The big idea: Every search for extraterrestrial life begins with one question: is there liquid water? Life as we know it cannot run without it.
The points to remember
- Water is the medium for life: the reactions of life happen in water, and the first cells formed in water.
- So the search for extraterrestrial life is a search for liquid water, present now or in the past.
- Three places to look: Mars (dry rivers and lakes), icy moons with oceans under the ice, and planets around other stars.
Remember it as: Follow the water: no liquid water, no search.
Real example: between 2008 and 2015 the Cassini spacecraft flew through jets of water vapour shooting out of Enceladus, a small icy moon of Saturn. It found salt and carbon-based molecules in the jets, which come from an ocean of liquid water under the ice. That ocean is why Enceladus is now a target in the search for life.
On Mars, the Curiosity rover drilled into mudstone in Gale crater in 2013 and showed that a lake of fresh water once filled it. Past water means life could once have lived there.
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The big idea: A planet can only keep liquid water if it is the right distance from its star. That band is the Goldilocks zone.
The points to remember
- Water is liquid only between 0 °C and 100 °C (at a pressure like Earth's). Hotter, it boils away; colder, it freezes.
- The Goldilocks zone is the band of distances from a star where a planet's temperature lets water stay liquid.
- Too close to the star, water boils away (Venus). Too far, it is ice (the outer planets). Earth sits in the zone.
- In a table of planets, pick the one whose temperature range allows liquid water, with air and gravity a living thing could stand.
Remember it as: Goldilocks: not too hot, not too cold, just right for liquid water.
Real example: TRAPPIST-1 is a small, cool star 40 light years away with seven Earth-sized planets found in 2017. The inner ones are too hot and the outer ones too cold, but three sit in the Goldilocks zone. TRAPPIST-1e, an exoplanet that receives about two thirds of the starlight Earth gets, could have liquid water if it has an atmosphere.
The exam gives you a table and asks which planet falls within the Goldilocks zone. Read the temperature column first:
Reading the table: Temperature decides it: the planet must be able to sit between 0 °C and 100 °C. An atmosphere marked unknown does not rule a planet out. A planet at 750 °C or -100 °C is out whatever else the table says; so is one with poisonous gases or a gravity many times Earth's.
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How this is tested: A one-mark multiple-choice item shows a table of planets and asks which one lies in the Goldilocks zone. A short written part can ask you to explain why a particular planet or moon is a target in the search for life.
TRAPPIST-1e orbits a small, cool star 40 light years from Earth. It receives about two thirds of the starlight that Earth receives, and estimates of its temperature range from -22 °C with no atmosphere to 11 °C with an atmosphere like Earth's. Explain why TRAPPIST-1e is a target in the search for extraterrestrial life.
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