So today we’re looking at Double Planets, and our big focus is going to be on the especially
rare case of two Earth Sized planets orbiting every 24 hours, since that’s the probably
the most interesting case, to us anyway.
But we’ll also take a look at a few other examples to including Rocheworlds, double
planets that have literally merged into a dumbbell shape, and as always we’ll take
some time to discuss how native life might arise on such worlds and what special aspects
of terraforming would be needed to make these habitable for humans.
The first thing to understand about double planets is that there’s no real cutoff at
which something ceases being a moon and becomes a double planet.
Our own moon, at an eightieth Earth’s Mass and a 1000 miles in radius is enormous as
natural satellites go, one of the largest in the solar system and all the others of
similar size are around gas giants much bigger than Earth.
Yet an Earth-sized moon around a Jupiter-sized gas giant, our topic a couple videos back,
would be much more like what we mean by planet yet would be proportionally much smaller compared
to that gas giant than the moon is to us.
Pluto and Charon are the closest to being double planets, and will actually be my own
cut off point.
Charon is about an eight Pluto’s mass, not the eightieth the Moon is to Earth.
Some would argue having the barycenter of a two-body system outside of the larger planet
is the proper definition, which is not true for Earth and the Moon but is true for Pluto
and Charon, but that’s also not my definition for today.
I will be arguing that it is because they are both tidally locked to each other that
makes for two double planets, in the context of what we think of as a habitable planet,
which is what this series is about.
Now we discussed tidal locking in detail on the video about that and before in this series
as well, but as a quick reminder, that is when a moon or planets slows its rotation
over time from tidal forces until it has the same side always pointing at its larger neighbor.
In a double lock, both planets point the same face at each other, and realistically any
two planets of vaguely Earth mass and temperature will be close enough in their orbit of each
other that they will have both tidally locked long before complex life could arisen, though
in point of fact the sheer tidal stresses involved prior to that double lock would probably
prevent complex life arising before that lock anyway.
What we haven’t discussed before is a barycenter, and mostly because it usually doesn’t matter.
We do not exactly orbit the sun, nor the moon orbit us, in terms of circling around it.
For any two-body system there is a center of mass that two actually orbit, and this
moves since two body systems usually have at least slightly elliptical orbits.
We’re mostly interested in circular or very low eccentricity orbit today though.
Now if the main body is infinitely bigger then the satellite that barycenter would be
exactly in the middle, but of course that’s never the case and for the Earth-moon that
barycenter is in Earth’s Mantle, whereas for Pluto and Charon it’s actually outside
Pluto.
And it will be in pretty much any case we’d start thinking of a planet and moon as a double
planet instead.
As the planets get closer and closer in mass, if you’re looking at them from above that
barycenter, they increasingly stop looking like one is orbiting the other and more and
more look like both are orbiting the barycenter, until at equal mass they seem to both orbit
on the same circle 180 degrees apart.
And if they are both tidally locked then the same face of each will always point at the
other.
This is great for space elevator if you’ve got a material strong enough because you can
run one right from the one to the other.
If they’re on elliptical orbits you’d need to have winches on either end playing
out or reeling in a little extra length since they would alter their total distance a bit.
Though you could, hypothetically anyway, use such a tether, if it’s just a little stronger
than it needs to be to avoid snapping, to slowly force the two into a perfectly circular
orbit.
Or one just as close to that as you like so as to keep some tidal forces in play, which
we’ll get back to in a minute.
Now in the case of two planets with approximately the same mass as Earth, the one most people
are interested in, once they’ve gotten tidally locked, in order for them to have a day 24
hours long they’d need to be just over 33,000 miles, or 53,000 km from center to center
for a circular orbit, only one seventh of the distance from center to center of the
Earth and Moon.
Different masses or day lengths would have different distances but you may calculate
such cases if you’re curious and in the video description below there is a link to
a convenient online calculator for that.
Further away and the days are longer, closer together and the days are shorter and we get
even more severe tides.
Now that’s actually only 25,000 miles apart, since 33,000 was from center to center, and
these are 4000 mile radius planets.
That’s more than nine time closer than the moon is and the planet is just under 4 times
wider, so the other planet is going to be about 40 times wider in the sky.
Here we’ve got our moon coming over the horizon, and here would be that double planet.
And a double planet could also have a moon, orbiting out far beyond the two, and it will
more or less look and act like a normal moon of that appropriate size and distance around
a planet of double the normal mass.
Of course that double planet never rises or sets, it stays in the same place in the sky
minus minor perturbations and libration like we discussed back in episode 2 in regard to
tidal locking.
It’s also going to be incredibly bright, over a thousand times as bright as the moon.
One side of the planet always has it hanging overhead, and the other never sees it, with
a band around the middle where it wobbles up and down over the horizon.
Now before they became mutually tidally locked the tides on these planets would be monstrous,
but would settle down to very low once that happened, relatively speaking anyway.
Two mutually locked bodies still exert tidal force on each other but it’s from various
types of libration and orbital eccentricity, not perfectly circular orbits and the like.
A very mildly eccentric orbit that saw the two planets just 5% further apart than when
they were closest would mean the gravity between them was 9.3% lower then when they were closest,
so the distortion caused by that 24-hour 9.3% drop in force pulling each other would get
you pretty decent tides, considering that change in force is a good deal stronger than
the total force the moon exerts on the Earth.
The moon is only 1.2% of the Earth’s mass, so even a twin planet that far away would
exert about 80 times the force that the Moon does, and ours are seven times closer.
The total force exerted would be around 4000 times what the moon exerts on us.
So the change in force from the moon being on one side, then another, of the Earth, when
compared to a 9.3% variation in force from a planet being 5% closer at perigee than apogee,
is a good 200 hundred fold higher.
Even if that apogee and perigee ratio was 100:101 we’d still be looking at 40 times
the force variation.
That’s vastly smaller then what would have been experienced by both before they tidally
locked but still pretty darn impressive.
For Earth like tides you’d need an eccentricity that kept them from moving more than about
a hundred miles closer or further apart.
That is one reason why a space elevator between them, either the classic one built on tensile
strength like we discussed in the first episode of the megastructures series or the alternative
pushing form, the space fountain we discussed in episode 3 of that series, would be very
nice for terraforming.
Besides the obvious value of a space elevator for getting material on and off those planets
when terraforming them, and traveling between them, you could use that elevator to slowly
bring down the eccentricity to the desired level, which might be preferable to some of
the other methods we discussed in the Terraforming video for tinkering with planetary orbits
and rotational speeds.
Now a common question about double planets is if they could share an atmosphere, and
the answer is yes but it depends.
The Rocheworld case will get to in a couple minutes can, but actual double planets like
our Dual Earth case really cannot.
These two planets will actually deform slightly toward each other, making them even a bit
closer from surface to surface then I previously mentioned, and there would be exchange of
material during major asteroid strikes, possibly allowing one world to seed another with life.
Especially early on in a solar systems life when you’d expect a lot more debris to be
careening around impacting stuff, a strike could send very basic and robust microbes
up into orbit and to fall back down on the other world.
We’d be talking very tiny and robust things though, so you wouldn’t expect them to have
the same flora and fauna, just to quite probably use the same basic building blocks.
And while atmosphere arguably extend hundreds of miles over a planet, what we think of when
we mean atmosphere is where you can still breath and fly, and that could never stretch
over that distance.
There’s no special tricks to terraforming these planets, they are in this specific case
obviously very earth like in mass and temperature, so just that linking space elevator trick
is unique to them and you can actually do something similar with any two bodies, tidally
locked or not, so long as you let the base of the elevator move on some track, though
an Earth-Moon one for instance would require a lot of extra energy to overcome the constant
massive drag of slamming the base through your own atmosphere at supersonic speeds,
making this a trick much better done from a top an orbital ring like we also discussed
back in the first megastructures video, since there’s no air up there to cause drag and
sonic booms.
Regarding life on such a double planet, they’d be forming in a very high tide environment,
which is not necessarily a concern, Earth used to have way worse tides back when the
moon first formed and it was closer and the earth spun around about once every 12 hours
not 24.
Only a double planet with any eccentricity is going to keep pretty impressive tides so
possibly amphibians would be a larger share of the planet’s animal life.
More tides also means erosion too, and this would effect storms, but if you’re just
thinking of this from a fictional standpoint, or one we’ve terraformed including altering
the orbital eccentricity, then it could be tailored to ape Earth tides by simply having
low eccentricity.
No flying critters between planets though, for that you need a Rocheworld.
The Roche limit is as close as an object holding itself together by gravity can get to another
large body before being shredded, as the larger body pulls on the surface of the smaller one
harder than the smaller one pulls on that surface itself.
This gets worse from both the deformation from gravity and the centrifugal spin near
the surface.
Equal sized, or near equal sized, bodies have much lower Roche Limits, so close they practically
have to be touching.
Even our moon if brought close to Earth would have to be within 6000 miles, which would
be skimming distance since the Earth is 4000 miles in radius and the moon 1000 so they’d
only have a 1000 miles of separation from closest points.
For our double planet the two can flat out overlap, and Robert L. Forward played with
this idea in his novel Rocheworld.
Your two planets are more egg shaped then Spheres, and can definitely share atmospheres.
They are also definitely tidally locked.
They can share an atmosphere, they can even share an ocean, and eventually they will deform
to share land and become a peanut shape.
This can take billions of years, but once that occurs they will begin merging into a
single sphere and that will be a very destructive process if you’re living on it.
But it is a geological timeline, not a short one.
This is also the sort of process that could give birth to a naturally occurring Hoopworld,
we looked at the artificial variety recently in the megastructures series.
While very close double planets should eventually merge together, if this process was interrupted
by a strike from a large rogue planetoid, similar to how our own Moon is thought to
have formed, and a strike that added a lot of new angular momentum to the system, it
could result in a torus shaped planet.
Neither a Rocheworld nor a Torus world are terribly probable or stable items, double
planets, even the big double pair of Earth masses, aren’t incredibly likely either
but would be much more likely than either of those.
Now a Rocheworld has another problem, the ones with Earth-like gravity would have very
short days.
These are double planets whose centers, if we’re talking Earth-like masses, will be
only about 10,000 miles from center to center.
So your day length is only 4 hours long.
I don’t think there’s any combination of worlds that would give you fairly Earth
like gravity and day length that wasn’t almost completely water.
What we often dub a Panthalassic world.
Water deforms, as a fluid, much easier than rigid bodies so you could have a rocheworld
that had a lot of water, and a vaguely sane day length and surface gravity, which might
actually connect oceans and potentially if just right have land masses on the far sides
of the two planets.
We’ll talk more about Panthalassic worlds in the future, ones who outside this case
are worlds with no land mass, or very little land mass, not including icebergs, potentially
whole ice continents, and oceans many miles deep.
But that’s going to wrap us up for double planets today, next week we’ll be returning
to the Fermi Paradox for a new, updated, and expanded version of Comprehensive list of
Fermi Paradox Solutions, so we can lay the groundwork to start exploring some of those
solutions in greater individual depth in their stand alone videos.
Questions and comments are welcome, and if you want alerts when those videos come out,
make sure to click the subscribe button.
In the mean time try out the other Habitable Planet videos or any of these other video
series by clicking on them to bring up that Playlist.
As always, thanks for watching, and we’ll see you next time.