Lecture 1: Why is Earth Habitable?

Earth Age

• 5.54 billion years old

Universe Age

• 13.8 billion years old

Stars in Milky Way

4x10^11 stars

Titan

• methane cycle, oceans of methane and ethane, mean temp -180C

Sun Luminosity

• ~10% per billion years, G-type star

Key Properties of Habitable Earth

• presence of liquid water over an extended period

• continuous protection from cosmic radiation

• not constantly bombarded by asteroids

Evolution of Life

• Archaea and bacteria appeared 3.8 Ga ago, eukaryotes evolved 1.6-2.1 Ga ago

Abiogenesis Habitable Zone

area where life can develop smaller than area where life can evolve, i.e life can adapt in a greater variety of conditions that it can originate it

Galactic Habitable Zone (6)

• has to be far from black holes and supernova due to intense cosmic radiation

• Earth’s magnetic field protects it to a degree, also far from galactic centre

• moderate metallicity: stellar system must have appropriate amounts of various elements to form rocky ‘terrestrial’ planets like Earth

• needs enough time to develop

• balance between life-destroying impacts and lesser impacts that stimulate evolutionary diversity

  • Jupiter and others ‘suck in’ asteroids and protect Earth, however also has enough gravity to pull asteroids out of the asteroid belt which creates a potential hazard

• Goldilocks Zone -> distance from the sun

  • certain elements we have deemed necessary to life only form/operate correctly in certain temperatures

Kepler Mission

• Gliese 581, red dwarf star

  • ~6 planets, 581c most earth-like exoplanet to be discovered

• probably billions of Earth-like planets in the habitable zone in our galaxy

  • most commonly ‘super Earths’, between Earth and Neptune in size

• Ground-based telescopes now capable of detecting light directly from exoplanets

Keeping Earth in the Goldilocks Zone

• global heat budget

• atmospheric density and composition and the ‘greenhouse effect’

  • +33C

• seasonality caused by axial tilt

• land-sea distribution (continental drift) + outgassing of volcanoes

Obliquity Variation

• over the past 5 million years, Earth’s obliquity has varied from 22.2° to 24.5°, currently 23.4°

Importance of the Moon

• tidally locked to the Earth

• progressively slows rotation

• limits the variation in Earth’s obliquity

  • high obliquity would result in ice around equator

  • Mars has large obliquity/climate fluctuations