Goldilocks, and other Habitable Zones for Life

Heard of the Goldilocks zone?
It’s the idea that an area of space around a star will be at the right temperature for life to exist. Not too hot, not too cold, hence Goldilocks.
It’s a bit like standing around a campfire on a very cold night. Stand too far away and you freeze, stand too close and you catch on fire and burn to death.
It’s the same with planets orbiting stars too, if they’re too far away then water freezes and life can’t emerge, and if they orbit too close the planet is roasting hot and nothing can live.
It gets a bit more complex than this though, but complex in a fun way. Oh and its also got some pretty big implications for the search for extraterrestrial life…

This Goldilocks zone is more usually called a habitable zone (HZ for short). It’s the distance around a star at which a planet can maintain surface liquid water.
Scientists care about liquid water, as all life on Earth needs liquid water to survive (life on Earth is basically bags of water with a few other ingredients thrown in). Scientists’ care about the idea of a HZ as it guides our thinking as to where in our Solar System life could potentially be found, and where it could be found in other solar systems too. And we all care about finding alien life, right?
Earth is in the HZ of our star, obviously, whereas Venus is too close to the Sun, as it’s surface is almost hot enough to glow, and Mars is probably right at the outer edge of the habitable zone, as its surface is too cold for water to remain liquid for long.

A simplified representation of our Sun’s habitable zone

A habitable zone is therefore defined as the region around a star between the distance at which water would evaporate and the distance at which surface water begins to freeze. (Sometimes the outer edge is set at the distance at which carbon dioxide would freeze out of an atmosphere, as CO₂ is a greenhouse gas that can heat a planet, meaning that planets rich in CO₂ could be warm enough to enjoy liquid water at a distance a bit further out than we would normally expect to find it).
The HZ doesn’t just depend on the distance from a star though; it also depends upon the features of the planet. If Mars had been slightly bigger it would have been able to maintain an atmosphere (it lost most of it’s initial atmosphere to space as its gravity isn’t strong enough to capture it permanently, more here) and if this atmosphere contained enough greenhouse gassed Mars could have a warm and wet surface today. Thus a HZ is typically defined as the region around a star in which an Earth-like planet could maintain surface liquid water.
A HZ also depends upon the star too. Larger stars emit much more heat, thus the zone in which an Earth-like planet could maintain surface liquid water would be much further out than for our Sun, and much closer in for stars smaller than ours.

Like this (click to enlarge)

Habitable zones are also affected by time. Over their lifetimes the heat output of stars changes. Our Sun has increased in luminosity since it first formed and is roughly 30% hotter today than it was 4.6 billion years ago. This means that the habitable zone must have moved outwards throughout the life of our star. Astronomers and astrobiologists believe that Earth has always been inside our Sun’s habitable zone, but it inspired a scientist called Michael Hart to come up with the idea of the Continuously Habitable Zone (CHZ). This is the region around a star in which an Earth-like planet can sustain surface liquid water for most of the lifetime of its star.
The idea of a CHZ is important, as the fossil record indicates that it took a long time for complex life to evolve on Earth. Palaeontologists have discovered that single-celled life emerged early in Earth’s history, possibly as far back as 4 billion years ago, but that it took more than 3.5 billion years for this bacterial life to evolve into the first animals. If the Earth had formed 5% closer to the Sun, or 15% further away, its likely that it would have been outside of this CHZ and thus animal life would not have been able to evolve on Earth (yes, that includes us).
This leads to a really cool habitable zone idea, that there may be different HZs for different types of life, an Animal Habitable Zone (AHZ) and a Microbial Habitable Zone (MHZ).
It’s likely that the AHZ would be very narrow, and would be confined to a star’s CHZ, as the planet would need to have surface liquid water for billions of years to allow animals time to evolve.

The animal habitable zone, narrow

The MHZ will likely be much wider, as microbial life may well take a mere few hundred millions years or so to emerge, thus can live on planets that may only spend a short time in a star’s HZ. Venus and Mars may well have had their own microbial life early in their histories, and thus may have been inside our Sun’s MHZ for a time.
Two other discoveries have also expanded the possible boundaries of a MHZ. The first of these was the discovery of extremophiles in the 1970s. Extremophiles are single-celled life forms that thrive in extreme conditions such as boiling water, sulphuric acid or inside rocks deep within the Earth’s crust. Extremophiles expanded the range of conditions in which life can be found and thus expand the range of the MHZ.
The second discovery is that liquid water can exist below the surface of planetary bodies that orbit way outside of a star’s HZ. Evidence suggests that some of the moons orbiting gas giant planets in our Solar System, such as Europa and Enceladus, may have vast subsurface oceans that could support life. I won’t go into the details here (if you want to know more than please see these posts; Europa, Enceladus) but it’s possible that these moons may have their own biospheres in underground oceans, but its more likely that these biospheres are microbial rather than animal. The existence of these moons suggests that the MHZ may be huge, and could potentially span between the orbits of Venus and Saturn in our Solar System.

Europa, within our Sun’s microbial habitable zone?

So what does this mean for the search for alien life?
Firstly, it means that if we hunt for advanced alien life, such as alien civilisations with radio technology, then we need to confine our searches to exoplanets that orbit in a very narrow CHZ around their stars.
Secondly, it suggests that microbial life may be relatively common in our Galaxy, as the MHZ is potentially so wide, but that complex life may be extremely rare, as it likely requires a planet of the right size and composition to orbit stars with a stable temperature at a precise distance. This means that planets that can support animal life in our Galaxy may be rare.
So maybe we aren’t alone in our Galaxy, but maybe most of our alien cousins are simple bacteria.
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