Overview

It is the only science that has no subject matter yet. We have never found a single alien microbe. But we can use what we know about Earth life to guess what could be out there. It is the border between biology and science fiction.

Core Idea

The core idea is Universal Biology. Are the rules of life (DNA, cells, carbon) universal, or are they just one option?

  • Carbon Chauvinism: The bias that life must be carbon-based.
  • Silicon Life: Silicon is chemically similar to carbon. Could rock-monsters exist on hot planets?

Formal Definition

The study of the origin, evolution, distribution, and future of life in the universe.

Intuition

  • Biology: Studying the fish in your aquarium.
  • Xenobiology: Asking “Are there fish in the ocean? And do they look like my fish?”

Examples

  • Europa: A moon of Jupiter. It has an ocean of liquid water under miles of ice. NASA is sending a probe (Europa Clipper) to sniff the plumes for signs of life.
  • Titan: A moon of Saturn. It has lakes of liquid methane (natural gas). Could life exist there that drinks methane instead of water?
  • The Goldilocks Zone: The distance from a star where water is liquid (not ice, not steam). Earth is in the zone. We look for other planets there.

Common Misconceptions

  • Aliens will look like humans: Unlikely. The humanoid shape is a specific result of Earth’s evolution. Aliens might look like octopuses or clouds of gas.
  • We will find radio signals: SETI listens for radio. But simple life (bacteria) doesn’t build radios. We are more likely to find “Biosignatures” (oxygen in the atmosphere) than a message.
  • Fermi Paradox: “Where is everybody?” If the universe is old and full of stars, why haven’t we seen anyone?
  • Shadow Biosphere: The idea that there might be a second, unrelated tree of life right here on Earth that we haven’t noticed because our tests only look for DNA.

Applications

  • Planetary Protection: NASA has strict rules to not contaminate Mars with Earth bacteria (Forward Contamination) and not bring Mars bacteria back to Earth (Back Contamination).

Criticism / Limitations

  • N=1: We only have one data point (Earth). It is hard to do science with a sample size of one.

Further Reading

  • Sagan, Carl. Cosmos. 1980.
  • Grinspoon, David. Lonely Planets. 2003.