Liquid water helps define planetary habitability because it supports chemistry for life, so astronomers examine habitable zones when searching for extraterrestrial organisms.
Coverage 2025–2025 · Updated 14 Jul 2026
The search for extraterrestrial life often prioritizes places where liquid water could exist because water supports the chemical interactions required by known life.
A star's Goldilocks zone is the range of distances where temperatures may allow liquid water at a planet's surface. A planet that is too close may be too hot; one too far away may be too cold.
When interpreting planetary data, first identify whether the planet lies in the Goldilocks zone, then state that this makes liquid water and habitability possible rather than certain.
If a table shows one planet at a distance compatible with surface liquid water, that planet is the best candidate for further investigation, not evidence that life is already present.
Being in the Goldilocks zone is not proof of liquid water or life; other planetary conditions also affect whether surface water can persist.
This objective is assessed through multiple choice, commonly using Identify / Interpret.
Identify / Interpret
Exam questions may provide planetary data and ask students to identify the best candidate.
Treating location in the habitable zone as proof that life exists.
Representative question
Which planet in the table falls within the "Goldilocks zone" and may be capable of sustaining extraterrestrial life?
Planet
Temperature
Atmosphere
Gravity / g
Kepler-22b
-11 to 16
unknown
2.4
WASP-39b
750
hydrogen, helium and
other elements
25.0
Gliese 876d
50 to 80
high levels of toxic gases
20.0
Kepler-16b
-100
unknown
0.9
