Goldilocks zone

The Goldilocks zone or habitable zone gets its name from the fairy tale, "Goldilocks and the Three Bears". Goldilocks is a little girl whose porridge has to be just right — neither too hot nor too cold. It’s the same with water-based life. For a planet to be "just right", or able to support life, it cannot be so cold that water only exists as frozen ice, and it cannot be so hot that the water all boils away. Only planets within a certain range of orbits in this zone are thought to be capable of supporting life. If a planet’s orbit takes it too close to its parent star then it will be too hot for liquid water to exist, and if it’s too far out it will be too cold.

Our own sun is a G-type yellow dwarf, and it's obvious where our habitable zone lies, with Earth orbiting around 150 million km from the star. The Earth is situated comfortably inside the habitable zone, but at the outer edge is Mars, which had plenty of water in the past but is a barren desert today, and at the inner edge is Venus, a boiling hot planet with a runaway greenhouse effect. In general, for M-type red dwarfs, which are smaller and cooler than the sun, the habitable zone lies much closer to the star. For a larger, hotter A-type star like Sirius, the Goldilocks zone is further out.

For astrobiologists dreaming about the origins of life on Earth and other planets, being in the habitable zone is just one of the factors they have to think about. Also, if conditions on a planet are exactly right for the existence of liquid water, this doesn’t necessarily mean it’s inhabited. In truth, we don’t know what other subtle ingredients are necessary in addition to water and an atmosphere.

With the daily discovery of new exoplanets, a much-studied red dwarf is Trappist-1 which is 40 light-years away. It has seven known rocky planets, with three of them lying within the star’s Goldilocks zone. It’s possible that water is present on all seven planets, though only in a liquid state on the three inside the habitable zone. The Trappist-1 system is one of the planned targets for the James Webb Space Telescope.