Course astrobiology astronomy Coursera

Astrobiology: Exploring Other Worlds

Week 1 - Planets in the Solar System and Beyond


Planet Formation

Planets and Moons

  • The surface of rocky planets is constantly changing
    • Geologically active
  • Magnetic shields are a product of differentiation
    • Shield the surface from harmful rays of space
    • Helps a planet retain its atmosphere
  • Planets near their star:
    • Hot
    • Cannot keep light gasses in their atmospheres
    • If they are very close, they tidally lock their star (one side always facing the star)
  • Planets far from their star:
    • Cold
    • May not have liquids on their surface
    • Can retain a thick atmosphere
    • May have subterranean liquids
  • Ices --> mostly frozen hydrogen compounds
  • Moons form around planetesimals
  • We haven't detected moons around exoplanets
    • Too small to detect

Our Solar System

Hot Jupiters and Planet Migration

  • Hot Jupiter
    • gas giant planet with mass equal to, or great than, Jupiter
    • Tight orbit close to their parent star
    • Surpising that they event exist
      • Simulations suggested they should not exist
      • Caused us to rethink planet formation theories
    • Planetary migration
      • Planets may drastically change orbital distances before settling into their final orbit
      • Explains how hot Jupiters would end up with an orbital path close to their star
    • The Grand Tack theory
      • How Jupiter may have experience planetary migration in our solar system
      • Cleared out some debris in the solar system and also caused the period of heavy bombardment
    • Extremely hard to verify models of the early solar system

Water Worlds

Week 2 - Hunting for Exoplanets

  • How do we find exoplanets?
    • Small and trillions of miles away
  • Two methods for finding
    • Wobble observation
      • Observing red and blue shift as it moves towards/away from us
    • Brightness of its star will dip as it orbits in front

Gravity and the Doppler Shift

  • Core technique used to find the first exoplanets
  • Reflex motion
    • Gravitational pull of massive planets (like Jupiter) is enough to exert gravitational influence on their star
    • We don't actually see the star move from large distances but observe a red and blue shift
    • doppler shift
    • Toward us --> blue shift (compressed waves)
    • Away from us --> red shift

The Radial Velocity Method

The Transit Method

  • Observing the light of a star
  • The light will dim as an exoplanet moves in front of the star (transiting)
  • Very difficult because the dimming is very short
  • Strongly prefers finding hot Jupiters
  • Limitations
    • Size
      • The larger the planet radius, the easier it will be to observe dips
    • Orbital distance
      • Planets with greater orbital distance from their star crease smaller dips

Learning from Observations

  • Relies on Johannes Kepler's laws of planetary motion
  • From the radial velocity method:
    • With radial velocity and time (radial velocity method) and Keper's first law, we can determine the orbiutal period and distance
    • With orbital distance and Kepler's first law, we can calculate the velocity
    • Once a planet's velocity is known, we can calculate the mass of an exoplanet
  • From the transit method:
  • From both:

Planet Composition

Microlensing, Imaging, and Selection Effects

  • We can't take images of exoplanets because they are too faint compared to their stars
    • Need to block out the star first if you want to take an image of an exoplanet (easier to do in IR)
  • Coronagraphs
    • Designed to block out the central cirlce of its field of view
    • Used to block out a central star so we can image exoplanets
  • Gravitational lensing
    • Massive objects warp the fabric of space-time
    • Light travels through this warped space
    • Better for more distant exoplanets
    • Alignments are very rare