Lately on this channel we’ve been exploring the idea that liquid water is not going to
solely be found within the circumstellar habitable zone of a star, call it the CHZ.
If our own solar system is any indicator, ice shell worlds with liquid oceans are very
common in the universe and many of them may well be teeming with everything from microbes
to complex life, or more.
We also can’t ignore the CHZ.
We live within one, here on our gorgeous jewel of a planet that no doubt has a twin somewhere
in the galaxy.
Earth might well be rare, but rare does not necessarily mean unique, and that twin or
twins may have life and civilizations upon them.
But blue jewel earth analogues within the habitable zone are not the only kinds of planets
within the zone that could harbor life and liquid water.
The first alternative type is the familiar desert worlds that have featured so heavily
in science fiction, most famously with Tatooine.
I’ve always enjoyed seeing the moisture farms and can just imagine Mr. Lucas asking
himself back in the 70’s “How would you make a living on a planet like that?
Ah hah!
Moisture farming!”.
It’s an interesting idea, and anyone that’s ever used a dehumidifier knows that it’s
possible, but on such a desert world it would be an exceedingly difficult and hard venture.
These planets would have far less water vapor in their atmospheres than Earth does, so it
would be pretty slim pickings as far as moisture farming goes.
But the strange thing here is that that may actually be helpful as far as life is concerned,
at least under certain circumstances.
Not so much because of a lack of water, but the conditions that low amounts of liquid
water create.
In a 2011 paper by Yutaka Abe, link below, low-water may extend the habitable zone in
both directions.
With less water vapor in the atmosphere, you have a reduced greenhouse effect.
This means that whatever water you do have, say a few lakes or oasis type springs, you
can keep closer in towards the star.
Whereas at those distances, worlds with lots of water would tend to turn into Venus.
The other advantage is again, with a general lack of water, you would have less ice.
Less ice means less heat is reflected back into space making for a potentially warmer
planet over all, and that pushes the habitable zone outward a bit for desert worlds.
Life on such a planet would need to be adapted to sustaining itself under those conditions,
perhaps living in subsurface aquifers or hanging around the springs, perhaps even something
like plants might survive on these worlds, and who knows what else.
Another type of world that might bear life in a CHZ is a moon world.
We’ve seen evidence of gas giant exoplanets within habitable zones already.
If such a gas giant had a very large moon, say the size of earth, interesting things
might happen.
The reason the moon would need to be big is that to hold onto its water, it needs to be
able to hold onto thick atmosphere through its own gravity.
Maybe even a super earth might be needed here, which may not even be possible around a gas
giant.
And it also would need to be able to contend with a very radioactive environment being
near a gas giant.
If such a habitable exomoon is possible, it would be a strange place.
Imagine the effects of having such a large gravity well nearby on tides, if you even
have them, since the planet would almost certainly be tidally locked to the gas giant.
On the other hand, if the exomoon orbits the gas giant distantly, given it is within the
CHZ of the star and the gas giant, then other possibilities open up.
But it can’t be too distant, or orbital stability becomes an issue.
But no matter what, tidal forces are likely to play a huge role on a world like this.
That can be a good thing though, that might allow for plate techtonics or the creation
of a very strong magnetic field through tidal heating in the planet’s core.
Also, axial tilt might be significant issue here.
Our moon regulates ours, but without that, the tilt can change pretty significantly.
I find myself skeptical that we’re going to find too many of these kinds of worlds.
While possible, they face an uphill battle because of the gas giant elephant in the room.
I think life at gas giants would generally go another route rather than arising on tidally
locked huge ocean moons.
It would happen at gas giants outside the habitable zone on small ice shell moons, just
like Europa and Enceladus.
One final alternative to an earth analogue involving a star’s CHZ is the concept of
a part-time life world.
This is a tough sell for a world bearing life because it’s pretty harsh.
But does seem possible and I include it especially because we have creatures on earth, such as
the tardigrade, that could survive those conditions by going dormant.
This idea stems from a paper by Stephen Kane and Dawn Gelino.
What happens here is that the planet has an elliptical orbit that brings it into the habitable
zone during part of its orbit of the star.
This would alternately freeze the planet and then thaw it forcing a sort of hibernation
cycle on any life that can arise there.
Now, it’s hard to envision life getting a start on a planet like that, but there are
a lot of options there, such as geothermal vents where life can get its start and sustain
itself, and then branch out from there and adapt to the cyclic changes in temperature
of the planet.
These three scenarios are complicated as far as to whether life can arise on planets like
this.
But it was complicate for earth as well, yet here we are.
In my next video, I’m going to complicate it even more.
As another nod to Tatooine, I absolutely adore the imagery of that world, I will ask the
question of what effect does living in a double star system have on the possibilities of life
arising on planets orbiting them?
The answer is surprising, so stay tuned.
Thanks for listening!
I am science fiction author and futurist John Michael Godier currently wondering if tatooine
like worlds are possible, then what else is?
How about a death star?
Jedi I am not and I for one welcome our new imperial overlords 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.