Are the moons of Jupiter old planets?

The large moons of Jupiter formed together with this planet as moons. In our solar system, aside from some smaller moons of the gas and ice giant worlds, Neptune has one larger moon, Triton (shown above), which did not form in this way but is a captured trans-Neptunian dwarf planet.

The story of how this happened is also extraordinary, and it might involve a huge ice giant planet that our solar system lost. It resided between Saturn and Uranus and was similar in size to Neptune and Uranus. It existed in our system for hundreds of millions of years, during which orbital resonances built up, causing the orbits of the large planets to become unstable.

This large world was ejected, causing Uranus and Neptune to move outward. Triton was at the time a transneptunian dwarf planet similar to Pluto, and when Neptune approached, it was captured into its orbit as a new moon. It now orbits it in the opposite direction from other moons, and it’s in a slow death spiral with its fate sealed. It will fall into Neptune in about 3.5 billion years.

The expulsion of this planet additionally explains the late heavy bombardment with asteroids between 4.1 and 3.8 billion years ago, which Earth and other planets and moons experienced.

Meanwhile, the lost planet became a rogue world floating in interstellar space without a star, and we will never find it. Not only did we lose it more than 4 billion years ago, but our system also moved about 16,000 light-years from where it might have formed. The Sun’s elemental makeup indicates that it might have been born in a Wolf-Rayet nebula or a nebula enriched by radiation from the Wolf-Rayet stars.

They are rare in the flat spiral disk of the Milky Way, where we are located 26,000 light-years away from the center, but common in the central bulge, which has a radius of about 10,000 light-years, and our solar system might have been born nearer to it.

Most stars form with siblings in stellar nurseries, and the Sun, being more massive than average stars, is more likely to have been born with a companion as well, but its binary system broke apart, and the sister star of our Sun is also forever lost, like the planet that was expelled. It’s probably drifting somewhere on the other side of the Milky Way galaxy, more than 100,000 light-years across, and we are unlikely to ever find it and the lost planet.

Clone moons of Saturn..

 

What you're seeing are three different moons of Saturn 🪐, named Tethys, Dione and Rhea.

It's very hard to distinguish between them. But that isn't the only reason we can consider them to be clones.

Tethys and Dione are very similar in size. A difference of just 30 km in their radii. Rhea is quite larger, though.

They're all mostly made of water ice 🧊, and heavily cratered, which means they’ve not changed since billions of years— old scars of impacts are still there.

Here are enhanced colour images of their surfaces:

It's obvious that all of them have one side much darker than the other.

There's an interesting reason behind it.

The three moons orbit in Saturn’s E ring, a faint ring of ice, created by another tiny moon Enceladus, whose geysers blast off water into space.

Being tidally locked to Saturn (keeping the same face towards their planet), their one side continuously faces the ring while other is shielded.

This creates a thin layer of ice on half of the moon, making it brighter than the other.

Here's a stunning image taken by Cassini spacecraft. Rhea passes behind Dione. They blend seamlessly into a snowman. ⛄

Are there any "gas giant" moons?

 

We have not yet discovered any exomoons. Many inevitably exist, as all planets in our system except Mercury and Venus have moons. Our instruments for detecting them elsewhere in the universe are too crude yet. Gas giant exomoons will likely be found in the future and will not be much smaller than the exoplanet they orbit.

As the above image shows, gas giant planets don't get much more significant in diameter with the increase in mass due to compression from high gravity. Brown dwarf stars start from 13 times the mass of Jupiter, while lowest-mass red dwarf stars are at least 80 times as massive as Jupiter, yet they aren't that much bigger in diameter. We found some outliers in this regard. Some gas giant exoplanets orbit their stars so close that they get puffed up from being fried by high temperatures.

A very massive gas-giant exoplanet that is nearly 13 times the mass of Jupiter can potentially have moons that are Neptune, Saturn, or even Jupiter-like, and they would be gas-giant moons.

In the past, we had doubts about whether such moons could form, but in October 2023, the James Webb Space Telescope discovered many Jupiter-like double rogue exoplanets floating in space without stars in the Orion Nebula. The current leading theory is that they formed in typical star systems and were ejected. Some planets can end up jettisoned from systems during planetary formation, collide, or fall into the star and be destroyed before the orbits settle on stable configurations.

If such double Jupiter-like planets indeed formed in star systems and not directly in space like suns form, then we will find double gas giant planets and massive gas giant planets with gas giant Jupiter- or Saturn-size exomoons in the future.

In our system, Titan, the moon of Saturn, is larger than the planet Mercury but less massive and has a thicker atmosphere than Earth. It is the closest we get to a gas giant moon in our backyard.