Binary Planets through Aerobreaking-Capture

In the case of habitable moon, I think the chances [of life] are higher on moons that were captured by the planet, not that formed with it. The type of moon I'm talking about would be much richer in metals from the core to the surface than the icy moons of Jupiter. It would be in the habitable zone.

It would be a silicate-metallic moon with volatiles on the surface, like Earth. I think a 1-mass Jupiter planet could capture at least 1 moon with considerable high mass (like Mars), if the moon pass next to the planet during the formation of the system. Jupiter may have enough gravity to capture such higher mass world. Jupiter did not maybe because there weren't any high mass worlds near it to capture. If Io and Europa were planets, they were captured and because they were just already near the orbit of Jupiter.

In the case of Europa, it couldn't have life on the surface. I was talking about life under the thick (or thin?) ice. Maybe also on Enceladus or even Titan interior, who knows?

The problem with capturing bodies is that a mechanism is needed to capture a body, not just a close flyby. Every spacecraft sent to other planets need to do something to get captured by the planet's gravity, they have to lose speed, or else they will fly right past it. Spacecraft have built in engines to do this but planets don't. So if an Earth-like planet flew past a Jovian planet, unless it has a mechanism to lose some speed, then the planet will just continue on.

Triton is suspected of being part of a binary Kuiper-Belt Object. It was allowed to be captured by Neptune because with Triton's motion around the barycenter it had with whatever other object allowed its velocity to change as the pair passed by Neptune. The other companion would have continued on alone. If I'm not mistaken, the outer moons of Jupiter (the cloud of rocks) are suspected to be that one part of a binary system that was pulled away in the flyby that separated the two.

Now the question is, how do you set up a habitable planet with a mechanism to slow it down enough to get captured by the gas giants gravity? Sure, you can invoke a moon for the planet, but unless the moon's mass is comparable to the planet's, the planet's velocity relative to the gas giant's, won't be cycling around much. It isn't known how common binary planets are.

One may suggest that, during the flyby, the future habitable planet may whack an already-present moon, losing some velocity to the impact. The problem is that unless the moon's mass is comparable to the planet's mass, the effect will be small.

It's going to be difficult to arrange, not impossible, but very difficult.

Anyone know if the circumplanetary disk can undergo gravitational collapse and form moons of higher mass than usual?

Well , it's teorized that most systems have their planets start a downward spiral after they have formed , so i supose it wouldn't be that far-fetched to accept that a super-jovian got slowed down by the close aproximation of a neptune-sized world also in his way to lower orbits. Tidal ballet stars a planetary tango where the netune sized world gets stripped of his mantle of H/He , leaving a heavy , super-earth sized core , orbiting the now also belitted jovian , both locked in a new , estable orbit around their star.

The super-earth moon around this gas giant would be tidally heated , and may be convinced to release compounds locked in its crust. With that and what remaindered of his original atmosphere , we could end up with a system ready to go the path of a garden world.

Sorry if i got a little carried away, but the thread merites it

The thing is, even with the migrating Jovian approaching the Neptune, the Neptune can't get captured into orbit without some mechanism, else it will just be scattered.

Run Gravity simulator and try getting planets to do anything more than just scatter each other.

well , i was counting in the stripped atmosphere of the neptunian to act as ballast... you know , dumping kinetic energy as well as angular momentum , anchoring the neptunian to the jovian.

Analog dumping of the uppermost layer of atmosphere from the jovian would slow down it too. Thus , with the anchoring provided by the ex-neptunian , the downward migration could be stopped.

Don't see how a gravity simulator would acoount for this kind of dynamics

Oh, wow I have absolutely no idea if that would work. I can imagine what you're saying though.

I would expect that to leave the ex-Neptune in a very close planetary orbit, probably subject to tidal decay.

To me, at least most of the high mass moons, when still being planet, had no life, but after orbiting the giant, the tidal stress would allow a stable environment and magnetic field, so life appear. What I'm saying is, to me, that most planets that become moons only develop life when are orbiting the gas giant, not before.

Maybe I could elaborate further into the scenario.

Picture this close encounter like an aerobraking maneuver , where the neptudian barrels thru the superjovian atmosphere , shedding speed and his own atmosphere in the process , losing speed and changing direction , thus entering a high ecc orbit.

In each orbit , the remmans of its own atmosphere and the ejected envelope from the tunnel it has bore in the superjovian (SJ from now on , and N for the neptunian) form a kind of soft barrier slowly dispersing in the interplanetary medium for the most part , a sizeable amount falling back into the SJ , and a tiny fraction (but massive enougth) acretes back in the N. This barrier dampens the eccentricity of the N body faster than tidal actions , warming and refacing the surface of the N. Like the primordial earth comet-bombing , but at an accelerated rate.

A few thousands years after , we could see a rapid spinning SJ (the momentun lost for the N can go as a gain of SJ spin , or get dispersed in the gas cloud , i don't care) , with a masive core of a N-class planet doing the Super-Earth Sized Moon , with an atmosphere mix of primordial N , primordial SJ , and compounds released from the planetary crust after warming and bombard.

As diakonov points out , theres nil chance of any prior kind of life to survive this , but i bet there is some kind of chemical reaction that could find a way to self-replicate in a high-variety energy source and hydrogen rich enviroment like this one. Tidal forces , High electron currents, the ocasional dust bombardment , and wathever energy the star sheds on the system. Inmense reservoirs of water are highly likely on the resulting moon, and his great gravity would ensure no atmosphere would be stripped away by the solar wind.

All in all , if this setup is located at a jupiter equivalent of the solar system , has a great chance to survive even to the first steps of the star going out of the main secuence, as the energy output of the star is mainly irrelevant.

Yeah, I get what you're saying. It's essentially aerobraking-capture for planets.

Here's an advanced hydrodynamics simulation (doodling in paint) to demonstrate.


The perijove of the planet would be the altitude at which the impact occurred (simply speaking). This is well within the Jovian planet's Roche limit. The ex-Neptune will probably be tidally disrupted. If not, then because it is orbiting inside the synchronous orbit limit, tides will lower its already very low orbit, and the gas giant will absorb the ex-Neptune.

I doubt the Jovian planet will be sped up much, only its outer atmosphere was hit. Any additional angular momentum delivered to the Jovian planet will be far too little to lower the synchronous-orbit radius to the altitude of the planet.

In all, this set-up is unstable.


Edit: I'm going to split the posts that are irrelevant to the original post out to a different thread.

Sirius_Alpha wrote:

The problem with capturing bodies is that a mechanism is needed to capture a body, not just a close flyby. Every spacecraft sent to other planets need to do something to get captured by the planet's gravity, they have to lose speed, or else they will fly right past it. Spacecraft have built in engines to do this but planets don't. So if an Earth-like planet flew past a Jovian planet, unless it has a mechanism to lose some speed, then the planet will just continue on.

I don't understand why you assume that a potential moon must be going too fast. I suspect that a planet is very unlikely to be traveling at just the right speed to drop into a stable orbit, but surely any relative speed and any trajectory is possible for a large body of unspecified origin entering a solar system?

jbjerk wrote:

I don't understand why you assume that a potential moon must be going too fast. I suspect that a planet is very unlikely to be traveling at just the right speed to drop into a stable orbit, but surely any relative speed and any trajectory is possible for a large body of unspecified origin entering a solar system?

Conservation of energy and momentum. If the orbit is initially unbound then the orbit will remain unbound unless you dissipate the energy somewhere.

If the smaller planet got that close to the larger planet, wouldn't they just collide? I would think that the tidal forces at that distance would be so large that the smaller planet would break up first before colliding with the larger.

Probably. It should start breaking apart after entering the Roche limit, a bit before closest approach, so the atmosphere would be lost before, followed by the tidal disruption of the core shortly afterward. Maybe?

Since the Roche limit for the Neptune's atmosphere would be at a higher altitude than the Roche limit of the Neptune's core, a significant fraction of the atmosphere could probably be lost before the core gets disrupted. Is it possible to lose enough momentum through shedding the atmosphere to capture the Neptune into orbit if the closest approach is above the Roche limit, but the atmosphere isn't? Sort of an extreme tidal circularisation of an orbit from hyperbolic (e > 1) to bound (e < 1)?

Careful here: bear in mind some of the assumptions that go into computing "the" Roche limit. For a start most estimates assume synchronous rotation of a satellite in circular orbit. For an object passing through on its first encounter, the orbit will be hyperbolic and the rotation will not be synchronous.

The planet that will be a moon don't need to hit the planet and lose its atmosphere and even have a very eccentric orbit. That don't seem to have happened to Triton. To me, its likely that all ex-planets will have considerable eccentricity, but not necessarily high eccentricity. Triton, that was captured by Neptune, is not so eccentrical in its orbit. It may be retrograde, but not much eccentrical as Nereid.

To me, a huge enough gas giant or brown dwarf could capture more planets, if they pass near the orbit of a super high-mass world.


Hypothetical Solar System:

Returning in the case of Venus. Let's see if our solar system had a super jovian in the habitable zone, lets call it Zeus. And if all the inner planets were moons of this massive world, let's see, that would have 7 or 13 Jupiter masses. Mercury would be the first moon, Venus would be the second, Earth the third and Mars the fourth. All these ex-planets in the supposed system would have some eccentricity in their orbits, but not much, because there's a balance between the orbits of such moons, like the balance of orbits of Jupiter moons.

In the case of Mercury, it would be a bigger Io with no craters and a tenue atmosphere due to intense and constant volcanism. If life's present, only microrganisms.

In that case of Venus, due to tidal stress, would have active core and volcanism to keep a balanced atmosphere. The rotation would be tidally locked to the planet, but much faster than the actual Venus. Let's say, a period of 3 days. It would be like a second Earth. And with an atmosphere a bit thicker than Earth, it would be no problem to have that long day. It would have magnetic field to protect the surface from the radiation of star and radioactive particles from the planet, and the thick atmosphere being an extra shield.

In the case of Earth, it would be not much different. The air would be thicker and day period longer, lets say about 7 days. Of course temperature variation would be higher from day and night, but with a thicker atmosphere, it wouldn't be a problem. Maybe the sleeping cycle would not depend on light, but on another factor, such as just being tired enough to sleep. Plants would have more time to absorve and liberate oxygen. So photossynthesis would not be much different. Ecology would be different, but still would have very complex organisms.

In the case of Mars, due to the higher distance, being the last moon, its day period would be... about 12 days or 16 days long. Mars would be active, because, even being the last moon, it still orbit a super jovian. It would have not much volcanism and life would be poor, mostly near many shallow seas. And most part of the surface would be cold and desert, but not so cold as the actual Mars. Just 15°C to -15°C. Also due to tidal stress would have some magnetic field, avoiding the fast loss of atmosphere. And volcanism would replenish CO2 with time. Much CO2 in the air would be needed to keep a thin aired world heated. But it would have also enough nitrogen and oxygen. The only problem would be the long days and nights, but in that case life may use a substance such as ammonia or alcohol to avoid freezing during nights. And there would have some air circulation between day and night side, avoiding the collapse of the atmosphere.

This is just an idea I had for a possible different Venus and Mars if they orbited a gas giant.

A gracefull capture would be really difficult to achieve , granted , it was just a possibility that crossed my mind. The approximation would be gradual , due to the mutual perturbation of their orbits , as they are orbiting the same star.

I don't really know why the downward migrattion occurs (tidal forces , drag from the remains of the primordial dusk disk ,whatever) , but it seems to be a fairly common occurence.

This said , it's unlikely life could exist without some ecually common process to stop it in their tracks. In our sun system , resonance appears to be the way to go. How that resonance was achieved is anybody guess. The aerobraking mechanism is a way out , but as i'm just a layman in the presence of brigth professionals , I defer to more knowleadgeable forum members (Thank you Sirius_Alpha for your kind effords with paint. I'm just unable to use paint )

Now , as for the disruption of the Neptunian core , it really doesn't matter. It may even be beneficial for the case of an habitable moon to have the big core disgreged in more manageable chunks , that could result in a re-accretion of an Earth analogue. Some matter would fall into the Super Jovian (bear in mind the momentum conservation here) giving some tiny push and an increase in momentum , maybe big enougth to stop dead in its track the already dampened downward migration.

Nothing i did said is backed from any studies i had followed or any degree , just things i'd read here and there and my own musings at nigth staring at the stars. No one should use my words as an authorized opinion

Diakonov wrote:

The planet that will be a moon don't need to hit the planet and lose its atmosphere and even have a very eccentric orbit. That don't seem to have happened to Triton. To me, its likely that all ex-planets will have considerable eccentricity, but not necessarily high eccentricity. Triton, that was captured by Neptune, is not so eccentrical in its orbit. It may be retrograde, but not much eccentrical as Nereid.

Triton is suspected of being part of a binary KBO system, where it can lose velocity (relative to Neptune) simply through its orbit with its companion. Triton got captured, the companion is out there somewhere. With Triton only being 1/5 of a Lunar mass, it's not hard to imagine it in a binary KBO pair.

Diakonov wrote:

To me, a huge enough gas giant or brown dwarf could capture more planets, if they pass near the orbit of a super high-mass world.

The capture of anything will require a mechanism to bleed off the planet's momentum, regardless of how massive the planet is. If Saturn were 10 M_j, Cassini would have still needed to fire its retro-rockets to get captured into orbit.

ciceron wrote:

The approximation would be gradual , due to the mutual perturbation of their orbits , as they are orbiting the same star.

While you can get the orbits of the two planets to gradually approach each other, once the planets themselves do, it becomes very un-gradual, very quickly, as the two planets begin to fall toward each other. The Neptune-mass planet will not simply nudge itself alongside the super-Jovian, since gravitational acceleration will kick in and increase all the way to closest approach.

ciceron wrote:

I don't really know why the downward migrattion occurs (tidal forces , drag from the remains of the primordial dusk disk ,whatever) , but it seems to be a fairly common occurence.

Imagine tides on Earth, they are formed by the gravitational pull of the moon. This creates two bulges on Earth, one of which face the moon. But since Earth rotates around its axis in less time than the moon completes an orbit, so the tidal bulge is carried ahead of the point on Earth's surface directly under the moon. Now this tidal bulge, having mass, will also have gravity. Since this gravitational source is ahead of the moon in its orbit, the moon will accelerate, and its orbital altitude will increase. Downward migration would happen if the tidal bulge is behind the point on Earth's surface directly below the moon. Mars + Phobos being an example. Since Phobos orbits Mars faster than Mars rotates on its axis, the tidal bulge raised by Phobos (which is really small, but non-negligible) is dragging behind the sub-Phobos point on Mars. This decelerates the moon's velocity, and the semi-major axis declines.

ciceron wrote:

resonance appears to be the way to go. How that resonance was achieved is anybody guess.

As planets migrate, they go through these resonances. Imagine two planets, the inner one having an orbital period of 10 days, the outer having an orbital period of 21 days, with the outer migrating inward. After a short while (astronomically speaking), the outer planet will migrate inward enough that its orbital period is 20 days. Assuming the migration is gradual enough, this 1:2 resonance will be locked in place, and the orbits of the two planets will evolve together, keeping that 1:2 resonance.

ciceron wrote:

Now , as for the disruption of the Neptunian core , it really doesn't matter. It may even be beneficial for the case of an habitable moon to have the big core disgreged in more manageable chunks , that could result in a re-accretion of an Earth analogue.

If the core was tidally disrupted into lots of little chunks, those same tidal forces will keep those chunks from join together again as long as they are still within the tidal disruption limit (as Lazarus pointed out, the "Roche limit" is a bit too simplistic for these cases). Recall that Saturn's rings have yet to join together into another moon.

ciceron wrote:

Some matter would fall into the Super Jovian (bear in mind the momentum conservation here) giving some tiny push and an increase in momentum , maybe big enougth to stop dead in its track the already dampened downward migration.

I'm not really sure I understand what you're saying. The matter that falls to the Jovian is simply the matter that, after the collision, failed to keep enough momentum to keep orbit, so it fell into the planet. The conservation of momentum is satisfied by the matter falling inward. The matter that didn't do so can remain in orbit of the planet, but will not migrate outward.