One could argue that almost any body in the solar system is habitable so long as you have
the technology to colonize it.
But that would involve this solar system’s elephant in the room, which is our civilization.
For the sake of this list, I leave us off it; we know that life on earth is possible.
But as we’ve learned more about our solar system, it is becoming increasingly evident
that other places could have evolved life independently of earth.
So here are ten places where life is possible in this solar system other than earth, plus
a few bonuses tossed in, since hypothetical habitability is by no means rare in this star
system, and probably isn’t rare in the rest of the universe either.
10.
Venus
This possibility is a long shot, though life can be tenacious and surprising so it remains
a possibility, at least for microbial life.
Venus was once like earth, it may have had abundant liquid water early in its history,
and seems to have had it long enough for life to arise.
But the story of Venus is a sad one; several contributing factors led to an eventual runaway
greenhouse effect on that world that turned it into a vision of hell.
But that transformation would have been a gradual process as far as we know and any
life that was present on the surface may have had time to adapt to the changing conditions
and find a niche in which to survive.
That niche lies in the upper reaches of Venus’ atmosphere.
There exists a zone that is temperate and cool that may allow for airborne life to exist
and it’s possible that we may have already detected it.
This option for microbial life a huge maybe, Venus’s atmosphere is quite acidic and this
life would be radically different from most life on earth, but there is a hypothetical
way for life to exist in that kind of acidity through a molecule known as an S8 molecule.
An organism could hypothetically incorporate S8 molecules as a sort of armor, as these
molecules are resistant to the effects of sulfuric acid.
S8 has been detected in the atmosphere of Venus, and could have been put there by life.
Such life could also use ultraviolet light from the sun as an energy source, and oddly
enough, when we look at Venus in ultraviolet, we see mysterious unexplained dark streaks
in the atmosphere.
One of the possibilities on the table for whatever causes those streaks is microbial
life.
9.
Pluto
For this option, we go from the inner solar system, to the outer solar system.
One of the most surprising discoveries of recent years was that Pluto and its moon Charon
were not merely frozen solid worlds but in fact dynamic bodies with active geology and
even abiotic organic chemistry occurring.
With this, Pluto has gone from seemingly being one of the last places you would expect to
find life to one where it is hypothetically possible.
For life, you need energy, organics and water.
Pluto has all three, though on the surface the water is frozen.
But it may have a liquid water ocean beneath the surface heated by radioactive decay in
the planet’s core.
This has potentially enormous implications for life in this solar system.
Pluto is thought to essentially be a nothing special Kuiper Belt object, one of hundreds
or thousands.
But if it does have that mix of radioactive decay and subsurface liquid water, then potentially
many Kuiper Belt objects have this as well.
If that’s the case, then we may someday discover hundreds of new candidates for life
in this solar system.
8.
Triton
Speaking of Kuiper belt objects, our next candidate is thought to also have originated
from the Kuiper belt.
There’s a good reason to think this, it orbits in the opposite direction of Neptune’s
rotation, suggesting that it did not form along with that planet.
Plus, it looks similar to Pluto in a number of ways.
But it does stand out in one very odd way, it’s currently geologically active but instead
of volcanoes, it has cryovolcanoes that spew out nitrogen contributing to a thin atmosphere.
Another thing that sets Triton apart from most of the other moons of the solar system
is that because it was likely captured, it would have been subject to tidal heating from
Neptune as it settled into a stable orbit.
This could have created a Europa-like situation where there might have been subsurface oceans,
and they would likely have persisted for a long period of time, but how long?
Are they still there?
One clue is that Triton appears to have a young surface, thought to be only about a
hundred million years old and may have resulted from an extrusion from the ocean below.
And something is driving the cryovolcanism, so one possibility is that there is still
a subsurface ocean present there.
If so, it would likely be rich in ammonia as well as water, and could contain life.
7.
The Moons of Uranus
As we work our way inward from the outer solar system, we next arrive at a planet that really
doesn’t get enough attention.
I’ll admit my own bias here; I’ve never talked extensively about Uranus or its moons
on this channel, despite having said the word Europa like a hundred times.
Perhaps it’s simply because Uranus is not exactly the most photogenic of the planets,
all Voyager II saw was a featureless blue-green sphere, though subsequently clouds and other
activity have been seen.
As a result, not a lot of attention has been paid to this planet.
But looks can be deceiving, Uranus hosts mysteries just like all other bodies in the solar system,
including how it came to be that this planet rotates on its side compared to the other
planets.
Best guess there is that early in its history Uranus got smacked by an earth-sized protoplanet.
The first candidate moon for liquid water is Titania, though unfortunately so little
is understood of this body’s evolution that it’s candidacy is up in the air.
But it is possible, at least as far as we know, for an ammonia rich or salty liquid
water layer to exist deep under this moon’s surface.
It’s much the same story for Oberon, another candidate; we just don’t know enough, but
a subsurface liquid layer is on the table.
Less likely is Umbriel, it’s been suggested, but there doesn’t seem to be much room there
for anything but ice and rock.
Uranus is a place where life is unlikely, but marginally possible, at least as far as
current thinking.
And with all candidates mentioned for Uranus, you need more than mere water for oceans to
persist, you need ammonia and or salt as a sort of antifreeze to keep everything liquid.
But, life is in principle still possible here, so on the list the Uranus system goes.
6.
Space Itself
That last place you might choose to look for alien life is space.
As in actual space itself.
We tend to automatically think that life must be intrinsically linked to planets or moons.
And that’s certainly a fair point; it’s hard to see life arising in the cold vacuum
of space.
But nothing says that life can’t leave its planet.
Stating the obvious, one way to do this are rockets and spacecraft, whether you are a
human intentionally going into orbit, or you’re a dog or chimpanzee being put there by a human.
Or even a stowaway bacterium, tardigrade or lichen unintentionally leaving earth, either
by way of a spacecraft, or through various methods of natural panspermia.
But the possibility of natural panspermia comes with another possibility: that life
could adapt, survive and live in space on its own.
We know that certain examples of earth life, rare though they are, can survive the conditions
of space.
But what of something that can go much further.
Freeman Dyson envisions one such possibility.
Termed Dyson’s sunflowers, these would be organisms that might originate on ice shell
moons, growing up through cracks in the ice with a connection to the water below like
the roots of a plant.
It might adapt itself to keep itself warm through natural mirrors, and might create
its own supply of liquid water.
It might even eventually be knocked off its planet and move into deep space and might
adapt.
I think this option is possible, but not probable.
I doubt we would see life like this very often when exploring the universe.
But it pays to remember that life often surprises us down here on earth, and I see no reason
that it wouldn’t surprise us in space.
5.
Titan
Here we move to a moon that not only has one possibility for life, but two.
It is a body different from all others in the solar system, save for earth.
On its surface, it boasts liquid hydrocarbons mimicking the water cycle on earth.
It has been advanced that hydrocarbons in liquid form could serve as a solvent for life,
like water does for us.
This life would be very different from what we’re used to, it would need to exist at
much lower temperatures than here, but Titan is also so sufficiently strange that the possibility
is worth consideration.
One thing in favor of life on the surface of Titan are, again, abundant organics.
And there may even be hints that there might be life there, such as difficult to explain
methane levels at the surface, though there also are natural possibilities for creating
that, life is merely one option.
But how might that life work?
One thing that’s been advanced are cell membranes involving acrylonitrile that could
work with liquid methane as a solvent.
And, lo and behold, acrylonitrile was found at Titan by the Cassini mission.
Only time and more study will tell if there is life on the surface of Titan.
But, it too is an ice shell moon, thought to have a liquid water ocean, high in ammonia,
below the surface where life may also have arisen.
Perhaps there are two separate forms of life on this little orange moon.
But Titan also presents an interesting future scenario where life is concerned.
Say it has arisen there or someday will.
In the future, as the sun goes red giant, Titan may get substantially warmer and undergo
a greenhouse effect.
In which case, the life on the former surface may extinguish, but the life below may get
a few hundred million years in the sun in the form of a surface ocean.
4.
Enceladus
Enceladus is a place where the possibility of subsurface water is far more than a maybe.
The stuff literally sprays out of surface cracks into deep space right before our eyes.
And, it’s very nutritious water for life as we know it, very likely made so by subsurface
volcanic vents.
This is a tantalizing environment, but there’s a problem.
It may not be old enough for life.
The problem here is that some of Saturn’s moons orbit in such a way that suggests that
they couldn’t have been doing it this way for long.
It places an age on Enceladus, at least as we know it now, of about 100 million years.
That doesn’t leave life much time to arise, but the question is open.
However, there is one particularly odd feature of Enceladus that may be related to number
one on this list, do you see those cracks on the surface.
Do you see how the extruded material is bluish, like clean ice?
Take note of that for later.
But in the end, even if there is no life there now, Enceladus seems well-suited for it to
arise, leading to the possibility that it may some day be a laboratory where the human
race watches the advent of alien life in real time.
3.
Ganymede
Moving from the Saturn system to Jupiter, we find a place where all four major Galilean
moons can hypothetically host life.
Least likely here is Io, it’s wildly volcanic, but it is warm and is thought to have once
had water, so maybe something clings to existence deep in a moist lava tube.
Also included here is Callisto, a rather ignored moon that could host a subsurface ocean.
Then there is Ganymede, the largest of Jupiter’s moons.
Current thinking is that it doesn’t just have one ocean, but several, all stacked one
on top of each other separated by layers of various forms of water ice.
Think of it like a high rise building with different floors.
Any of these oceans may harbor life, and they may even host different ecosystems that interact
through cracks in that ice.
I’m sure you’ve noticed that I ignored the obvious candidate for life in the Jupiter
system.
I did that on purpose, as it tops this list.
2.
Mars
In both science, and the world of science fiction, Mars has always occupied a prominent
place as far as ponding extraterrestrial life is concerned.
There is something alluring about it, the red planet, that in some ways looks very much
like Earth, the solar system’s ultimate abode for life, but also strangely alien.
It’s like an alternative earth in a way, a twin that isn’t identical, but similar
enough to be a little spooky.
But that similarity extends to the scientific, Mars was once a habitat very much like earth
with abundant liquid water.
Now it is cold and dry.
And in fact, the famous streaks that were thought to be evidence of salty seeping water
on the surface is now in question.
Still, liquid water could persist on this planet deep below in aquifers allowing a refuge
for any life that might have survived the great drying.
But it would almost certainly be microbial in nature, but in the end, life is life.
But what hints have we seen that something could still be hiding out on this world?
First and foremost were the inconclusive Viking experiments that gave positives for metabolism
in samples in an experiment designed to detect life.
But unfortunately, the results were called into question, and remain so today.
It’s anyone’s guess if they did detect life.
And it’s also debatable as to how surface life could exist in such a harsh radioactive
environment.
But there’s more.
It’s been known for a while that something weird is going on with Mars and the gas methane.
It was detected in the Martian atmosphere back in 2009, but the trouble is that methane
is destroyed in that environment, so something must be replenishing it.
The mystery deepened when the Curiosity rover detected a massive ten fold rise in methane
levels at its location at Gale crater, only to then see the levels drop back to normal.
There are natural ways for this methane to be released geologically, but it also happens
to be that methane is also a gas associated with active life.
1.
Europa
Europa probably has the best chances of harboring life in the solar system other than Earth,
and that life may even be more than microbial.
It has a subsurface liquid water ocean that interacts with its surface, it has geysers
that periodically spew materials from the ocean below into space, it has an energy source
in the form of geothermal energy from tidal flexing, essentially it has, as far as we
know, the right mix for life to arise.
And we may have already seen evidence of it.
In 2003, a team lead by Brad Dalton looked at the infrared signatures of the cracks in
Europa’s surface where odd discolorations are visible.
He then compared that with microorganisms here on earth located around geothermal springs
at Yellowstone.
They matched, opening the possibility that the discolorations are due to the presence
of frozen microorganisms from the ocean below.
Taking it further, Dalton took extremophile microorganisms native to earth and put them
in conditions similar to what’s found on Europa.
He looked again in infrared, and there were still correlations, but not a perfect match.
A mineralogical answer was looked at, yet, no combination of salts fit either.
Yet, the pink and brown discoloration of the cracks itself might be telling, salts should
appear white.
Yet the extremophiles used in Dalton’s experiments are colored pink and brown.
While this is by no means conclusive, the discolorations could well be mineralogical,
the mystery probably won’t be solved until we can get samples of Europa’s ice or drill
into the ocean itself.
But if I had to bet on any body in the solar system hosting life other than Earth, I’d
bet on Europa.
But back to enceladus for a moment.
Europa and Enceladus are very similar moons.
One has cracks with staining present, the other does not.
What is so radically different between these otherwise very similar objects?
Thanks for listening!
I am futurist and science fiction author John Michael Godier currently musing.
If you watch a lot of space science press conferences like I do, you notice that they
often consist largely of thanking people.
But they never thank the object they’re studying, so let me take a moment to publically
thank Jupiter for its awesomeness, keep up the good work champ, be sure to check out
my books at your favorite online book retailer and subscriber to my channel for regular,
in-depth explorations into interesting, weird and unknown aspects of this amazing universe
in which we live.