The Goldilocks Zone

StarsGoldilocks went into the three bears house and found three bowls of porridge. One was too hot, one was too cold, and one was just right. ‘How does this apply to astronomy?’ you ask. Let’s have a look.


As of this writing, 3,767 exoplanets have been identified by scientists, according to Wikipedia. That’s only a fraction of the number of planets in our neighborhood. The Milky Way — the galaxy we are in — is calculated to contain at least 100,000,000,000 planets . That’s quite a lot.

Actually, the Milky Way is just one galaxy out of hundreds of billions. Some galaxies are smaller than ours, some much bigger. That means there are probably some 10,000,000,000,000,000,000,000 planets in our universe. Although there are probably a couple of zeroes too few in that number.

That’s a staggering number, but they range from hot gas giants to tiny rocks covered in ice. Focusing on possibly habitable planets we come to the Goldilocks zone.

The Goldilocks Zone

When you start to look for habitability of planets, the first question that you have to ask is this: is there liquid water? Life on Earth could never have appeared if there had been no liquid water. It’s what separates our planet from Venus.

Liquid water means that the surface temperature of a planet must be between 0 and 100 degrees Celsius – the freezing and boiling temperatures of water. And that is where Goldilocks comes in.

If a planet is too close too its star, it will be too hot to have liquid water. If its too far away from its star, it will be too cold. There is a certain orbital zone around a star that is ‘just right’ for liquid water: the Goldilocks zone.

Other factors

The Goldilocks zone isn’t the end-all of habitable planets. There are a lot of factors that can still make life impossible, even if a planet is in the right place. If the planet is  a huge gas giant, then the internal pressure will not be conductive to life. The same goes for planets which are tidally locked — one side of the planet always faces its star — causing one side to burn and the other to freeze.

On the other hand, we’ve found evidence of liquid water on the moons of Jupiter. There are some weird things out there that can result in even weirder planets. One could even question if other types of planets might host other types of life that don’t require liquid water at all.

Habitable planets

If you look at the available numbers and do some statistical analysis, you can start estimating the number of earth-sized planets in the Goldilocks zone that exist in the Milky Way. Scientists did exactly that, coming to as many as 40,000,000,000 planets. If we take that and take the rest of the universe into  account we come to an even larger number:


That’s an insane amount of potentially habitable planets. Note again that this number could vary by several zeroes in reality. But that’s still an enormous amount.

It does beg the question: how many of these planets actually contain life? And how many contain intelligent life at this moment? So far, we don’t have a clue.

Martin Stellinga Written by:

I'm a science fiction and fantasy writer from the Netherlands