Life in the Universe


  • The main questions: Are we alone or is there life elsewhere? Did life on Earth come from space?
  • Need to ask first: What is life? What does it need in order to develop? How can we find out where it did?

What Happened on Earth?

  • 4.6 Gyr ago: Formation of the Earth.
  • 4.5-4 Gyr ago: Chemical evolution: - At first, atmosphere formed from H, N, C, O degassing; Then, those gases combined into CH4, NH3, CO2, H2O molecules.
  • 3.9 Gyr ago: Heavy asteroid bombardment that resurfaced the Earth.
  • Shortly afterward, 3.85-3.65 Gyr ago: Life and biological evolution started. Blue-green algae.
  • How did it happen? Possibilities: 1: Energy from radiation, lightning, volcanoes, meteorites led to complex molecules (aminoacids, nucleotides), the basis for life; 2: Complex organic molecules came from space, possibly on comets or dust grains (panspermia) – after all, we do have meteorites from Mars.
  • 2.1 Gyr ago: Amoebas, multicellular organisms in the oceans.
  • 400 Myr ago: First life on land.
  • A few Myr ago: Early Homo sapiens.
  • 100,000 yr ago: Homo sapiens sapiens.
  • 10,000 yr ago: Agriculture; then cultural evolution, aided by language, toolmaking, ...
  • Extinctions: It is generally believed that the K/T extinction 65 Myr was caused by an impact, and others may have been as well; Another astronomical possibility is UV radiation from the Sun after ozone layer depletion by a nearby supernova's gamma rays (has been suggested for the Ordovician extinction 440 Myr ago).

What Do We Know about Elsewhere in the Solar System?

  • Summary: There's probably no carbon-based life in liquid water anywhere outside the Earth now, but...
  • Mars? Almost certainly not now; Maybe there was microbial life in the past; We have no firm evidence yet, despite the Viking landers and the famous meteorite.
  • Europa? Probably has liquid water under the icy crust.
  • Enceladus? 8th moon of Saturn; Seems to have liquid water under an icy crust.
  • Titan? 12th and largest moon of Saturn; Has an atmosphere, but may be too cold.

  What Can We Find Out from Studies on Earth?

  • What could alternative lifeforms be like? Turn to astrobiology and the search of exotic life forms on Earth for clues; The field is rapidly expanding; See NASA's Astrobiology Institute, for example; Could life develop with ammonia instead of water, Si instead of C? Some bacteria found 2 miles below ground derive all their energy from radioactively produced chemicals, and none from the Sun.
  • Urey-Miller experiments: In 1953, produced aminoacids in the lab.
  • Later experiments: Produced protein-like droplets similar to some fossils.
  • Other observations: Study composition of comets and meteoroids.

Is there Life Around Other Stars?

  • First condition: Existence of extrasolar planets; We now know there are lots of those.
  • Further conditions: Environments suitable for life (not too close or too far from the star, planet not too big nor too small, right chemistry); What is the likelihood that in a suitable environment life will actually arise? How wide is the "Goldilocks zone"?
  • How can we find out if there's life there? Look for the simultaneous presence of oxygen (or ozone) and a gas like nitrous oxide or methanes, in a planet's atmosphere.
  • Intelligent life? That's a whole different issue...

page by luca bombelli <bombelli at>, modified 29 sep 2012

"This porridge is too hot," Goldilocks exclaimed.
So she tasted the porridge from the second bowl.
"This porridge is too cold."
So she tasted the last bowl of porridge.
"Ahhh, this porridge is just right!" she said happily.
And she ate it all up.

"Goldilocks and the 3 Bears" children's story