The Goldilocks zone, also called the habitable zone, is one of the most useful ideas in the search for life beyond Earth. It is the ring-shaped region around a star where conditions are, in the words of the fairy tale, just right: not so hot that water boils away, and not so cold that it freezes solid. A rocky planet orbiting inside this zone could, in principle, hold liquid water on its surface, and liquid water is the one ingredient every known form of life requires.

The idea is simple, but the reality is complicated. The zone's location depends on the star: around a hot, bright star like our Sun it sits roughly at Earth's distance, while around a dim red dwarf it hugs close in, sometimes closer than Mercury is to the Sun. Around half of the stars in our galaxy are red dwarfs, so many of the potentially habitable planets we find orbit close to small, cool stars.

Where the zone sits for different stars

For a Sun-like star, the habitable zone is usually drawn from about 0.95 to 1.4 times the Earth-Sun distance, with the exact edges depending on the planet's atmosphere. Earth sits comfortably inside it, Venus sits just inside the inner edge, and Mars sits just outside the outer edge. That neat arrangement is a little misleading, though: Venus was probably once mild enough for oceans, and Mars shows clear evidence of ancient rivers, which means the zone's edges have shifted over billions of years as the Sun has brightened. The habitable zone is not a fixed address. It moves as the star ages.

Around red dwarfs, the zone is much closer in, and that creates problems. A planet orbiting that close is likely to be tidally locked, always showing the same face to its star, just as the Moon always shows the same face to Earth. One side would bake in permanent daylight while the other froze in endless night. Red dwarfs are also prone to violent flares that can strip a planet's atmosphere away. So a planet can sit squarely in the Goldilocks zone and still be a terrible place to live, which is why astronomers treat the zone as a starting point, not a verdict.

What else a planet needs besides the right distance

Distance is only one ingredient in habitability. A planet also needs an atmosphere thick enough to keep water liquid and to shield the surface from radiation, but not so thick that it turns into a runaway greenhouse like Venus. It helps to have a magnetic field to deflect the charged particles of the stellar wind, and it helps to have plate tectonics, which recycles carbon dioxide and stabilises the climate over geological time. Earth has all of these, and that combination may be rare.

This is why the most exciting exoplanet discoveries are not just planets in the zone, but planets in the zone with the right size. Rocky planets between roughly half and one and a half times Earth's size are the best candidates, and astronomers have found dozens of them in or near the habitable zones of their stars. The TRAPPIST-1 system is the most famous example: seven Earth-sized rocky planets around a cool red dwarf, with three of them in or near the habitable zone, all packed into orbits smaller than Mercury's.

How Webb is testing the zone

The James Webb Space Telescope is the first instrument that can test whether Goldilocks-zone planets actually have atmospheres, and what those atmospheres contain. When a planet passes in front of its star, a little starlight filters through the atmosphere, and the gases in it leave fingerprints in the light that Webb can read. So far the results have been a mix of excitement and caution: some small rocky planets appear to have lost their atmospheres entirely, while others show hints of gases that need further observation to confirm.

The most talked-about case is K2-18 b, a planet in its star's habitable zone where Webb detected methane and carbon dioxide, and a possible hint of a molecule called dimethyl sulfide, which on Earth is produced by marine life. That hint is far from proof and remains hotly debated, but it shows what Webb can do. The telescope cannot see continents or oceans or life directly. What it can do is narrow the list of candidates, one atmosphere at a time, until the day a future telescope finds a world whose air looks unmistakably alive.

Frequently Asked Questions

Q: Is Earth the only planet in the Sun's Goldilocks zone?

A: Earth is the only planet in the Sun's habitable zone with liquid water on its surface today. Venus sits near the inner edge and Mars near the outer edge, but neither currently has surface oceans.

Q: Does being in the Goldilocks zone mean a planet has life?

A: No. It only means the temperature could allow liquid water. A planet also needs a suitable atmosphere, a magnetic field and a stable climate, and even then life is not guaranteed.

Q: How many potentially habitable planets have been found?

A: Astronomers have found dozens of rocky planets in or near the habitable zones of their stars, and the total grows as detection methods improve. Most are too far away for Webb to study in detail yet.