What makes a planet habitable?

Water in liquid form is thought to be a necessity for life on Earth.

Naturally, some say that life may flourish under other conditions, and perhaps even in the absence of water.

While that may be true, take a look around - life seems to do quite well here on Earth and we've yet to find it elsewhere in our Solar System. 

Based on this, let's look at the classical definition for the habitable zone as the region around a star, such as our own Sun, where the temperature of any orbiting planet permits water in liquid form.

Astrophysicists are extremely good at calculating the temperature of a star and then, taking into account the distance of a planet from its host star, it is easy to work out the planet's "equilibrium temperature".

The starlight (in our case, sunlight) that falls onto the planet is reradiated as heat and, hey presto, we have our actual planet temperature - simple. Except it isn't.

Greenhouse gases

What if the planet sports a blanket of white clouds? Clouds are reflective and therefore will cool the planet, acting to push the habitable zone closer to the star. 

Amusingly, if we calculate this "equilibrium temperature" for the Earth, taking into account its beautifully reflective clouds, then it turns out that we live outside the classical habitable zone!

The same calculation for Venus gives an expected equilibrium temperature of about -10°C, but in reality it is more like 450°C.

What happened?

Clouds' reflective qualities cool the planet and mean the habitable zone can be closer to a star

Both these planets have greenhouse gases present in their atmospheres, warming the planet up and driving the outer boundary of the habitable zone further away from the star (while clouds drive the inner boundary closer to the star). 

The very latest habitable zone definitions use simulations of these cloud and greenhouse effects - widening and blurring the crude classical definition. 

Throw into the mix that we currently can't study the atmospheres of rocky terrestrial exoplanets (and therefore have no idea whether they have clouds, greenhouse gases, or even an atmosphere at all!) - then to say "that planet is habitable" is impossible, for the time-being at least.