# Could binary gas giants have ring and moon systems?

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Would any moons/rings around either of the gas giants stay intact or get ripped apart by the tidal forces between the two giants? If any moons/rings are able to form around either of the giants, is there a limit to how large a ring system could be, or how far a moon could orbit?

It would depend how close the gas giants were too each other, but in theory it would be possible. Similar to the binary star system (image below), provided you ignore the AUs and scale this down from stars to planets, the same principal applies to orbits around two gas giants orbiting each other.

Source article.

They would have relatively small hill spheres depending on how close they were and corresponding, even smaller true regions of stability. But planetary ring systems tend to be quite close to the planet anyway and larger ring systems need to be almost exclusively inside the Roche Limit.

The binary gas giant orbits would have to be outside each other's respective Roche Limits anyway and provided there was sufficient distance between then, like maybe 4-6 times the larger of the two Roche limits (ballpark guess), then it's possible to have both gas giant planets with their own ring systems.

What's not possible is to have two orbiting disks around two bodies where the orbiting disks intersect each other, like they had in the Startrek Discovery premier episode. And that was forming star systems, not gas giants, but it's still impossible for any orbiting ring or debris disk to intersect the way they did it. It was a cool looking image, but clearly impossible.

I also want to point out that a ring system around both planets like the top of the first image with the writing "disk around both stars", that's not possible with a typical ring system. Ring systems need to be in tight orbits around their planet, though faint/disburse ring systems like Saturn's E ring can exist further out. The further out a ring system, the more likely it would be disturbed by the binary planet.

## Binary gas giants

Are these possible? And how would the orbits of moons around binary gas giants work? And should I approach binary gas giants like binary star systems? Or more like terrestrial binaries?

To look at the stability of binary systems, look at the moons of Pluto and Charon. They are binary dwarf planets and they have moons surprisingly close in. For habitable moons I would put them considerably further away. And make sure your orbits are in the barycentric reference frame, not the reference frame of the largest object.

The gas giants would likely be tidally locked, and thus would rotate much slower. This would mean that they would probably loose some of their cloud banding. They might be slightly egg shaped due to their low densities and the fact that theyɽ be tugging fairly heavily on one another.

The moons, especially the irregular ones, would be tossed around and have irregular, seemingly random rotation. They might not have a constant day length, which would make calendars with days much more difficult to make work. That might be very interesting to see done right.

The gas giants might also be spaced much further apart instead of close in like Pluto and Charon. In that case they could hold onto their own moons. Irregular-orbit satellites (like the 50+ tiny bodies which orbit Saturn or Jupiter) could orbit the barycenter of the further-spaced gas giants, but only if the barycenter is far away from the parent star and has a larger hill sphere. These further-spaced binaries might have tidally-unlocked gas planets, which will preserve the cloud banding.

You could also do gas moons. This would be a case where a Superjupiter M>5J is parent body to Saturnian-mass or Uranian-mass gas or ice giants. Like all moons, they could have their own hill spheres, but you can place them more similarly to rocky moons around terrestrial planets. The planet Uranus, for example, is about 4 times more massive compared to Jupiter than Moon compared to Earth. (Moon/Earth =

0.04) So if you have a planet with 4 Jupiter masses, an Uranus-mass moon would behave gravitationally very similar to a Luna-mass moon of an Earth-mass planet. You could put in some other icy or terrestrial moons in their like you would minor moons around an Earth-like planet.

Play around in Universe Sandbox 2 to get an idea of how the gravity stuff works.

It's a matter of how "day" is defined.

Wikipedia's article on Jupiter cites this IAU/IAG paper for the length of a Jupiter day. In it, footnote (e) of table I has the following:

The equations for W for Jupiter, Saturn, Uranus and Neptune refer to the rotation of their magnetic fields (System III)

The assumption is that whatever's generating the magnetic field forms a reasonably coherent mass that's rotating at a uniform speed. This produces a periodic variation in the radio emissions of the planet, which is used to measure the rotation speed of that object.

We're reasonably certain the Sun doesn't have a coherent core, so measuring the rotation speed of the magnetic field doesn't provide a useful definition of the Sun's rotation speed.

It's a matter of how "day" is defined.

Wikipedia's article on Jupiter cites this IAU/IAG paper for the length of a Jupiter day. In it, footnote (e) of table I has the following:

The equations for W for Jupiter, Saturn, Uranus and Neptune refer to the rotation of their magnetic fields (System III)

The assumption is that whatever's generating the magnetic field forms a reasonably coherent mass that's rotating at a uniform speed. This produces a periodic variation in the radio emissions of the planet, which is used to measure the rotation speed of that object.

We're reasonably certain the Sun doesn't have a coherent core, so measuring the rotation speed of the magnetic field doesn't provide a useful definition of the Sun's rotation speed.

It's a matter of how "day" is defined.

Wikipedia's article on Jupiter cites this IAU/IAG paper for the length of a Jupiter day. In it, footnote (e) of table I has the following:

The equations for W for Jupiter, Saturn, Uranus and Neptune refer to the rotation of their magnetic fields (System III)

The assumption is that whatever's generating the magnetic field forms a reasonably coherent mass that's rotating at a uniform speed. This produces a periodic variation in the radio emissions of the planet, which is used to measure the rotation speed of that object.

We're reasonably certain the Sun doesn't have a coherent core, so measuring the rotation speed of the magnetic field doesn't provide a useful definition of the Sun's rotation speed.

It's a matter of how "day" is defined.

Wikipedia's article on Jupiter cites this IAU/IAG paper for the length of a Jupiter day. In it, footnote (e) of table I has the following:

The equations for W for Jupiter, Saturn, Uranus and Neptune refer to the rotation of their magnetic fields (System III)

The assumption is that whatever's generating the magnetic field forms a reasonably coherent mass that's rotating at a uniform speed. This produces a periodic variation in the radio emissions of the planet, which is used to measure the rotation speed of that object.

We're reasonably certain the Sun doesn't have a coherent core, so measuring the rotation speed of the magnetic field doesn't provide a useful definition of the Sun's rotation speed.

Probably not.

A direct quote from here would be nice, but I'm not sure if there's any relevant copyright on the text, so I'll just summarize. Here are the steps to forming a contact binary star system:

1. A binary star system forms
2. After billions - perhaps only millions, in larger stars - of years, one of the stars becomes a giant.
3. The giant grows and grows until its outer layers reach its Roche Lobe.
4. One of two things can happen: If the other star, too, is large, the stars may quickly form a contact binary. If the other star is small, matter transfer from the larger star to the smaller star may take place and then a contact binary may form.

Here's why I don't think this could happen in planetary systems:

• Planets don't grow much once they become full-fledged planets. They can shrink and release energy via the Kelvin-Helmholtz mechanism, but they generally can't grow. They can grow when they're in the protoplanet stage, or when they've just reached planethood - whenever that happens - and are still accreting excess material
• Gas giants generally collect all the gas in their immediate area. This means that its more likely that one will form first and take all the gas from another . . . unless matter transfer happens, as detailed above.
• There are a lot of collisions among protoplanets. It's unlikely that two massive planets will form near one another without colliding. They'll each have sustained collisions with other protoplanets and planetesimals I would think these collisions should make them be at a higher risk for collision with each other.
• Gas giants often migrate from the outer reaches of the stellar system, so in their early days, their orbits may be unstable.

Maybe a temporary common envelope could form at some point, though the system would be unstable.

We can calculate the approximate radius of the Roche lobe. Let's assume that the planets are both the same mass, so $M_1=M_2$ and $frac=1$. Given the distance between the planets to be $A$, the radius of the Roche Lobe $r_R$ is $r_R=left(0.38+0.2 log frac ight) A$ $r_R=(0.38+0)A$ $r_R=0.38A$ Let's say that the gas giants are just large enough and close enough that they reach the outer edges of their Roche lobes. We'll also say that they are about the same mass and radius as Jupiter. $r_R=r_J=0.38A$ $A=2.61R_J$ That's problematic, because we can also calculate the Roche limit of the planets - the point at which one will be torn apart by the other. In this case, the more massive of the two has a mass $M_p$, and the less massive has a mass $M_2$. $d=1.26 R_J left(frac ight) ^<3>>$ $d approx. 1.26R_J$ So they're way to close! The more massive one will tear the less massive one apart - in essence, they'll collide.

Regarding what TimB wrote in the bounty - I don't have the expertise necessary to do an analysis like that, so I'll leave that to someone else. I wouldn't be able to do it as well as it deserves.

## What moons in other solar systems reveal about planets like Neptune and Jupiter

Exomoons may reveal secrets about how gas giants like Jupiter formed and what is in their core.

Bradley Hansen is a UCLA professor of physics and astronomy. He wrote this article for The Conversation.

What is the difference between a planet-satellite system as we have with the Earth and Moon, versus a binary planet &mdash two planets orbiting each other in a cosmic do-si-do?

I am an astronomer interested in planets orbiting nearby stars, and gas giants &mdash Jupiter, Saturn, Uranus and Neptune in our solar system &mdash are the largest and easiest planets to detect. The crushing pressure within their gassy atmosphere means they are unlikely to be hospitable to life. But the rocky moons orbiting such planets could have conditions that are more welcoming. Last year, astronomers discovered a planet-sized exomoon orbiting another gas giant planet outside our solar system.

In a new paper, I argue that this exomoon is really what is called a captured planet.

Is the first detected &lsquoexomoon&rsquo really a moon?

True Earth analogues, that orbit Sun-like stars, are very hard to detect, even with the large Keck telescopes. The task is easier if the host star is less massive. But then the planet has to be closer to the star to be warm enough, and the star&rsquos gravitational tides may trap the planet in a state with a permanent hot side and a permanent cold side. This makes such planets less attractive as a potential location that could harbor life. When gas giants orbiting Sun-like stars have rocky moons, these may be more likely places to find life.

In 2018, two astronomers from Columbia University reported the first tentative observation of an exomoon &mdash a satellite orbiting a planet that itself orbits another star. One curious feature was that this exomoon Kepler-1625b-i was much more massive than any moon found in our solar system. It has a mass similar to Neptune and orbits a planet similar in size to Jupiter.

Astronomers expect moons of planets like Jupiter and Saturn to have masses only a few percent of Earth. But this new exomoon was almost a thousand times larger than the corresponding bodies of our solar system &mdash moons like Ganymede and Titan which orbit Jupiter and Saturn, respectively. It is very difficult to explain the formation of such a large satellite using current models of moon formation.

In a new model I developed, I discuss how such a massive exomoon forms through a different process, wherein it is really a captured planet.

All planets, large and small, start by gathering together asteroid-sized bodies to make a rocky core. At this early stage in the evolution of a planetary system, the rocky cores are still surrounded by a gaseous disk left over from the formation of the parent star. If a core can grow fast enough to reach a mass equivalent to 10 Earths, then it will have the gravitational strength to pull gas in from the surrounding space and grow to the massive size of Jupiter and Saturn. However, this gaseous accumulation is short-lived, as the star is draining away most of the gas in the disk, the dust and gas surrounding a newly formed star.

If there are two cores growing in close proximity, then they compete to capture rock and gas. If one core gets slightly larger, it gains an advantage and can capture the bulk of the gas in the neighborhood for itself. This leaves the second body without any further gas to capture. The increased gravitational pull of its neighbor drags the smaller body into the role of a satellite, albeit a very large one. The former planet is left as a super-sized moon, orbiting the planet that beat it out in the race to capture gas.

A remnant core as a look back into history

Viewed in this context, the captured planet is unlikely to be habitable. Growing planetary cores have gaseous envelopes, which make them more like Uranus and Neptune &mdash a mix of rocks, ice and gas that would have become a Jupiter if it had not been so rudely cut off by its larger neighbor.

However, there are other implications that are almost as interesting. Studying the cores of giant planets is very difficult, because they are buried under several hundred Earth masses of hydrogen and helium. Currently, the JUNO mission is attempting to do this for Jupiter. However, studying the properties of this exomoon may enable astronomers to see the naked core of a giant gaseous planet when it is stripped of its gaseous envelope. This can provide a snapshot of what Jupiter may have looked like before it grew to its current enormous size.

This exomoon system Kepler-1625b-i is right at the edge of what is detectable with current technology. There may be many more objects like this that could be uncovered with future improvements in telescope capabilities. As astronomers&rsquo census of exoplanets continues to grow, systems like the exomoon and its host highlight an issue that will become more important as we go forward. This exomoon reveals that the properties of a planet are not solely a consequence of its mass and position, but can depend on its history and the environment in which it formed.

Exomoons may reveal secrets about how gas giants like Jupiter formed and what is in their core.

## Was the Sun once part of a binary star system?

The Sun is single, a solo star traveling around the galaxy.

But… did it once orbit another star? In the distant past, could it have temporarily had a traveling companion, been part of a binary system?

That's not a silly idea. Half the stars in the galaxy belong to binary or multiple systems, so a star is just as likely to be in one as not. This idea of the Sun once being in a binary has been around a long time, but a new paper takes a look at the possibility of the Sun having a companion for a while shortly after it formed as a way to explain a couple of odd things about our solar system, including the presence of Planet Nine, a theorized ninth planet orbiting the Sun far out past Neptune.

Astronomers think Planet Nine (or just P9) exists due to an alignment of the orbits of several smaller bodies also very far out from the Sun. But it's weird it would have to be much more massive than Earth, and it's not easy to form such a planet that far out (it would orbit the Sun on an elliptical path about 75 billion km from the Sun for comparison Neptune is about 4.5 billion km out).

Artwork depicting Planet Nine, a theorized super-Earth orbiting the Sun several tens of billions of kilometers out. Observations of distant icy worlds imply this planet exists. Credit: Roberto Molar Candanosa and Scott Sheppard, courtesy of Carnegie Institution for Science.

It's possible P9 once orbited another star that passed close to the Sun, and the gravity of our star stripped it away, the Sun keeping it for its own. But the physics of that makes it hard to do as well usually in an encounter like that the planet gains so much energy that it gets flung away from both systems.

But that's if the Sun is alone. If instead it were part of a binary system when it was young the capture physics actually gets easier in many cases the two stars work together to minimize the added energy to the planet, allowing it to be captured by one of the stars.

Artwork depicting a second sun, a binary companion to the Sun that may have existed billions of years ago. Credit: M. Weiss

Assuming that's the case with the Sun, there are some things you can say about what the possible companion star would have been like. In general, the star would have to be about three times farther from the Sun than P9 for the planet's orbit to be stable (otherwise the gravitational influence of the second star will destabilize the planet's orbit). That means the Sun's alleged companion would had to have been at least 225 billion km out. Assuming it had the same mass as the Sun (most binary star components have roughly equal mass that happens naturally in the way they form) that makes capturing and keeping P9 twenty times easier than if the Sun were solo.

In fact, the astronomers in the paper note that according to their work a lot of other large bodies would've been captured along with P9, which is a testable prediction. The Vera Rubin Telescope — a monster 8.4-meter mirror telescope equipped with a staggering 3.2 gigapixel camera — will go online in a few years, and is the odds-on bet to find P9 if the planet exists. If it finds other objects in similar orbits, that will give the solar binary hypothesis a big boost.

Schematic of the proposed early solar system: Planet Nine orbits along with many other such objects about 75 billion km out from the Sun, with a second Sun-like star about 225 billion km out, and the Oort cloud of icy bodies over a trillion km away from the Sun (note: 1 AU = 150 million km). Credit: Siraj and Loeb

They also point out that a binary companion also solves some other problems in our solar system. For example, the icy bodies orbiting past Neptune come in different groups. One is called the scattered disk, and is comprised of objects that have highly elliptical and tilted orbits, probably flung out into that region of space by encounters with the gas giants, most notably Neptune. Another is the outer Oort Cloud, a huge spherical volume of space about a trillion (!!) kilometers from the Sun. There are roughly 10 times as many outer Oort Cloud objects as there are in the scattered disk, but according to most hypotheses on the solar system formation that number should be somewhat lower. In the paper, the astronomers find that the binary idea naturally produces the correct ratio. Interesting.

So if the Sun had a binary companion, where is it? It's clearly not there now a star like the Sun 200 billion km away would be as bright as the first-quarter Moon! You'd think we'd have noticed by now.

If it ever existed, it's long gone. Most stars are born in stellar clusters, groups of hundreds or even thousands of stars, so it's not a stretch to think the Sun was born in one 4.6 billion years ago as well. Encounters between stars are very likely in such a crowded volume of space. If even a low-mass red dwarf with one-tenth the Sun's mass passed about 300 billion km away it could disrupt the system, ejecting the Sun's erstwhile companion. It's probable the Sun would've only kept the companion for about a hundred million years before losing it, a short period compared to the Sun's current age.

So for the moment, this is a very interesting idea, but highly theoretical. Hopefully it won't be too long before Planet Nine is found, and then maybe we'll have some observational evidence too. We still don't know a lot about the actual conditions and environment for the young Sun. Maybe soon we will.

## Single-Moon Systems Possible around Gas-Giant Planets

An artist’s impression of several moons forming around a young gas-giant planet. Image credit: Nagoya University.

“Many of the moons we see in the Solar System, especially large moons, formed along with the parent planet,” said Nagoya University’s Dr. Yuri Fujii and Dr. Masahiro Ogihara from the National Astronomical Observatory of Japan.

“In this scenario, moons form from the gas and dust spinning around the still forming planet.”

“But previous simulations resulted in either all large moons falling into the planet and being swallowed-up or in multiple large moons remaining.”

“The situation we observe around Saturn, with many small moons but only one large moon, does not fit in either of these models.”

In the new study, the researchers created a novel model of circumplanetary disks with a more realistic temperature distribution.

They simulated the orbital migration of moons, taking in to consideration a pressure from disk gas and the gravity of other satellites.

They found that dust in the circumplanetary disk can create a ‘safety zone’ where a moon is pushed away from the giant planet.

In this zone, warmer gas inside the orbit pushes the satellite outward and prevents it from falling into the planet.

“We demonstrated for the first time that a system with only one large moon around a giant planet can form,” Dr. Fujii said.

“This is an important milestone to understand the origin of Titan.”

“It would be difficult to examine whether Titan actually experienced this process,” Dr. Ogihara added.

“Our scenario could be verified through research of satellites around extrasolar planets.”

“If many single-exomoon systems are found, the formation mechanisms of such systems will become a red-hot issue.”

The findings were published in the journal Astronomy & Astrophysics.

Yuri I. Fujii & Masahiro Ogihara. 2020. Formation of single-moon systems around gas giants. A&A 635, L4 doi: 10.1051/0004-6361/201937192