The Goldilocks zone: Definition and characteristics

The Goldilocks area takes its name from the fairy tale “Goldilocks and the Three Bears”. Goldilocks is a difficult little girl whose porridge must be perfect, neither too hot nor too cold. It’s the same with life itself – or at least, the kind of aquatic life we ​​know of on Earth.

Because a planet must be “just in point”, or capable of supporting life, it cannot be so cold that water exists only as frozen ice, and it cannot be so hot that the water boils completely. Only planets within a certain range of orbits called the “Goldilocks zone” – or officially known as the “habitable zone” – are considered capable of supporting life.

Definition of the habitable zone

If a planet’s orbit brings it too close to its parent star, it will be too hot for liquid water to exist, and if it is too far away, it will be too cold. However, the actual distances involved, which define the habitable zone, vary from star to star.

Our own Sun is a G-type yellow dwarf, and there’s no doubt where its habitable zone is because Earth occupies that area, orbiting 93 million miles (150 million kilometers) from the star.

But for M-type red dwarfs, which are smaller and cooler than the sun, the habitable zone is much closer to the star. For a larger, hotter A-type star like Sirius, the Goldilocks zone is further away, according to Nasa.

For astrobiologists, who search for life on other planets, being in the habitable zone is just one of the factors they need to consider. Take our own moon, for example. It is obviously in the Goldilocks area as it is so close to Earthbut there is no liquid water on its surface.

Indeed, atmospheric pressure and composition must also be taken into account, according to This makes the moonwhich has no atmosphere to speak of, unable to support life on its own.

It is also important not to over-interpret the word “habitable”. Even if the conditions on a planet are perfectly suited for the existence of liquid water, that does not necessarily mean that it is inhabited. Scientists have yet to pinpoint the exact origins of life on Earth, so we don’t know what other subtle ingredients are needed besides water and atmosphere.

Our own solar system is the most studied of all planetary systems. Theorists have determined where its Goldilocks zone should be, by estimating a planet’s surface temperature based on the amount of solar heat it receives.

The results so far agree with what we know from the observations. Earth – a very watery planet teeming with life – sits comfortably inside the habitable zone. March, which had plenty of water in the past but is now an arid desert, lies just on its outskirts. At the inner edge is Venus — a hot planet, thanks to both its proximity to the sun and its ultra-thick atmosphere, according to Nasa.

Exoplanets in the Goldenilocks Zone

The discovery of new exoplanets orbiting distant stars has become almost commonplace. But it’s always exciting when you’re in the Goldilocks zone of its parent star.

This happened in 2016 in the case of Proxima B, which orbits the sun’s closest neighbor in space – the red dwarf Proxima Centauri, just over 4 light years away. It’s so small and dark that its habitable zone is located a very short distance away, but Proxima B – which orbits the star once every 11 days – is safe inside, according to the European Southern Observatory (ESO).

Trappist-1 is another well-studied red dwarf. At around 40 light years, it is more distant than Proxima but still a close neighbor in cosmic terms. Trappist-1 is notable for having seven known rocky planets, three of which are in the star’s Goldilocks zone, according to NASA.

It is possible that water is present on all seven planets, but only in a liquid state on the three within the habitable zone. It would take the form of atmospheric water vapor on the planets closest to the star, or of ice on the most distant of them. Astronomers are understandably keen to learn more about the Trappist-1 system, and it’s one of the planned targets for the James Webb Space Telescope.

The ultimate engineered solar system

When a solar system forms, there is no reason for planets to be preferentially in the habitable zone, and the TRAPPIST-1 system is unusual in having up to three planets there.

But theoretically, is there an upper limit to the number of planets that can be stuck in the Goldilocks zone? It’s a question that astrophysicist Sean Raymond addressed on his blog. .

This carefully tuned system has no less than 416 planets in the habitable zone. (Image credit: SeanRaymond(

It turns out that there is indeed a theoretical limit, beyond which the system becomes unstable because the planets are too close to each other. Raymond came up with what he calls “The Ultimate Engineered Solar System”, with a grand total of 412 planets in the habitable zone, arranged around eight concentric orbits that spin in alternately prograde and retrograde directions.

Build theoretical solar systems

Here, Sean Raymond, originator of the “Ultimate Engineered Solar System,” discusses his work.

Raymond writes about the interface of science and fiction at (Image credit: Sean Raymond SeanRaymond (

How did you come to create such a strange arrangement of planets?

“My daily job is to understand how planetary systems form, what makes the solar system different from the exoplanet systems we have discovered, and what kinds of orbital configurations are stable and which are not. I wanted to understand what kind of orbital architecture would maximize the number of planets in the habitable zone – remaining stable but not having to worry about how the system would have formed. Fortunately, I was able to draw inspiration from several recent articles by scientists. I also tested the craziest systems using computer simulations to make sure everything worked.

Why do you call it the quintessential “manufactured” solar system?

“Originally, I built two systems, each with about 30 planets in the habitable zone. These systems could plausibly form in the wild, if the right series of events occurred (such as rolling sixes on two dice ten times in a row). But I can’t imagine how the Ultimate Engineered Solar System could form naturally. A system with equidistant planets, spread along rings orbiting in opposite directions, is simply impossible as far as I know. So if such a system exists, I’d say it must have been built on purpose, presumably by the engineers of a super advanced civilization.

If such a system really existed, how could we detect it from Earth?

“It would be quite difficult to detect, because the signals we measure to infer the presence of exoplanets – typically, radial or transit velocity signals – could end up being so confusing that they could be mistaken for noise in the case of the ultimate engineering. system.”

Additional Resources

For more information on the Goldilocks Zone, see “The Goldilocks Zone: Conditions Necessary for Extraterrestrial Life (Search for Other Earths)” by Laura La Bella and this habitable zone video produced by ESA.


  • Michele Johnson, “Habitable Zones of Different Stars”, NASA, December 2009.
  • Paul Sutter, “We Don’t Really Understand the Habitable Zones of Alien Planets”,, February 2021.
  • Sean Raymond, “The Ultimate Engineered Solar System,”, accessed April 2022.
  • NASA, “What is the habitable zone?”, 2021
  • Sara Rigby, “What does it mean if an exoplanet is ‘habitable’?”, Science Focus, 2020
  • ESO, “Planet found in habitable zone around nearest star”, August 2016
  • NASA, “Largest Batch of Earth-size Habitable Zone Planets Found Orbiting TRAPPIST-1,” accessed April 2022.
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