Double Planets and Rocheworlds


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.

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