Astrobiology and the Search for Extraterrestrial Life

Posted on Jun 12, 2026 in Geology

Search for Life in the Solar System

Slides: Day 12–16

General Requirements for Life

  • Life needs: organic molecules + energy source + liquid medium.
  • Water is the best-known liquid medium: it dissolves chemicals, transports substances, and participates in metabolism.
  • Other possible liquids include methane, ethane, or ammonia, though these typically involve colder, slower chemistry.
  • Liquid water is the easiest search target, but life may exist in subsurface oceans or non-water liquids.

The Moon

  • The Moon has essentially no atmosphere; its density is much lower than Earth’s.
  • If surface water existed, it would evaporate or escape into space.
  • There is no recent geological activity to resupply volatiles.
  • Water ice may exist in permanently shadowed polar craters, likely deposited by impacts.
  • The Moon’s night side is much colder than Earth’s because there is no atmosphere to redistribute heat.

Mercury

  • Often described as “the Moon, but hotter.”
  • Essentially all water and volatiles have been lost to space.
  • It is very hot during the day and very cold at night.
  • Ice in polar craters does not make Mercury broadly habitable.

Venus

  • Venus: 0.949 Earth radii, 0.815 Earth masses, 0.723 AU.
  • If Venus had an Earth-like atmosphere, the average temperature would be about 95°F.
  • Reality: surface temperature is about 880°F, pressure is about 94× Earth’s, and the atmosphere is 96% CO₂.
  • The main reason it is hotter than Earth is the thick CO₂ atmosphere causing a runaway greenhouse effect.
  • Surface conditions rule out ordinary life.
  • Possible present habitability exists only high in the atmosphere/cloud layer, where temperatures are more Earth-like, though sulfuric acid and cloud chemistry remain harsh.
  • Venus may have been habitable in the past if it once possessed liquid water.

Mars

  • Mars: 0.533 Earth radii, 0.107 Earth masses, year = 1.881 Earth years.
  • The most promising terrestrial planet besides Earth.
  • Today: no stable liquid water on the surface due to a thin atmosphere and low pressure.
  • Seasons are caused by axial tilt, not primarily by distance from the Sun.
  • Polar caps include CO₂ dry ice.
  • Evidence for ancient liquid water:
    • Meandering riverbeds and channels
    • Tributary networks
    • Minerals and rocks formed in liquid water
    • Rounded pebbles and sediments
    • Lakebed-type deposits
  • Polar ice caps by themselves are not evidence for ancient flowing liquid water.
  • Early Mars could have been habitable.
  • Small planets cool faster: heat stored scales with volume (∝r³), while heat loss area scales with surface area (∝r²).
  • Mars cooled faster than Earth, leading to weaker geology, atmosphere loss, and less long-term habitability.
  • Possible underground life: it is warmer below the surface, water is less likely to evaporate, and extremophile analogies exist.

Giant Planets

  • Jupiter and Saturn: no solid surface, mostly H/He, not ideal life candidates.
  • Jupiter atmosphere problem: strong vertical convection and winds make it hard to stay at the right altitude.
  • Uranus and Neptune: mostly H/He with methane and strong winds.

Why Icy Moons Matter

  • Outer Solar System moons formed beyond the snow line, where rocks and water ice were available.
  • Water ice is common in the moons of giant planets.
  • Although cold, they can have internal heat from:
    • Radioactive decay
    • Tidal heating/tidal dissipation
  • Tidal heating: orbital resonances keep orbits eccentric; the planet stretches the moon, and friction heats the interior.
  • The habitable zone is not necessary because liquid water can be heated by sources other than the Sun.

Io

  • The closest Galilean moon.
  • Extremely volcanic due to tidal heating from Jupiter.
  • No atmosphere; outgassed volatiles are lost to space.
  • A good example of tidal heating, but not a good life candidate.

Europa

  • Icy shell, few craters, and cracks indicate a young, resurfaced exterior.
  • Possible subsurface ocean.
  • A conducting fluid below the surface has been detected through magnetic field measurements.
  • Possible water plumes.
  • Europa Clipper: launched in 2024, arriving in 2030 to determine ocean depth, surface composition, and geology.

Enceladus

  • A Saturn moon with active plumes.
  • Lack of craters, presence of cracks, and plumes indicate active geology.
  • Cassini flew through plumes and measured their composition.
  • Evidence of a global, deep ocean, organics, simple molecules, and water vapor.
  • Liquid water exists due to internal heating and ammonia antifreeze.
  • A strong target for life searches because ocean material is ejected into space, making it easier to sample.

Ganymede vs. Callisto

  • Both have exterior ice shells and possible subsurface oceans.
  • Ganymede: shows some geological activity.
  • Callisto: no clear surface geological activity; heavily cratered and older.
  • If asked which has more recent geological activity, the answer is Ganymede.

Titan

  • Titan has a thick atmosphere; surface pressure is about 1.6× Earth’s.
  • Atmosphere includes nitrogen, methane, argon, and organics.
  • Surface temperature is about −290°F.
  • Features methane clouds, rain, and liquid methane/ethane lakes.
  • Organic haze from UV chemistry provides a protective atmosphere.
  • Water ice is abundant; possible cryovolcanism could provide heat or liquid water.
  • Question: can methane be life’s solvent? Chemistry, metabolism, and evolution would be slow due to the cold.
  • Dragonfly mission: a dual-quadcopter to sample surface material and study the atmosphere and seismic activity (estimated launch 2028).

Textbook Quick-Quiz Traps for Chapter 7

  • Oxygen and carbon are the 3rd and 4th most abundant elements.
  • At twice the Earth-Sun distance, sunlight is 1/4 as strong.
  • Liquid methane is colder than liquid water.
  • Frozen lakes have liquid water below because ice floats and insulates.
  • Mercury: hot day, cold night.
  • Venus: liquid water, if any exists now, is only high in the atmosphere/clouds.
  • Venus is hot mainly because of the thick CO₂ greenhouse effect, not just proximity to the Sun.
  • Jupiter’s atmosphere is poor for life mostly because strong winds and convection prevent organisms from staying at the right altitude.
  • A random small Jovian moon likely has water ice.
  • Cassini orbited Saturn.

Future of Life on Earth

Main slide file: Day 17; also Day 16 ending

Habitable Zone Basics

  • Habitable zone: the region where liquid water could exist on the surface of an Earth-like world.
  • Depends mainly on distance from the star and stellar luminosity.
  • Assumptions: Earth-sized, Earth-composition, and Earth-like atmosphere.
  • The HZ is useful but not perfect:
    • Not sufficient: Moon/Mars are in/near the HZ but are not broadly habitable.
    • Not necessary: Europa/Enceladus have subsurface oceans; Titan has alternative liquids.
  • Requirements include size, composition, atmosphere, geology, and energy sources.

Sunlight and Distance

  • Energy from a star follows the inverse-square law: flux ∝ 1/r².
  • Venus at 0.72 AU receives about 193% of Earth’s sunlight.
  • Mars at 1.5 AU receives about 44% of Earth’s sunlight.
  • Jupiter at 5.2 AU receives about 3.7% of Earth’s sunlight.

Atmosphere and Greenhouse

  • The atmosphere keeps surface temperature more uniform and prevents oceans from boiling away.
  • Earth is warmer than a naive calculation suggests because greenhouse gases trap heat.
  • The Moon has extreme day/night temperature variations because it lacks an atmosphere.

Inner Edge of the Habitable Zone

  • Runaway greenhouse inner edge: maybe around 0.84 AU.
  • Moist greenhouse inner edge: maybe around 0.95 AU.

Runaway Greenhouse

  • Higher temperature → more evaporation.
  • More water vapor → stronger greenhouse effect.
  • Stronger greenhouse → more heating → more evaporation.
  • Oceans evaporate; carbonate rocks decompose; more CO₂ strengthens the greenhouse.
  • Earth at Venus’s orbit would likely undergo a runaway greenhouse and become extremely hot.

Moist Greenhouse

Key sequence:

  1. Higher temperature evaporates more water.
  2. Water vapor reaches the high atmosphere.
  3. UV dissociates water molecules.
  4. Hydrogen escapes to space because it is light.
  5. Repeat until oceans are lost.

Common exam trap:
Moist greenhouse = water loss to space after UV splitting + H escape.
Runaway greenhouse = positive feedback heating/evaporation.

Outer Edge of the Habitable Zone

  • Outer edge: maybe around 1.7 AU, where even a strong greenhouse cannot keep a planet warm.
  • Could be around 1.4 AU if CO₂ freezes/snows out.
  • Methane might extend the outer edge in some cases.

Future of Earth

  • The Sun is not static; it increases fusion and luminosity over time.
  • Earth will receive more sunlight in the future.
  • The CO₂ cycle has kept Earth’s temperature relatively stable so far.
  • A moist greenhouse may end Earth’s habitability in roughly 10⁸ years.
  • Eventually, oceans will evaporate and Earth will become uninhabitable.
  • The long-term Sun red giant stage may engulf or severely heat Earth.
  • Jupiter/Saturn moons may later enter the habitable zone.

Star Habitability and Exoplanets

  • Different stars have different temperatures, luminosities, and lifetimes.
  • Massive bright stars have short lifetimes, leaving less time for life.
  • Low-mass stars live long but can have UV, flare, or tidal locking issues.
  • The habitable zone can be calculated around any star if assumptions are fixed.

Textbook Quick-Quiz Traps for Chapter 10

  • Earth receives more sunlight in the future because the Sun brightens.
  • Moving Earth inward toward Venus’s orbit would cause a runaway greenhouse.
  • The HZ does not guarantee habitability.
  • Liquid water can exist outside the HZ if internally heated.
  • Atmosphere matters as much as distance.

Exoplanets

Slides: Day 18–21 + early Day 22

Favorable Traits for Habitable Systems

  • Enough heavy elements for planets/life: oldest stars are disfavored.
  • Enough time for life to develop: massive O/B stars are disfavored.
  • Low enough UV radiation: solar-type stars are favorable; red dwarfs have issues.
  • Stable orbits: multiple-star systems can be harder.
  • Only ~7% of stars are G-type like the Sun; we should not only search Sun-like stars.

Exoplanet Detection Overview

Main methods:

  • Transit
  • Radial velocity
  • Direct imaging
  • Microlensing
  • Astrometry/timing

Detection bias:

  • We have discovered many planets unlike Solar System planets.
  • We are not sensitive to all types; a lack of detections does not mean a lack of planets.
  • The current sample is biased toward close-in planets, big planets, and transiting geometry.

Radial Velocity Method

  • Planet gravity makes the star wobble.
  • Detect periodic Doppler shifts in the star’s spectral lines.
  • Measures motion toward/away from us.
  • Gives only minimum mass/lower bound because the inclination is unknown.
  • True mass can be larger than M sin i.
  • Stronger signal for massive planets, close-in planets, and lower-mass stars.
  • Earth causes a Sun wobble of only ~10 cm/s, which is very hard to detect.

Transit Method

  • Planet passes in front of a star, causing a brightness dip.
  • Transit depth: drop in flux ≈ (Rp/Rs)².
  • Earth around the Sun → ~0.01% drop.
  • Easier for large planets and small stars.
  • Need multiple transits to confirm.
  • Only a small fraction of systems are aligned to transit.

Kepler

  • Stared at the same field of ~150,000 stars.
  • Measured brightness every 30 minutes for 4 years.
  • Goal: Earth-sized planets at Earth-like periods.
  • Kepler measured the probability that Sun-like stars host Earth-sized planets in the HZ, called η⊕.
  • Roughly ~10% for Sun-like stars, but uncertain; could be much higher or lower.
  • Kepler did not directly measure water, life, or detailed atmospheres.

TESS / TRAPPIST-1 / JWST

  • TESS finds planets around bright nearby stars, which are good for atmospheric follow-up.
  • TRAPPIST-1: a small star where an Earth-sized planet causes nearly 1% dimming.
  • TRAPPIST-1 has multiple HZ planets, but we do not know what they look like.
  • Concerns: high UV, tidal locking, and atmosphere survival.
  • JWST is versatile for atmospheric spectroscopy but was not originally designed for it.
  • So far: no clear atmosphere detection for TRAPPIST-1 planets in the slides.

Transmission Spectroscopy

  • During transit, some starlight passes through the atmosphere.
  • Molecules absorb specific wavelengths.
  • The planet blocks more light at wavelengths absorbed by atmospheric molecules.
  • Can detect H₂O, CO₂, CH₄, Na, etc.
  • Star spots can contaminate interpretation.

Secondary Eclipse Spectroscopy

  • Measure the drop in total light when the planet goes behind the star.
  • Allows measurement of light coming directly from the planet.
  • Useful for hot planets and day-side temperature.
  • Day-side temperature can indicate whether a planet has an atmosphere.

Direct Imaging

  • Hard because star glare overwhelms the planet.
  • Like seeing a firefly around a streetlight from miles away.
  • A coronagraph suppresses starlight.
  • Current direct images are mostly young, hot super-Jupiters in the infrared.
  • Current telescopes cannot take useful images of terrestrial exoplanets around Sun-like stars.
  • Future: Roman coronagraph/Habitable Worlds Observatory aim toward exo-Earth imaging.

Biosignatures

Types:

  • Gaseous: O₂, O₃, CH₄, H₂O, CO₂.
  • Surface: red edge/vegetation-like reflectance.
  • Temporal: seasonal changes, Keeling-curve-like gas cycles.

Oxygen Biosignature Caveats

  • Oxygen alone is not definitive.
  • Abiotic oxygen can form from CO₂ photolysis: CO₂ + photon → CO + O; O + O + M → O₂ + M.
  • Life does not necessarily use or produce oxygen.
  • Earth itself had little oxygen for much of its history.
  • Need context: water, methane, CO₂, CO, and disequilibrium.

Disequilibrium Chemistry

  • Life pushes an atmosphere out of chemical equilibrium.
  • Example: O₂ + CH₄ together can be suggestive because they react away without replenishment.
  • Need combinations of gases unlikely to coexist without life.

Red Edge

  • A surface biosignature.
  • Photosynthesis absorbs shorter wavelengths and reflects strongly in the near-infrared.
  • Could directly suggest vegetation-like life.
  • Alien photosynthesis may use other pigments/colors.

Technosignature Crossover

  • Industrial climate change, smog, artificial lights, and megastructures are technosignatures, not just biosignatures.

Density / Mass / Radius

  • Density = mass/volume.
  • Same mass but larger radius → larger volume → lower density.
  • A larger radius at the same mass means a puffier/less dense planet.

Textbook Quick-Quiz Traps for Chapter 11

  • The HZ can be identified around nearly any star.
  • Radial velocity = lower bound/minimum mass.
  • Kepler = probability of Earth-sized HZ planets around Sun-like stars.
  • Transmission spectroscopy = more light blocked at certain wavelengths.
  • Red edge = surface biosignature.
  • Climate change from industrialization = technosignature.
  • Twice the distance = 1/4 the sunlight.
  • Same mass + larger radius = lower density.

Search for Intelligent Life

Slides: Day 22 after SETI starts, Day 23, Day 24, Day 25

Intelligence and SETI Framing

  • “Intelligent life” is hard to define.
  • Practical SETI question: search for life capable of transmitting detectable signals across interstellar distances.
  • Humans have been radio-detectable for only about the last 100–120 years.
  • Intelligence may not be inevitable; human-like intelligence arose very late on Earth.
  • Intelligence has benefits but requires high energy/resources.

Drake Equation

N = R* × fp × ne × fl × fi × fc × L

  • N: number of transmitting ET civilizations in our galaxy.
  • R*: star formation rate.
  • fp: fraction of stars with planets.
  • ne: number of suitable planets per system.
  • fl: fraction where life develops.
  • fi: fraction where intelligent life develops.
  • fc: fraction capable of interstellar communication.
  • L: lifetime of a communicating civilization.
  • Not exact; estimates can differ by huge factors.
  • Exam trap: The Drake equation estimates currently transmitting/communicating civilizations, not all life ever.

Technosignatures

  • Signs of alien technology, distinct from biosignatures.
  • Examples: climate change, smog, city lights/LEDs, solar arrays, megacities, planet-scale engineering, radio/optical beacons, Dyson spheres.
  • Need imaging or spectroscopy of terrestrial planets, which is hard.

SETI Signals

  • Modern SETI searches for radio or optical signaling beacons.
  • Targeted known planetary systems vs. all-sky surveys.
  • SETI has been mostly privately funded historically.
  • Leaked communication signals get weaker as distance².
  • With current technology, human-like leakage would not be detectable from Proxima Centauri.
  • Intentional beacons are easier because they are designed to be detectable.
  • 1974: Arecibo message sent toward M13 globular cluster.

Narrowband Signals

  • Narrowband signals are efficient and artificial-looking.
  • Natural astrophysical sources are usually broader or explainable.
  • SETI looks for signals distinct from astrophysical sources.
  • Repeating pulses/patterns can be interesting but can be natural (e.g., pulsars).
  • Wow! Signal: 1977, sharp narrowband signal, never repeated.

Kardashev Scale / Astro-Engineering

  • Type I: planetary civilization.
  • Type II: stellar civilization.
  • Type III: galactic civilization.
  • Type II → look for Dyson spheres.
  • Type III → maybe interstellar/galactic-scale transmissions.

Dyson Spheres

  • Civilizations need energy.
  • Most energy in a star system comes from the star.
  • Dyson sphere/swarm goal: harness all or part of a star’s energy.
  • Search signs: partially blocked starlight/artificial transits, waste heat/infrared radiation.

UFOs / UAPs

  • UFO = unidentified flying object: unknown, flying, object.
  • Unknown does not mean alien.
  • About ~90% are explained as stars, planets, aircraft, gliders, rockets, balloons, birds, ball lightning, or meteors.
  • Roswell: likely a classified military balloon experiment.
  • Crop circles: man-made.
  • Alien abductions: often explained by sleep paralysis/hallucinations.
  • UFO study should be systematic and evidence-based.

Interstellar Travel

  • Modern spacecraft take years to reach the outer Solar System.
  • Voyager 1/2 entered interstellar space.
  • Human-built spacecraft are just beyond the Solar System edge, not at the nearest star.
  • Voyager-speed travel to Proxima Centauri would take thousands of years (Day 24 says ~16,700 years).
  • Relativity: nothing with mass can travel faster than light; energy needed approaches infinity near c.
  • Time dilation means the crew may age less at near-light speed, but Earth time still passes.
  • Future propulsion ideas: nuclear pulse, ion engines, light sails, matter-antimatter, interstellar ramjets.
  • Slowing down is also hard.

Outer Space Treaty / Colonization

  • Outer Space Treaty signed in 1967.
  • Outer space exploration/use is for the benefit of all.
  • Any state can explore.
  • Territory in outer space cannot be claimed by any country.
  • Celestial bodies must be used for peaceful purposes.
  • States are responsible for government and non-government space activities.
  • Colonization is not a near-term solution to overpopulation or climate change.
  • Earth is not easily disposable.

Fermi Paradox

  • If life is abundant, there should be older/more advanced civilizations.
  • The Milky Way formed billions of years before Earth.
  • If some civilizations survive and expand, why haven’t we seen them?
  • Possible answers: Earth/humans are rare; civilizations destroy themselves or do not last; interstellar travel is too hard; they are avoiding us (zoo hypothesis); we are not looking correctly; signals are too weak/short-lived; technology may not always lead to communication.

“Could We Be the Most Advanced?”

  • This is one possible Fermi paradox solution.
  • It assumes Earth is rare or we are special.
  • Historically, “we are central/special” has usually been wrong.
  • Still possible because we do not know how common plate tectonics, the origin of life, mass extinction history, intelligence, and technology are.

Textbook Quick-Quiz Traps for Chapter 12

  • The Drake equation estimates the number of civilizations capable of radio/interstellar communication now.
  • SETI looks for narrowband signals.
  • Dyson sphere goal = collect a star’s energy.
  • UFO reports are mostly misidentifications, not mostly hoaxes.
  • The Outer Space Treaty prohibits territory claims.
  • Farthest spacecraft = just beyond the Solar System edge, not another star system.
  • Red lights preserve dark adaptation at an observatory.