So far we’ve discussed planets like Pluto that orbit far from their sun, and tidally
locked worlds which orbit very close to their sun.
Today we’ll be looking at worlds that don’t orbit suns at all.
Moon is a pretty broad concept, as a term it just means any decently sized object in
a reasonably stable orbit around a planet.
They can vary in size from a rock a few miles across to things that are bigger than other
planets.
Our own moon is bigger than Pluto and Jupiter’s moon Ganymede is larger than the planet Mercury.
The only size limit is that they can’t be bigger than the planet they orbit since if
they were the relationship would be backwards, but in practice we would normally deem any
moon of fairly similar mass to its planet to be a double planet.
There’s no rigorous definition of a double planet, but we’d usually say anything less
than a tenth the other planet’s mass counts as a moon.
We’ll look at double planets another time.
Life on moons like frozen Europa or Saturn’s moon Titan are also interesting possibilities
but today we won’t be looking at those either.
Our subject for today is going to be pretty long just by itself.
We’ll be focusing on worlds which differ from ideal planets of Earth like mass only
in that they are moons, presumably in orbit around gas giants.
Same mass and composition, same day-night length.
To avoid confusion we’ll refer to this a Gaian Moon.
We’ll save the other cases for another day.
Regarding them briefly though, before moving on to our main subject, the key difference
between a moon and an asteroid or dwarf planet in terms of habitability is that they will
receive tidal heating.
As we’ve mentioned previously, the single biggest factor we look at for life arising
on a world is if there’s enough energy coming in to as a power supply to biological life
and permit temperatures in which the right kind of chemical interactions are commonplace.
We touched on that more in the video on Pluto and we’ll talk about it more in the future.
If you have two otherwise identical large asteroids much further away from the sun than
Earth is, but one is a captured moon, then that one has all the extra power input of
tidal heating to supplement its meager supply of sunlight.
Which also applies to Earth-like worlds that are moons of gas giants.
They can get away with being a bit further from the Sun than Earth is and still be warm
enough for life.
Our plants don’t use most of the sunlight that hits them a noon-time, so if there’s
another power source keeping the moon warm the moon can be further from the sun than
Earth is.
Indeed they probably can’t exist as close to the sun as we are or they’d be baked
by the extra influx of energy from both sources, tidal and solar.
Now the difference between a moon and a planet, in terms of habitability, doesn’t stop there.
Two major factors which also apply.
The first is tidal locking, which we discussed in detail both last time and in the companion
video on Tidal Locking.
Like all the video links I use in these videos, you can click on this one and it will just
pause this video and open the other in a new window so if you may want to do so now.
So let’s talk about Gaian Moons.
If a planet becomes tidally locked to a star, it has eternal daylight on one side and eternal
night on the other.
Moons locked to a planet are different.
As we’ve discussed, there’s no dark side of the moon, just a side we can never see
from Earth.
We’ve also discussed why almost every moon is eventually going to end up tidally locked.
Again click the tidal locking video link if you’re not familiar with that.
So a moon tidally locked to a planet is going to have a day and night cycle.
Our moon’s day-night cycle is a month long, which is also a Lunar Day and a Lunar Year.
Same concept applies to an Earth-sized moon.
Throughout this video we will use the term day to mean the moon’s day night cycle length,
rather than saying year or month, since in this case they are all the same length.
Now that moon wouldn’t have to have a 24-hour day for life to be possible but the table
I’ve got up shows the distance a moon of Earth Mass needs to be from a planet to have
a 24-hour orbit.
Our Gaian Moon case.
Here’s also a simplified equation you can use if you want to calculate Gaian Moon Distance
from planets of different Masses, or if you want to calculate it for an Earth Mass moon
with different day-length in hours.
Pause the video if you want to review these, as I don’t want to turn the video into a
math lesson, they’re just Kepler’s law simplified for this specific type of case,
an Earth Mass moon.
The distance up there are only accurate for a roughly circular orbit, for eccentric orbits
those would be the semi-major axis.
Similarly in a moment I’ll bring up the diameters of the planets and that is their
equatorial diameter.
Planets are rarely perfect sphere and gas giants in particular tend to be wider at their
equator then poles but we’re working in approximations here so I will just be calling
these distance and diameter.
As you can see in the table I have our four gas giant worlds, Jupiter, Saturn, Uranus,
and Neptune listed.
But I’ve also included 6 other examples.
3 at the beginning which are essentially double planet scenarios, which again we’ll discuss
more in the future, and three at the end for a Larger Gas Giant, a thousand times Earth’s
mass, then one of about 13 Jupiter Masses, generally considered the point at which gas
giants become brown dwarf stars, and finally a minimum mass red dwarf star, one just massive
enough to ignite stellar fusion.
We’ll talk about life around brown dwarf stars another day as well.
In a moment I’m going to talk about how big these objects are in the night sky of
their Gaian Moon, compared to how big the Sun and Moon are in our sky.
But let me mention the Roche Limit briefly since it occasionally gets mentioned in regard
to habitable moons.
The Roche Limit is the minimum distance a moon can be from a planet, or a planet from
a star, before it gets shredded by gravity.
The more massive the moon, the closer it can be because its own gravity holds it together
better, even though its bigger diameter makes the tidal strain on it higher.
Simply put, the Roche Limit is essentially never relevant to planetary habitability when
we’re talking about close Earth parallels like Gaian moons, because the distance they
have to be to have an orbital period of a day or so is well beyond the Roche Limit,
and the distance a planet needs to be from even the dimmest main sequence star and not
be roasted is also well outside the Roche Limit.
It can be a concern for moons where we aren’t contemplating surface life, but that’s not
our concern today.
So what does the planet look like from the surface of our Gaian Moon?
First, since it is tidally locked you can only see the planet from one side of the moon,
where it will always be visible, and not from the other side, though moons on fairly eccentric
orbits will have a wide band around the poles where it will sometimes be visible and sometimes
not, due to Libration, as we discussed last time in the Tidally Locked Planets video.
For those who can see it, the planet will be very large.
Our table here show how large, in terms of how big our own moon appears to be in our
night sky.
What’s interesting is that while the planet will appear larger and larger as it gets more
massive, it will actually peak out when you get to about Saturn’s size.
Not only is the distance the Gaian Moon is from the planet rising as the planet’s mass
rises, so that objects appear smaller, but the density of gas giants starts rising about
then too.
Earth is the densest planet in the solar system, because the bigger planets are mostly made
of ice and gas.
Saturn is less dense then Neptune and Uranus because it is mostly gas whereas Neptune for
instance is slightly smaller than Uranus but slightly more massive.
It is denser because it has a rocky core slightly more massive than Earth then a thick mantle
composed of ices of water, methane, and ammonia.
But Jupiter is much denser than Saturn, yet is still almost all hydrogen.
Why?
Well simply put at a certain point gravity kicks in and compresses planets a lot more.
Brown dwarves, even though they are a dozen to several dozen times more massive than Jupiter,
are all about the same size as Jupiter, just much denser.
So to people living on a Gaian Moon, the visual size of their planet in their sky begins to
slowly rise until you get to be about Saturns’ size.
It then peaks out and begins to drop off slowly as the planet’s stop getting much bigger
as their mass increases while the moon’s distance continues to get larger to keep that
24 hour day.
Then at roughly Jupiter masses the apparent size is going to drop off even more steeply.
This is very like the effect we noted when discussing red dwarves last time, that the
smaller stars look bigger on their habitable worlds because you have to be so much closer
to them to stay warm.
Now this massive planet-moon in the sky has a big effect on night time brightness too.
While on the back side of the Gaiain Moon they don’t see the planet, though they will
likely see the planet’s other moon, on the front side they always see that moon and it
is terribly bright.
Roughly on par with the square of the diameter.
Our own full moon is nowhere near as bright as our sun, even less than it seems because
our eyes are logarithmic to light, but if you made it 20 times wider in the sky it would
be 20-squared times brighter, 400 times brighter than now.
Gas giants also have higher albedos than our moon, about four times higher on average,
so they would look even brighter.
If we plopped Saturn down next to us and stuck Earth in a 24-hour orbit around it, it would
appear about 20,000 times brighter than our own moon.
Now brightness is a very weird concept and hard to compare but simply put, in that case
the planet would appear nearly as bright as the sun.
Especially considering that in order for the Gaian moon to be habitable with all that reflected
light and tidal heating from the planet, as well as infrared emissions, the Gaian Moon
needs to be further from the sun anyway.
So the planet in the sky is going to stay more or less stationary in the sky, day or
night, wobbling a bit from libration, and go through phases like our own moon does.
Both sides of the planet are habitable, but the planet side will be warmer than the ‘dark
side’, which is again not actually dark, but will be a bit cooler.
Now if that were all of it, we could stop and say that any moon of similar mass to Earth
with a day length reasonably similar to Earth’s would be just as habitable as Earth.
Unfortunately gas giants are massive sources of radiation, which as a side note would make
for some truly massive auroras on Gaian Moons.
We mentioned last time how planets around red dwarves need strong magnetic fields to
avoid having their atmosphere’s stripped off by their sun and the same concept applies
to moons of gas giants.
In the case of a Gaian Moon it would have a strong magnetic field but it will be further
shielded from its sun by its planet’s own huge magnetosphere.
Which is good except that they also get inundated by radiation from that planet’s magnetosphere.
Our Magnetosphere shields us from the sun’s radiation but a byproduct of that is the Van
Allen Radiation Belt, where radiation is actually a lot stronger than it normally is.
Gas Giants have a similar but stronger setup and many of their moons are right in that
radiation belt.
It’s not really possible for me to give you even a loose rule of thumb for how bad
that radiation will be, because there are so many variables involved, even a planets
rings can kick in to partially reduce the radiation for instance, but we can say it
is a big problem for moons but ironically a smaller one for the two types of possibly
habitable moons we’re interested in.
One like Europa where the life would hypothetically exist deep under the frozen ice in seas protected
from radiation and needing no atmosphere, and ones like our hypothetical Gaian Moons
where a strong magnetosphere is very likely to exist and shield the atmosphere from being
stripped away.
It’s quite possible that wouldn’t be enough, and as we mentioned with red dwarf planets
it may be that none can maintain an atmosphere, but we do have the example of Saturn’s Moon
Titan, which has a thick nitrogen atmosphere even though it is far less massive than Earth.
Though it also six times further than from Saturn than a Gaian Moon would be which reduces
the radiation it gets.
Still the habitability of Gaian Moons around gas and ice giant planets looks fairly promising,
not just for simple life but quite possibly for complex life like us.
So the Star Wars style forest moon of Endor or Yavin IV are definitely in the realm of
the plausible to the best of our knowledge.
In terms of colonization, we obviously don’t have any Gaian Moons in our solar system.
Ganymede is the largest moon, but it’s still tiny compared to Earth and has a rotation
period of a week, Titan’s is about two weeks incidentally.
Both are far too cold for Earth-like surface life too.
But if we did find a Gaian Moon around another star, it would be a fairly straightforward
colonization.
Hard to find much better than something with an Earth-like mass, composition, and day-night
cycle after all.
And as I mentioned last time that you can boost a planet’s, or a moon’s, magnetosphere
by placing large solar powered magnets in orbit around the thing.
These would actually work very well in tandem with something like an Orbital Ring if you’ve
been following our Megatructures Series.
Speaking of Megastructures, we’ll be returning to that series next to discuss Lofstrom Loops
and other construct for launching objects into orbit before returning to this series.
In the next episode of this series we’ll be looking at the possibility of life on Rogue
Planets, objects that aren’t in orbit around any star at all.
If you want to be alerted to when that or other videos come out, hit the subscribe button,
and don’t forget to like and share this video if you’ve enjoyed it, and try out
the other videos in this series or in my other two series, the Fermi Paradox and Megastructures.
Comments and questions are welcome below, and please let me know which type of planets
you’d like us to look at next.
As always, thanks for watching and have a great Day!