it doesn't meet (3b). Vesta is not a planet because it doesn't meet (2) or (3b).
Until we see their surfaces, we cannot class Eris, Haumea or Makemake as planets because we do not know that they meet (3b).
This definition returns us to the nine-planet solar system. It allows for Kuiper belt objects to be defined as planets if it turns out that they have been geologically active, but until we send probes to them they remain dwarf planets. Finally, in my view it is not an arbitrary criterion: having an active self-generated geology is a significant factor.
Here is my suggestion on the 'what is a planet' question in light of what we saw at Pluto.
A planet is a celestial body that:
1) Orbits the sun, not any other body.
2) Has achieved hydrostatic equilibrium (i.e. its own gravity has squashed it to be round).
3) Has one of the following properties:
3a) Is a gas giant*; or
3b) Has a surface modified by self-generated geological processes.
(*A more formal definition might be along the lines of 'more than half its mass is not in solid phase')
Point (1) excludes geologically active moons such as Io, Triton or Enceladus.
Point (2) excludes comets.
Point (3a) is probably obvious but ensures that Jupiter etc count despite not having a 'surface' for the purposes of point (3b).
Point (3b) is what distinguishes a planet from a large asteroid and also excludes Io etc as their geological activity is generated externally (tidal forces from their primary body).
On this definition, Jupiter, Saturn, Uranus and Nepture are all planets by virtue of point (3a). Mercury, Venus, Earth, Mars and Pluto (as seen in the last couple of days) are all planets by virtue of point (3b).
Pluto and Charon probably count as a 'double planet' given that Charon seems to meet (3b) too.
Ceres is not a planet because it doesn't meet (3b). Vesta is not a planet because it doesn't meet (2) or (3b).
Until we see their surfaces, we cannot class Eris, Haumea or Makemake as planets because we do not know that they meet (3b).
This definition returns us to the nine-planet solar system. It allows for Kuiper belt objects to be defined as planets if it turns out that they have been geologically active, but until we send probes to them they remain dwarf planets. Finally, in my view it is not an arbitrary criterion: having an active self-generated geology is a significant factor.
