Life on a Low-Gravity Planet

Life on a Low-Gravity Planet

This episode is sponsored by Audible. In the future humanity may come to reside
on strange new worlds, and likely many will have lower gravity than Earth. We may find native life on those worlds or
adapt to them ourselves. But what would those adaptations be? There is a fairly common assumption in science
fiction that folks living on low-gravity planets or on spaceships would tend to grow tall and
willowy while folks on high-gravity worlds would tend to be squat and muscular. On initial inspection this seems to make sense. Trees have evolved to reach up high to get
sunlight, above their neighbors and other plants, and are limited in height largely
from issues like weight and fluid pumping which are obviously reduced in lower-gravity,
so it stands to reason they’d be able to get taller if there was less gravity. Of course they also need to be strong enough
to resist wind, so where there is more of that they can’t grow as tall. When we consider places like Mars, with its
infamous dust storms, we sometimes forget that those aren’t actually very strong for
the speed the air moves at, since the atmosphere is so thin. If the atmosphere is thinner, as we often
expect on low-gravity worlds, this makes it even easier for a tree to grow tall. However, thin atmospheres are not a guaranteed
thing on low-gravity worlds, and neither is a tree a very good analogy for an animal let
alone a human. Our catalogue of planets besides Earth has
grown massively in recent decades, numbering a thousand times what it was in my childhood,
but not only are most of the new worlds around distant suns little more than vague blobs
we know little of, but they are almost all planets that are vastly more massive than
Earth. It’s easiest to detect the bigger worlds
after all, because they block more light when they pass in front of their sun, make their
sun wobble more with their mass, and noticeably change the gravitational lensing effects when
their sun lenses a more distant star. But smaller planets may prove far more numerous
when we can better detect them. And so far we really only have Mars, Venus,
and Mercury to look at in terms of places with less gravity and obviously none host
any known life. There are four basic factors that make a planet
keep or lose atmosphere. High gravity holds it in place, high temperature
boils it off, ionizing radiation zaps it away a little at a time, and geomagnetic fields
protect it from that ionizing radiation. But remember that an atmosphere is generally
a mix of several gases, and the high-molecular weight gases are easier to hold onto than
light ones. Venus illustrates this very well, because
compared to Earth it’s worse on every one of these factors—lower gravity, high temperature,
more solar radiation, and no magnetic field—and yet is has a thicker denser atmosphere than
Earth. To explain how low gravity works on Venus,
we have Peter, the voice of What If. What If is a science show here on YouTube
where you explore epic hypothetical scenarios. Well when low and high molecular weight gasses
are mixed, they reach the same temperature, but the high weight gasses aren’t moving
nearly as fast. This means, the escape velocity, which is
the speed needed to break out of a planet’s gravity well, is the same for any molecule
mass, but far fewer of the slow molecules are needed to break out of it. Earth actually loses little bits of elemental
hydrogen and helium to space all the time, but it manages to hold onto heavier elements
like nitrogen and oxygen. But Venus has lost those gases and holds onto
much heavier ones like carbon dioxide, sulfur dioxide, trioxide and some nitrogen. So in theory, a low-gravity planet could have
a dense atmosphere, but this means it would have to be composed of heavier gases than
what we have on Earth. A magnetic field would definitely help keep
the atmosphere in place, but as Venus shows it’s not completely necessary. Thanks Peter. And Folks, make sure to check the link in
the description and subscribe to What If. Back to low gravity… Given the timelines involved, a planet that
has developed complex life is probably either going to have a pretty thick and stable atmosphere
or have long since lost all but a trace of it. There’s no real reason it would have a thin
atmosphere compared to Earth and yet be thick enough to permit surface water. Moreover, we can’t really assume our solar
system is a good model for this, as our Sun is bigger than most stars and also rather
stable as these things go. Many stars will be far more volatile, while
others might have less output in the more dangerous bands of radiation that would tend
to ionize particles. For that matter, under lower atmospheric pressure,
water boils at a lower temperature, putting more water vapor in the air that can be broken
into hydrogen and oxygen and that hydrogen lost. Whereas if the atmosphere is higher pressure,
water evaporates at higher temperatures. You also need more air on low-gravity worlds
to get the same surface pressure due to the lower gravity. Pressure is the cumulative result of weight
of air and weight is a function of gravity, so the same quantity of air on a lower gravity
planet as Earth has would be at a lower surface pressure. We would also expect that atmosphere loss
is a bit of a runaway process, as you lose more and more air it goes faster. The pressure drops and the oceans evaporate
easier, and particles hit by high-energy particles can travel farther in the thinner air without
hitting another particle to lose speed, and so you lose more pressure and lose air faster. Add to that, hotter planets are nearer their
Sun, and hit by more ionizing particles and solar wind. So too, they’re more likely to become tidally
locked and have their metallic inner core slow down as a result, and it’s those big,
spinning balls of molten metal that generate those magnetospheres, if that weakens, they
lose air even faster. So we do indeed have reason to think low-gravity
worlds and hotter worlds will lose air faster and be less likely to have an atmosphere by
the time life of complexity had developed, but we also can’t assume that’s a hard
and fast rule. Large Moons around gas giants might be shielded
in part by the magnetosphere of their massive parent world, for instance. Worlds around less volatile suns might get
less pounded by solar radiation and wind. Planets might get hit more often by comets
bringing more water and other materials in, and so on. They will be less likely overall to have thick
atmospheres than bigger worlds, but they are also likely to vastly outnumber those bigger
worlds, so it may be that the majority of worlds with oxygen-nitrogen atmospheres are
actually worlds less massive than Earth, simply by raw numbers. As I mentioned a moment ago, Venus is hot,
has virtually no magnetosphere, and has slightly less gravity than Earth, yet possesses a very
dense atmosphere – though much of this is heavier carbon dioxide. Beyond not wanting to assume life definitely
needs water to live, or oxygen to breathe, a world heavy in carbon dioxide might retain
a higher pressure atmosphere while also being warm farther from its sun, as carbon dioxide
is a greenhouse gas. Many other permutations of atmospheric composition
are potentially plausible and viable too, and needless to say this only applies to worlds
with surface oceans and atmosphere, many moons and lesser planets will have oceans beneath
the layer of ice in which life might originate. We’ll focus on the liquid water under an
atmosphere case though. Now this all assumes a natural setup, when
it comes to terraforming worlds we could probably keep an atmosphere even on our own moon simply
by setting up a powerful magnetic field around it. And it’s easy enough to do too, you don’t
need a big ball of spinning molten iron for this and it’s far easier to simply create
a big solar-powered electromagnet ring around such a moon or planet, as we discussed doing
in Springtime on Mars. It’s a big task to be sure, but less than
folks tend to think, it doesn’t require that much power, and is far easier than trying
to drill down to a core and dump millions of megatons of atomic bombs down there, as
folks often suggest for Mars. For comparison, it’s like deciding your
backyard gets too much wind and building a wall or hanging tarps to block the wind, versus
trying to set off Earthquakes and volcanoes to make a new mountain arise to block that
wind. And of course you can always just go the construction
route and cover a world with vast numbers of domes to hold air in, what we call para-terraforming. It sounds like quite a project too but it
is tiny compared to actual terraforming. So too, bringing in the occasional comet to
hit the world, or get detonated right before hitting to avoid a big impact, allows you
not only to add an atmosphere but refresh it if it’s leaking a bit too much. And since it is mostly leaking hydrogen, and
a little bit of helium, the first and second most abundant materials in the Universe, you’re
hardly in short supply of replenishment sources. As to oxygen, it is the third most abundant
element in the Universe and most rocky planets are overflowing in it, tied up in those rocks,
it usually makes up the first or second most abundant material of those places. You can easily remove it from those rocks
for your atmosphere and atmospheres are only a tiny part of the total mass of planets like
Earth anyway. Needless to say this only is allowed under
artificial circumstances, planets we come to inhabit with our technology and industry,
though we might imagine any number of interesting phenomena that might allow it to be natural. As an example, planets around red dwarf stars,
the most common kind of star, might easily have their own robust Kuiper Belts or Oort
Clouds full of icy comets, water is ridiculously common in this Universe due to hydrogen and
oxygen being so abundant, and any such comets entering in close would have a shorter path
between the frostline where they melted and impacting the planet, which also takes up
a bigger effective cross-section for collision in that solar system since it’s more compressed
around that smaller, dimmer star. So they might get hit by comets much more
frequently and replenish their gases that way. Now as to life living there, however it arose,
natural or transplanted, what really is different? Again it’s likely to have a decently thick
atmosphere or none worthy of note, so we shouldn’t assume the natives need giant lungs, or that
trees can grow however tall and broad they want without fear the wind will knock them
over. What about the critters though? Are they more tall and willowy from the low
gravity? Perhaps but we have to ask why they would
get taller. A tree has a reason to get taller. Tall is only advantageous in nature because
it makes a critter look more threatening or reach food that’s higher up, and there’s
a reason we don’t really have as much megafauna as we used to. As to thinner bones and less muscle, one might
ask why? Birds have thinner bones to make them lighter
for flight, though hardly lack in muscle. That comes at a price of being less sturdy
against blows. We need thick skeletons to support our mass
and absorb the shock of falling and walking but also to support us against inertia of
hitting or being hit and inertia is not affected by gravity, just weight. Nature also cooked up ways for critters to
get big, like having air pockets in their bones, as large dinosaurs often had. Peter, are there any alternatives? Well, a cool thing you’ll find on a low
gravity planet is that you can have thicker bones, as well as more muscles compared to
what you might have on Earth. This is due to your body weighing less. It will still take effort to alter your enertia
but you’ll weigh a lot less and be able to carry way more mass. Heh, think of how cool you’ll look when
you’re at the gym. We can look at Megafauna in the past who also
benefited from higher oxygen levels. If we wanted to achieve similar pressure than
we have on Earth we’d need more air, which would significantly impact the weather, as
well as the biology and chemistry on the planet. Flight is easier in lower gravity as well. Though flight mostly has to do with speed
and air density, we could see massive critters flying about. Ew, gross. Since gravity would affect swinging and jumping
as well we could also see a lot more creatures living in trees, engaged in branch hopping
or even gliding. These wouldn’t just be tiny little squirrels
either. The creatures living in these trees could
be massive. The reason why they aren’t up there right
now is because it’s too dangerous for large predators to be in up in tall trees, as it’s
too risky for them. With this low gravity we could also see creatures
who could parachute as well as ones with wings! However, this low gravity world could hurt
smaller flying creatures, like insects. These creatures rely on the air being thicker
at small scales which helps for flight and maneuverability, even in lower gravity. The new insects we find on a lower gravity
planet would be kind of like dragonflies, only smaller, or they might be microscopic. But what would happen to these insects if
we lived on a planet with much higher gravity? Well you can check out our episode “What if
Earth was as big as the Sun?” right here. Back to you Isaac. That part about bones having air pockets in
them from a moment ago might result in quite the opposite effect on tree height too, on
higher-gravity worlds. As we said earlier hydrogen and helium escape
fastest from planets, and gravity really does play a role in retaining gas. What’s more a bigger planet is likely to
have a stronger magnetosphere to hold on to those gases. Such being the case, we might see higher gravity
worlds featuring tree-equivalents that made use of biochemistry to separate the lighter
gases from the atmosphere and used internal bladders of lifting gas to let them grow far
taller, potentially even photosynthetic bladders as a leaf analogy. Such plants would still be vulnerable to wind
but they might be able to deflate themselves in wind and storms then re-inflate. Of course something like that could have evolved
under any gravity, it really depends more on if the atmosphere’s composition is rather
spread out in terms of the mass of air particle types. Virtually all of our atmosphere is diatomic
nitrogen and oxygen and they are about the same mass, only a 14% difference, as opposed
to diatomic hydrogen and carbon dioxide, where the latter masses 22 times as much as the
former. If it had larger portions of gases that differed
by much in molecular mass, such lifting-gas approaches to weight might show up, creatures
that were essentially organic blimps that sucked in the air and retained the lighter
molecules to serve as a lifting gas. We do after all see it in regard to oceanic
creatures which frequently use air and water ratio changes to alter their buoyancy. Indeed it’s possible you might not simply
end up with two ecological layers, ground and treetop, but an entire biosphere layer
suspended in the air, if that lighter gravity world was maintaining its atmosphere in large
part from having a big molten core and much tectonic activity, you might have a good deal
of dust and moisture in the air and organisms floating up there to maximize light and feed
off those atmospheric contaminants to achieve both nutrient and buoyancy. In lower gravity it’s easier for wind to
pick up dirt and soil and loft it into the air too. But speaking of dirt, we’re not limited
to going upward either. Much of our own ecology is beneath the ground,
and is partially limited by the rising pressure and density of the dirt. You might have deeper topsoil just from a
reduction in gravity and weight compressing things. Such worlds might have far more caves too. The Moon has many large underground lavatubes,
as probably would many smaller worlds, because they don’t collapse as easily, and it is
easier to dig through dirt or gravel and shore up tunnels in lower gravity. You might have far more burrowing creatures
and far more large caverns as well. What we’re seeing though is that the gravity
itself, while a major factor in how a world is setup, still leaves a very broad ranges
of options available. Taken as a whole, I don’t think we can make
the assumption that critters would evolve to be skinny and large, though large seems
plausible. Just in this case large might also be musclebound
brutes carrying a lot of bone. You might also tend to see more internal storage
too, akin to the camel and its hump of water, motion still takes energy and is mass-dependent
but a lot of critters spend a lot of their energy just standing, you burn more calories
standing than sitting or lying down for instance, and that’s purely a function of weight. But big also moves slow, even if it’s got
the muscle to run quick, because it’s not very agile, it’s very hard to turn on a
dime when you’re an elephant after all, compared to a mouse. As I said, many of the planets, and most large
moons that we will encounter in the Universe will be smaller than Earth. We’d expect to find more worlds with less
gravity than more. I suspect life, at least surface based land-life,
to be more likely to exist where gravity was a bit higher but it could be that the larger
number of smaller worlds will make up for them being less likely to host atmospheres. But I also mentioned that while we were focusing
on worlds with surface oceans and atmospheres, we would have a lot of such worlds where the
water was trapped under a layer of ice, places like Jupiter’s Moon Europa. Such worlds would be scanty and meager in
their life, there’s just not much energy to run an ecology without the Sun, but they
may be quite prone to having life develop regardless. It’s also a good reminder that our oceans
are affected by gravity too, and not just tidal forces. Pressure is a function of weight above you
and that’s lower if gravity is lower, so you might see quite an expansion in oceanic
layers. Now, the top thin layer of water where light
can reach abundantly is still where most of the action would take place, with much beneath
it being those organisms living on debris, marine snow, descending from that top level. That might see some interesting adaptations
toward the vertical. Near the shore we often see plants that grow
on the seafloor, getting nutrients there, but reach up to the top for light. Out in the deeper sea things either live on
the bottom without light or up top with light but little nutrients. In lower gravity that shore-zone where they
can reach from the seafloor to the top is expanded as plants can grow taller and reach
from seafloor to surface further from the shore. Possibly a great deal as traits that improved
that option would probably be more easily favored by evolution. You might see plants growing rather tall in
the sea, moving your coastal ecologies out much deeper. However, nutrients that are heavier than water
also sink faster in higher gravity so we might tend to see more abundant ocean life. Most of Earth is covered in deep sea, life
is not terribly abundant there compared to the shores or inland because sunlight is up
at the top and nutrients far below, or scarcely diffused into the water near the surface. If it’s easier for nutrients floating in
the water to persist near the top, then more biomass would arise in those areas away from
the coastline. Combined with the lower pressure, this might
result in very verdant oceans, potentially clogged with life on the surface so that you
could literally walk across the water. Hard to say of course, and this is all very
speculative, but we see a door open to far more ecological options than we might initially
expect, compared to the usual notion of thin, dusty, dry worlds of low gravity. Of course that is very dependent on having
a thick atmosphere and as mentioned earlier, once one of those begins dissipating it probably
is a bit of a chain reaction, losing air faster and faster. Such things would still be on geological timelines,
so life might adapt as the air thinned and oceans diminished. Until the early 20th century, we thought Mars
had canals left over from ancient oceans. And while better telescopes have revealed
that those barely-visible lines were just surface fissures, it also appears that Mars
did have oceans and an atmosphere, long ago. Hopefully someday soon, we’ll get to do
some serious excavating and geology there to see how long ago it was and how quickly
it ended once that atmospheric loss hit critical tilt. There’s a lot of books set on Mars of course,
and while modern ones usually focus on us colonizing it and bringing life there, those
of a few generations back tended to assume life was already there, but diminishing, that
Mars was dying off. We see one such example in C.S. Lewis’s “Out of the Silent Planet”,
the first book in his Space Trilogy, our Audible Book of the Month. C.S. Lewis is best known for his classic fantasy
series “The Chronicles of Narnia”, which along with his friend J.R.R. Tolkien’s Lord of the Rings helped breathe
life into the emerging fantasy genre. But over a decade before Narnia he wrote the
Space Trilogy, and its protagonist, Philologist Elwin Ransom, is said to have been modeled
on Tolkien as an inspiration. I happened to have gotten an omnibus edition
of the trilogy as a Christmas gift from my fiancée Sarah, who promptly borrowed it to
read on a trip, leaving me to need to grab the audiobook version instead, which to be
fair is my preference as I quite prefer listening to tales anyway. Unsurprisingly it inspired today’s episode. While the science is of course dated and never
was C.S. Lewis’s strong point anyway, the first book
paints a fascinating portrait of the Red Planet and the life that emerged there, the culture
and language that developed, and the philosophy of colonization of other worlds. Audible has an amazing catalog of audiobooks
and Audible members can choose 3 titles every month: 1 audiobook and 2 exclusive Audible
Originals you can’t hear anywhere else, including access to news, original audio shows,
and guided fitness programs, and since you can listen to your audiobooks anywhere on
any device, and seamlessly pick up where you left off, they’re great for commuting, running
errands, or going to the gym. You can start listening today with a 30-day
Audible trial. Choose 1 Audiobook and 2 Audible Originals
absolutely free. Just visit the link in the episode description,, or text “Isaac” to 500-500. So we have our monthly livestream coming up
this weekend, Sunday March 29th at 4pm Eastern Time, and then we’ll move into April’s
line up starting with a look at New Technologies that might be in the cards in the coming decades. Then two weeks from now we’ll flip that
around, and ask ourselves what things might be like if Technology came to a stand still,
and what might cause that, in “Technological Stagnation.” Don’t forget to check out the video “What
if Earth was as big as the Sun”, over on What If and subscribe to their channel and of course don’t forget to hit the like
button and subscribe to this channel if you enjoyed today’s episode, and remember we’ve got our monthly Livestream
coming up this Sunday. Until then, thanks for watching, and have a great week!

86 thoughts on “Life on a Low-Gravity Planet

  1. Thank you for adding something to fascinate me during this quiet time; I’m looking forward to this and making a snack. Had a piece in the Daily News last week on the COVID:
    and have another one on Harvey Weinstein in editing with a different place right now. Things aren’t too different for me and I’ve yet to don a mask or anything. You are well I hope ?

  2. I am glad to hear about the interesting realistic things that life and people would have to deal with. Hope you are doing good.

  3. So, Sir Isaac, I take it we’ll all be donning medieval suits of armor when traversing far off worlds to keep us firmly planted on the ground while providing some protection against certain forms of radiation. The images in my mind a the moment would make for some sweet cover art…

  4. This is one of the things I wondered when watching The Expanse. If someone who'd spent their entire life on Earth moved to the Ceres Station, they would almost be freakishly strong compared to native Belters, right?

  5. On the low gravity planet pressure would decrease slowly, so at the same pressure at ground will be thicker and reach higher altitudes, a single pressure bubble around the planet would solve the problem rather than thausen of doms.

  6. Thank you Issac for posting wonderful content that no one else usually tackles

    on another note will you also be featured in one of what-if's podcasts

  7. My good sir, There are fewer planets since your childhood not more. You have made the common and forgivable error of categorizing exo planets as planets.. But they are not.

  8. I dispute Pete's assertion about low gravity not holding onto lighter gasses. Titan has a very low gravity, but has a thick nitrogen atmosphere heavier than earth's.

  9. 11:35 Megafauna such as large dinosaurs or mammals did not live during a time with a higher oxygen concentration in the atmosphere. The animals did grow to larger sizes because of higher oxygen concentration were land arthropods like insects, arachnids, and myriapods in the Carboniferous.

  10. Please never have what if back on. They are a low grade inaccurate channel
    They may too many mistakes in the videos they make. And not to mention the mispronunciations that are constant.
    Curios Droid on the other hand the total opposite. Very professional channel such as your own.

  11. Isaac is the David Attenborough of the space/science/futurism sphere. You can instantly recognise who is talking and know just as quickly that you are going to get premium quality.

  12. I love your content. I haven't searched through all your videos but if you haven't covered the topic yet, would you cover life for a planet tidally locked with it's own Sun? My first guess would be one half of the planet would be very hot and bright, while the other half would be freezing cold and dark. But I wonder about the implications for aquatic life in oceans, the parity of life if any on the dark side of the planet contrasted with life on the mostly bright side. Would a tidally locked planet to it's star make for a habitable planet for life to form on it, much less sustain itself for billions of years?

  13. I was thinking of writing a novel about how living on different planets would change human physiology so this has been a very useful video.

  14. I don't think we would really see animals bigger then we have on Earth in low gravity. For example, the biomechanics of the sauropod bodyplan could probably have gotten twice as big as it did. The limiting factor for size in life on Earth seems to be the ability to get enough food to sustain their body size, not gravity

  15. congrats on the engagement – maybe that book is a ransom held to ensure you complete the agreed upon task lol

  16. World's with 2X the gravity wondering if life with half their own gravity is even possible.
    Then comes a wandering ship of human colonists.

    Plot of one of my WIP.

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