Up until now, I’ve really only spoken extensively of three basic types of habitable planets
on this channel.
Earth-like worlds, oceanic super earths and ice shell moons.
The common theme among these types of worlds is that water can exist as a liquid on them.
Of course, there are possibilities for alternative biochemistries that can make other, very different
types of worlds viable, such as Saturn’s moon Titan where you have liquid hydrocarbons
available as a solvent for life rather than water.
But with a water model, there may be other classes of worlds that can sustain liquid
water in a less intuitive manner.
An example of this would be an eyeball world, where the planet orbits a star very closely,
say a red dwarf, but within the habitable zone where temperatures can allow for liquid
water.
The problem with this model is that such a planet is likely to be tidally locked to the
star, like the moon is locked to earth and always presents the same face to us.
This might mean that such a planet experiences too much heating on the facing side, and not
enough heating on the night side for liquid water, meaning that a ring of habitability
could form on the planet in the twilight edges of the world.
More recently, it has been suggested that the dark sides of these worlds might experience
enough atmospheric circulation that enough heat could reach the dark side to keep water
liquid there allowing for habitability in complete darkness.
But there are other possibilities for habitability in a limited state.
In a paper by Adiv Paradise and colleagues, link below, they outline an idea for a snowball
planet where the oceans are frozen permanently at the surface, but with bare continents of
land that experience warmer temperatures could support liquid water in lakes separate from
the conditions of the oceans.
A way to imagine this is if earth’s oceans were completely frozen, but the continent
of North America was not and remained somehow warm enough to keep the great lakes liquid.
Their models suggest that an unfrozen continent on one of these exoplanets could see temperatures
that exceed 10 degrees celsius, maintain a water cycle and a carbon cycle releasing carbon
dioxide, and essentially experience stable, moderately warm climates over land even though
the planet should, by all rights, be entirely frozen solid.
Many factors would need to come together for an exoplanet like this to exist, such as placement
within the habitable zone of its star, how much carbon it can release, sea levels and
so on.
But on the other hand, we may have already seen a snowball planet sustain life.
It’s earth, and there may be indications that at least twice in its history it may
have gone completely snowball and froze over completely.
We know that earth experiences ice ages, where the glaciers advance periodically and cover
at least part of the planet for long periods of time.
But has it ever completely frozen over?
The main evidence that it might have comes from deposits laid down at various times in
earth’s history that show evidence of glaciation, but are thought to have been located in the
tropics at the time they were laid down.
Whether those rocks were indeed in the tropics at the time is a matter of open debate still,
but there are other potential indicators.
Another is geologic evidence that show that the oceans went through a period of low oxygen,
which might have been due to them being sealed in by surface ice.
The main criticism of the snowball earth hypothesis is that once you get locked in that state,
it’s very difficult for a planet to get back out of it due to global ice reflecting
sunlight back into space.
To do it, you need a greenhouse gas, in earth’s case carbon dioxide.
It’s estimated that to break out of a snowball state, earth would have needed carbon dioxide
levels 350 times what they are today.
That might have happened however.
Volcanoes, and microbial life under the ice, might have produced enough carbon dioxide
and methane for earth to break out of the snowball state, if the carbon cycle were interrupted
by ice cover and carbon dioxide could no longer be removed from the atmosphere.
In the end, snowball earth is a contentious idea, still very much debated in scientific
circles, mainly because earth’s climate is ridiculously complicated and there are
many unknowns.
It may have never gotten that bad as to completely freeze over, and an alternative less extreme
slushball earth hypothesis exists.
But whatever happened, life on earth survived it.
I have no doubt habitable exoplanets will be equally complicated and very special conditions
might exist that allow for counterintuitively habitable planets like snowball worlds with
warm continents.
Thanks for listening!
I am futurist and science fiction author John Michael Godier currently worried about snowball
aliens studying earth as a exoplanet and snickering about how the humans can’t even keep their
oceans frozen and be sure to check out my books at your favorite online book retailer
and subscribe to my channel for regular, in-depth explorations into the interesting, weird and
unknown aspects of this amazing universe in which we live.