Planet Definition and Classification

Planet Definition and Classification

• A planet is a celestial body orbiting a star, like our Sun.
• Stars are astronomical bodies primarily composed of plasma, with immense gravity enabling planets to orbit.
• Early Greeks referred to planets as "wandering stars" ("asteres planetai").
• Planets form from leftover debris in a protoplanetary disk, which is crucial for planet creation.

Planet Characteristics

• Orbit a star: Essential for planet classification.
• Sufficient gravity: Sufficient gravity to force the object into a spherical shape.
• Orbital dominance: Large enough to clear its orbital neighborhood of smaller objects.
• Moons orbit planets but lack the force to clear or dominate their orbital neighborhood.

Types of Planets

• Rocky (Terrestrial) Planets: Predominantly composed of rock, water, carbon, and silicate.

  • Form closer to the star due to solar heat.
  • Examples: Mercury, Venus, Earth, Mars

• Gas Planets (Gas Giants): Primarily composed of gaseous substances like hydrogen and helium.

  • Tend to be larger and further from the star.
  • Examples: Jupiter, Saturn, Uranus, Neptune

Solar System Planets

• Order: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune
• Mnemonic: "My Very Excellent Mother Just Served Us Nachos"

Dwarf Planets

• Are smaller than standard planets, and round.
• Orbit the Sun but do not clear their orbital neighborhood.
• Example: Pluto (previously considered a planet).
• Other dwarf planets include Ceres, Eris, and Sedna.
• Pluto resides within the Kuiper Belt which is a region beyond Neptune.

Exoplanets

• Planets orbiting stars outside our solar system.
• Some exoplanets are free-floating ("rogue planets").
• Located within the Milky Way galaxy.
• Identification: by detecting wobble in star position (a technique called 'dimming') or by changes in the star's light intensity during an exoplanet's orbit.
• Example: TRAPPIST-1 exoplanets.