PX282 - H1 - planets

image: IAU

• 8 planets in 3 classes

classes

terrestrial

• primarily iron and rock (silicates)
• deficient in volatiles

gas giants

• primarily H/He gas around a rocky/icy core
• rotate rapidly with strong magnetic field
• have a rich system of moons

ice giants

• rocky/icy core surrounded by a thinner H/He envelope
• rotate rapidly with strong magnetic field
• have a rich system of planets

others

• dwarf planets, asteroids, and comets
• mainly found in the asteroid and kuiper belts, and the oort cloud

formation

• all have approximately circular orbits in the same direction in ecliptic plane, aligned with the spin of the sun
• this shared angular momentum led to the nebular hypothesis, which is a natural consequence of conservation of angular momentum during gravitational collapse
• this proposes that the sun and the planets formed together from the gravitational collapse of gas with shared angular momentum

image: NASA/JPL-Caltech/T. Pyle (SSC)

definition of a planet

• IAU, 2006 set out three criteria that a celestial body must satisfy to be classified as a planet

• it must be in orbit around the sun

• it must have sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium, basically, must be spherical

• it must have cleared the neighbourhood around its orbit

• if only the first two are satisfied, it is called a dwarf planet, eg: pluto, ceres

• small solar system bodies satisfy just the first criteron, eg: asteroids and comets

• there are issues with this definities:

  • the third criterion has border line cases, even for earth
  • does not include exoplanets
  • has no upper limit on the size or mass, so stellar companions can be included