Last time we talked about life on planets very far from their stars, like Pluto. This
time we’ll be looking at life very close to stars instead. Now that can mean two different
types of planets, ones which are very hot because they’re very near a star, like Mercury,
or ones that aren’t hot because their sun is very dim. We’ll look at the first case,
hot or Cthonian planets, another time. In that latter case the worlds have to be
so close to their sun that they could become tidally locked to it, so one side always got
sunlight and the other never did. The biggest factor in Tidal Locking is how close the object
is to the thing it is orbiting and planets around red dwarves have to be very close to
be warm enough to be classically habitable. These worlds, sometimes called Twilight Worlds
or Eyeball Earths, will be our focus for today. Incidentally if you’re not too familiar
with tidal locking you may wish to pause this video and click the link to short Tidal Locking
companion video I did for this. It explains what Tidal Locking is, how and why it happens,
and why we think planets near red dwarf stars would tend to be tidally locked.
Also incidentally, if you’re newer to my videos and have a hard time understanding
me, you should turn on the closed captions for the video which ought to help. I’ve
got a bit of a speech impediment. As I’m sure you’ve noticed.
So you have a tidally locked planet. This tends to get envisioned as desert on the sun
side, dark ice on the night side, and a thin habitable area around the rim lit by twilight.
Hence the name, twilight worlds. The whole light side lit by a tiny glowing red star.
But how accurate is that notion? Like a lot of things seen in science fiction,
accuracy is a bit hit and miss. First we don’t know that all, or even most, worlds near red
dwarves would be tidally locked. Red Dwarves are the supermajority of stars in the Universe
and cover a pretty broad range, from ones more than half as massive as our sun to ones
less than a tenth of its mass. That’s a far wider proportional variation in mass than
most of the stellar types and on the more massive end we wouldn’t expect habitable
worlds to be tidally locked. More over, atmospheres and oceans, as well as moons, can impede the
tidal locking process. Again we discuss that in the companion video.
So the bigger red dwarves can have habitable worlds with orbits that take 2 or 3 months,
while the smallest it can be two or three days. For these smaller ones tidal locking
is a lot more plausible since not only is distance the biggest factor in tidal locking
but also because it’s hard to have a big moon on a planet so close to a star that might
destabilize its orbit. The other big misconception, or partial misconception,
is the appearance of that star to those living on the planet. First, while these stars are
smaller, they are way, way closer so they look much bigger in the sky than our own sun
does. They would seem to be giant blobs in the sky. Second, while they would look red
from twilight band for the same reason our sun looks red at twilight, red dwarves aren’t
really red. Our color classification system for stars was a very early and loose system
that’s just stuck around. All stars look white. Our own ‘yellow’
sun, which is actually green, or emits its peak wavelength in green light, is fundamentally
white. It is too bright in too many frequencies to have a color. Red dwarves have their own
peak frequency in the red range of the spectrum, but so do incandescent light bulbs. On any
tidal locked world, red dwarf or white supergiant, the sun will look red from the twilight band
because of all the air it passes through acting like a prism in what’s called Rayleigh Scattering.
Everywhere else you see it on a world with an Earth-like atmosphere – and most other
types you might expect – the sky will look blue and the sun will be big white glowing
orb. The exception would be a habitable world around
a red giant because those are simply so big and dim the sun would just blanket the sky
in red light rather than being a dense bright object. For most stars though the sun and
sky on most of the planet will look more or less like our own does and it’s the atmosphere’s
makeup that would have the big effect on color, and will vary a lot just as our own does with
time of day and weather. Mars’ has at times a bluish-grey sky, a red one, and a butterscotch
one. [show curiosity pics] Now what do we mean by this twilight band?
How wide is it? Is it the only place life can be? Is one side really desert and the
other massive sheet of ice miles deep? And the answer in all those cases is a mix
of maybe and depends. We imagine them as deserts on the sun side because if the light is always
shining down on one spot, growing a bit weaker as you get away from latitude and longitude
zero, you ought to have a constant wind emanating out pushing moisture away and never carrying
it back, until it deposits on the dark side in an ever thickening sheet till at last there’s
no water anywhere except on a very thin band around the rim.
However, tidally locked objects do sort of squirm around. We say our own moon only shows
us one face all the time, so we see half of it, but that’s not quite true. Libration,
an effect of the eccentricity of orbits and of things not being point-like objects, makes
slightly more than half of a tidally locked object visible to the object it’s locked
to. We can see about 59% of the moon’s surface over the course of a month, not 50%. For a
tidally locked planet with similar libration, this would mean the sun will actually dip
over and under the horizon in some places of the twilight band.
And again most habitable zone planets around red dwarves have planetary years lasting a
few days to a few months, with the ones most likely to be tidally locked on the shorter
end of that. So you’d have spots, for the smaller dwarves,
where the sun dipped over and under the horizon every few days, or on a weekly cycle. For
them it’s a yearly cycle since they have a very short year and their day is exactly
as long as their year, but we’ll try to keep this in comfortable Earth units when
we can hopefully without causing confusion. And that short year has another effect, because
while our own seasons have to do with our axial tilt, the Earth does get more or less
light when it is closer or father from the sun. Amusingly, at least for those of us in
the northern hemisphere, the Earth is closest to the sun not long after winter solstice
in January and furthest in July. That’s pure coincidence, but it does mean Earth gets
about 7% more light in January than in July, and Earth has a very low orbital eccentricity.
Mercury, the one closest to the sun, has the most eccentricity of any planet, and it get
more than twice as much sunlight when closest to the sun than it does when furthest away.
So a tidally locked world with a decent eccentricity might experience seasons, on say a two week
orbit, where the eternal days cooled or warmed rather noticeably. That effect could easily
result in weather moving moisture around in peculiar ways.
Next there’s the dark side. It’s worth remembering that Antarctica is hardly lifeless
even in the places where its dark six months out of the year and we’ve found life buried
under the ice down there. Tidally locked planets don’t start tidally locked and as we discussed
with Pluto, where we do suspect there are oceans under the ice there, or Jupiter’s
moon Europa, you can have liquid water where you wouldn’t expect it to be. There’s
hundreds of lakes buried under the ice in Antarctica. So we don’t want to write off
even the dark sides of these worlds as utterly barren and sterile.
You’ve got this massive miles deep block of ice there, constantly melting on the sides
and having snow come from the sun side adding back to it. But that’s not the only cause
of melting. Ice has weight, and that means pressure, and that also means sideways motion.
If you’re familiar with Game of Thrones, where they have this giant wall made of ice
and gravel, you might have heard people mention that it ought to be way wider at the bottom
and that the bottom should expand as it gets taller and melt more near the bottom. Obviously
that is supposed to be a somewhat magic wall and it’s a fantasy novel but when the laws
of physics apply a giant ice cube is going to squish down, get wider at the bottom, and
melt in the lower layers so the water isn’t going to stay totally trapped there and depending
on how much water the world has you get a thin world-spanning ocean at the rim with
icebergs constantly snapping off into it. Also, oceans transport heat around too, so
they could expand this habitable band quite a bit.
Taken as a whole, these twilight worlds with their thin habitable bands might be a lot
wider than we used to think. So wide that twilight band might not be properly descriptive
for some and the term ‘Eyeball Earth’ might be more apt.
So could we colonize such a world? Yeah, sure, on the grand list of screwed up planets with
weird conditions these sorts are a heck of a lot easier to terraform than most we’ll
discuss, for a given value of the world terraform, and probably easier than some place like Mars
or Venus. Adapting plants to perpetual daylight is probably feasible, and most plants only
use a smaller fraction of the light that hits them at noon anyway so eternal twilight isn’t
a big deal. It wouldn’t seem very dark to us either, your eyes are logarithmic in their
sensitivity which is why your light bulbs in your home seem perfectly bright at night
time but during the day with the windows open, even though only a small fraction of sunlight
gets in those windows, you’d barely notice the lights were on. Most house lights illuminate
a room at about one thousandth the brightness of the noon time sun, around as bright as
the sun on Pluto, so a twilight area of tidally locked world isn’t going to seem dim to
you anyway. It will be red from all the air, and the water will be a lot darker, but by
and large it will be fairly unexceptional from our point of view.
Plus, as we mentioned in the video on Pluto, you can always use orbital mirrors, thin light-weight
ones, to light the darkside up and shades the sunny side. You could even institute a
normal Earth 24 hour day night cycle if you were ambitious. After all a thin mirror can
be shiny on one side and opaque on the other and you can make the thing tilt parallel or
perpendicular to a star and twist their direction as needed. All things being equal, a tidally
locked world is probably the easiest kind to terraform closest to Earth’s own conditions.
Now how about life evolving there in the first place? Well there’s some challenges there.
First off, life, at least any life based off normal chemistry even if it isn’t our style,
doesn’t handle UV and X-ray radiation well and red dwarves do give that off, especially
when they’re younger. You need a magnetosphere to protect that life. Very thick atmospheres
or life living under water helps but for classic land life you really do need that magnetic
solar shield. We get ours on Earth from a giant molten metal
core spinning around. Tidally locked worlds do rotate, their day and their year are the
same but they do spin. Ganymede, Jupiter’s largest moon and the
largest moon in the solar system, actually bigger than the planet Mercury, is tidally
locked to Jupiter and does have a powerful magnetosphere. As I recall it’s the only
moon with a strong one and it’s still so weak that Jupiter’s own field dominates
it at that distance. Without that magnetosphere, a planet is not just going to have life on
it irradiated, but could get its whole atmosphere stripped off by the charged particles its
sun deluges it with. Realistically they don’t just need a magnetic field nearly as strong
as Earths, they need one decently stronger because red dwarves are fairly unstable as
stars go and can really oscillate their radiation a lot.
On the terraforming end, that’s not a barrier that can’t be overcome. You can simulate
a magnetosphere by having big solar powered magnets orbiting the planet just like you
can do orbital mirrors. On the natural life end though, that is a
worry, air protects from ionizing radiation and our twilight bands will have a lot of
air protecting them, that’s why the twilights here on Earth are red, light has to pass through
far more air at twilight than at noon. And again the world could have a natural magnetosphere
of great strength but while that’s not entirely dependent on rotational speed of a planet,
and tidally lock worlds still rotate, once a year which for them might be only a few
days or weeks, it doesn’t help matters having less rotation.
Another consideration is collision. Red Dwarves aren’t all that much less massive than our
own sun, even if they are way dimmer, so they could have fairly massive collections of planets
like we do. Those planets might be more densely packed, but they also ought to have a lot
of comets, and since they are dimmer these won’t evaporate nearly as quickly. There's
good odds we owe our own oceans and atmosphere in large part to getting whacked with tons
of comets. This is probably a bigger threat to life-bearing worlds around a red dwarf
since there could be a lot more comets hitting the thing and more gravitational perturbations
by the other planets knocking things into collision orbits and collisions are going
to be more common anyway since things are packed in a bit tighter. Circumstances depending,
that could be a benefit or hindrance to life evolving there.
We don’t know for sure how most of these factors add up as pros or cons since we don’t
exactly have a plethora of life-bearing worlds to examine or accurate models of how durable
life is, but that aspect alone might make red dwarf worlds barren and sterile. Or it
might not, in this videos, unlike the Fermi Paradox videos, we take a more optimistic
approach to where life might show up. The other good news is we probably won’t have
to wait terribly long to learn more about such planets, in our hunt for exoplanets worlds
close enough to be tidally locked are among the easier kind to detect.
In that optimistic regard, tidally locked worlds around red dwarf stars look like pretty
good candidates not just for simple life, as we discussed with Pluto, but complex life
too. And as science has rolled on we’ve found a lot more positives, and negatives,
for these kind of worlds hosting life, more than we can cover here today and we just do
know yet. But fundamentally, while most of the planet
isn’t very habitable for large land animals like us, probably about 5-20% would be, and
even if that area was mostly ocean like Earth is that’s still a large continent worth
of land. Probably enough to support a lot of biodiversity and the numbers needed to
be putting together a technological civilization from hunter-gatherer levels. So Twilight worlds
get the stamp of ‘plausible’ for possible home worlds of intelligent life.
That’s it for today. If you’ve enjoyed the video hit the like button and try some
of the other videos, and don’t forget to hit the subscribe button to be alerted when
new videos some out. Thanks for watching, and have a great day!