Detailed Notes on Earth's Astrophysical Context and Exoplanet Search

Liquid Water on Earth
  • Effective temperature recalculated for Earth using albedo (α = 0.60) for a sun 40% fainter:
    • Teff=203extK=−70ext°CT_{eff} = 203 ext{ K} = -70 ext{ °C}
    • Modest greenhouse warming of 11 °C from atmospheric CO2 results in a -59 °C average surface temperature.
    • This would render Earth's oceans frozen, leading to the snowball Earth scenario.
Snowball Earth and Habitability
  • Snowball Earth:
    • An ice-covered planet reflects solar radiation, which leads to lower temperatures and reduced greenhouse heating.
    • The snowball Earth scenario defines the cold edge of the Habitable Zone (HZ).
  • Habitable Zone Considerations:
      • Cold side: water exists as ice & snow.
    • Hot side: defined by boiling oceans (example: Venus).
Climate Feedback Mechanisms
  • Ice has a high albedo, meaning more ice leads to lower temperatures and further ice growth, creating a positive feedback loop.
  • Once on the cold edge of the HZ, recovery from this state is unlikely.
Geological Evidence of Water
  • Earth's geological records (2-3 Gyr ago) show sedimentary deposits confirming liquid water presence.
  • Fossils indicate life existed in oceans, and vegetation existed on land.
Evolution of Earth's Atmosphere
  • Early Earth must have had a greenhouse gas-rich atmosphere to maintain above-freezing temperatures and allow liquid water.
  • The carbon-silicate cycle regulates Earth's surface temperature in response to the Sun's increasing luminosity.
The Faint Young Sun Problem
  • The Sun’s luminosity has increased by about 40% since Earth’s formation; the stability within the HZ maintained by the carbon-silicate cycle is key to liquid water existence.
The Carbon-Silicate Cycle Explained
  • The cycle includes:
    1. Increased evaporation → higher precipitation → more erosion of silicate rocks.
    2. Silicates dissolve in oceans (CaSiO3), forming insoluble minerals (CaCO3, SiO2) that sink to the ocean floor.
    3. Eventually, CO2 is released back into the atmosphere via volcanic activity.
CO2 Balance Mechanisms
  • CO2 emissions and consumption are balanced by the temperature-dependence of silicate weathering:
    • Warmer temperatures lead to more CO2 being removed, thus cooling the Earth.
    • Conversely, cooler temperatures result in decreased CO2 removal, warming the Earth.
Extrasolar Planets and Exoplanet Search
  • Earth is the only known planet in the HZ; search for exoplanets involves looking at stars beyond our solar system.
  • Detecting exoplanets:
    • Astrometric wobble, radial velocity variations, and transits as main techniques.
Astrometry Approach
  • Astrometric wobble detects small orbits or wobbling of host stars due to orbiting planets.
Radial Velocity Methods
  • Changes in spectral lines are indicative of the star's reflex motion due to orbiting planets.
  • The UBC group's approach focused on observing small variations in the star due to planet impacts.
The UBC Exoplanet Search Program
  • Post 1987, they reported a planet candidate around γ Cephei but faced skepticism and misinterpretation issues that nearly destroyed the project.
Hot Jupiters and Their Discovery
  • First confirmed hot Jupiter (51 Peg) was found by Mayor & Queloz in 1995 and has since catalyzed extensive searches for more exoplanets.
    • Hot Jupiters orbit closely to their host stars with periods shorter than 10 days, contrary to typical gas giant formation expectations.
The Kepler Mission
  • Aimed to find Earth-sized planets in the HZ, monitored 530,506 stars, discovering over 2,327 confirmed exoplanets before its conclusion.
The TESS Mission
  • Launched in 2018, TESS searches broader areas for exoplanets and is anticipated to discover over 20,000 new planets.
Direct Imaging and Planet Formation
  • Few exoplanets have been detected through direct imaging, primarily in infrared.
  • Notable images of young stars in protoplanetary disks illustrate planet formation dynamics.
Gravitational Scattering and Exoplanet Eccentricity
  • Gravitational interactions influence the orbits of planets, leading to eccentric orbits near host stars. This phenomenon explains various exoplanet characteristics.
Conclusions on Earth-like Exoplanets
  • True Earth analogs in the HZ are rare; observational biases highlight limits to detection of Earth-like planets.
  • Most planetary systems do not mimic the solar system, particularly in terms of orbital stability and distances.