Every living thing on Earth needs liquid water. It dissolves and carries the chemicals of life and lets them react. So when astronomers ask whether a planet could support life, the first question is whether it could have liquid water on its surface — and that depends mostly on how much starlight it gets.
Too close to its star, a planet's water boils away. Too far, it freezes. In between is a band of distances called the habitable zone— often nicknamed the "Goldilocks zone," because it's not too hot and not too cold. For the Sun, it runs roughly from 0.95 to 1.7 AU. (One AU, an astronomical unit, is Earth's distance from the Sun.) Earth sits comfortably inside it.
A brighter star heats planets from farther away, so its habitable zone sits farther out. A dim star's zone huddles in close. Because starlight spreads out with distance squared (the same inverse-square law from Unit 4), the zone's distance scales with the square rootof the star's luminosity:
Here L is the star's luminosity in Suns, and the edges come out in AU. A star 4 times as luminous as the Sun has √4 = 2, so its zone runs from about 1.9 to 3.4 AU. A red dwarf with 1/100 of the Sun's luminosity has √0.01 = 0.1, so its zone runs from just 0.095 to 0.17 AU — far closer than Mercury is to the Sun.
Pick a star (or slide its brightness), then move a planet in and out. The green band is the habitable zone. The distance axis is logarithmic, so each tick is 10 times farther than the last.
Our own Solar System proves that the right distance isn't enough. Venusis almost exactly Earth's size, but it orbits just inside the inner edge, and its thick carbon dioxide atmosphere trapped heat in a runaway greenhouse effect; its surface is about 465 °C. Marssits near the outer part of the zone, but it's only about half Earth's width. Its weak gravity and lack of a global magnetic field let most of its atmosphere escape into space, leaving it cold and dry on the surface.
| Factor | Why it matters |
|---|---|
| Size and density | A habitable surface needs a rocky planet. Planets much bigger than about 1.6 Earth radii usually carry thick gas envelopes — you can check with the density method from Activity 6.1.1. |
| Atmosphere | Too thin and heat escapes (Mars); too thick and heat is trapped (Venus). A moderate greenhouse effect keeps Earth about 33 °C warmer than it would be otherwise. |
| Magnetic field | Shields the atmosphere and surface from the star's charged-particle wind. |
| Stable orbit | A very stretched orbit can swing a planet in and out of the zone. |
Unit 4 comes back here. A star's type decides how long its habitable zone lasts and how friendly it is:
| Star | Habitable zone | Lifetime | The catch |
|---|---|---|---|
| O, B, A | Far out and wide | Millions to about a billion years | Dies before complex life would have time to evolve (Activity 4.2.1) |
| F, G (like the Sun) | Around 1 AU | Several to ~10 billion years | Very few — the Sun is our proof of concept |
| K | A bit closer in | Tens of billions of years | Few; often called the "sweet spot" |
| M (red dwarfs) | Very close in | Trillions of years | Powerful flares; close-in planets may be tidally locked |
Red dwarfs are the most common stars in the galaxy, and their small size makes planets easy to find by transit — so many of the best-known candidates orbit them. But a planet that close to its star is probably tidally locked, with one side always facing the star (like the Moon always showing Earth the same face), and red dwarfs often blast their planets with flares that could strip away an atmosphere.
Put it all together and you have a checklist for any candidate world: Is it in the habitable zone? Is it likely rocky? Is its star calm and long-lived? Could it keep an atmosphere? No real candidate yet checks every box with certainty — and weighing what's known against what's still uncertain is exactly the skill scientists use to decide which planets deserve telescope time.
Run real worlds through the same four-part checklist. Start with our own neighbors, then try the exoplanets.