3
$\begingroup$

Could a planet with the size and characteristics of the Earth orbit a gas giant and would the earth-like moon be able to maintain its orbit if the gas giant went rogue and was launched from its host star?

I am interested in setting up a moon that had life on its surface until its host planet left its star system at which point the moon cooled. Once in interstellar space life hung on but around hydrothermal vents fueled by remaining geologic activity. Eventually the gas giant and its moon drift into a new star system and new life forms evolve to use the energy provided by their new host star.

Thanks in advance

$\endgroup$
4
  • $\begingroup$ What mechanism do you propose for the ejection of the gas giant? Or do you actually ask "Is it possible to eject a planet and its moon so that the moon orbit is unaffected"? $\endgroup$ – Adrian Colomitchi May 25 '20 at 6:01
  • 1
    $\begingroup$ It is certainly possible if unlikely for such a situation to arise. The rapid approach of another star on the far side of the planets sun could produce such an effect $\endgroup$ – Slarty May 25 '20 at 7:28
  • $\begingroup$ @Slarty, you should really make an answer from your comment. Don't want to "steal" your idea! $\endgroup$ – ksbes May 25 '20 at 9:56
  • $\begingroup$ @ksbes 3 feel free - no time at the moment $\endgroup$ – Slarty May 25 '20 at 13:57
1
$\begingroup$

Alternative: the moon-to-be was gladly orbiting its star until a rogue jupiter, coming from behind, "gently" pulled it out of the orbit and wandered off.

Yeah, gently is sorta relatively speaking, some tidal sloshing may have occurred in the process, their Venice wasn't quite happy about it.

Ah, yes, it has been rediscovered in 2013

$\endgroup$
1
$\begingroup$

Not unintentionally

Let us take a look at our solar system. A body in Earth orbit (ie in the Goldilocks zone where life is most likely to evolve) needs an additional 11 km/s in order to escape into interstellar space. Note that this is the best case scenario, ie where a fast-moving, sufficiently massive body ploughs into the planet from "behind" to provide an extra kick of velocity. If the force is applied from a different direction then the change in velocity required to break solar orbit is much greater.

11 km/s is pretty slow in terms of traversing the interstellar wastes, but it is really moving in planetary orbit terms. Any moon in orbit around a planet that suddenly has 11 km/s of delta v added is probably going to be left behind in a wobbly version of its original planet's orbit around the star. If the moon is really unlucky then the planet will plough through it instead and destroy it.

Ah, but what if the fast moving body had a fragment break off and was travelling parallel to it (defying the laws of gravity), with the ratio of mass between the fragment and the main body being exactly the same as the ratio of mass between the moon and the planet. Then, if the moon is in exactly the right position "next to" the planet and the main body hits the planet and the fragment hits the moon then both planet and moon change velocity by the same amount and can remain as a little orbital system going off into interstellar space. Yes? Well, sort of, but the moon will not be harbouring any life any more. Assume that somehow the impacting body transfers its momentum to the moon over a period of 10 seconds of uniform acceleration (it won't), then it means that the moon is being accelerated at 110 G for those 10 seconds. Goodbye atmosphere, goodbye hydrosphere, goodbye crust, mantle etc, so long and thanks for all the fish.

The only way that such a massive change in velocity can be imparted to both the gas giant and its moon without sterilising both is with many close-but-not-too-close gravitational encounters with massive bodies on optimal trajectories that will gradually tug both the gas giant and moon towards interstellar space. This really cannot happen by accident - to get both planet and moon moving without a Roche limit catastrophe occurring (or even a lesser disaster that eliminates the atmosphere and hydrosphere) would require intelligent design. If this did happen, then a Europa-type situation might be possible, with liquid water under a frozen crust allowing the possibility of life.

Deliberate intent is also needed in order to get the planetary system moving towards a star that they can reach before the planet cools so much that the hydrothermal vents have gone cold. To quote Douglas Adams again - space is big. Really big. The odds against this particular unlikely planet + moon combination reaching a star at the right stage in its fusion cycle and entering a stable orbit in the Goldilocks zone of that star are literally astronomical. Look at Voyager 1 - after being deliberately launched into the cosmos it will pass within 1.6 light years of one star in 40,000 years and within less than 1 light year of another in 300,000 years. For a randomly wandering planet to not just enter the gravitational influence of another planet but to gain a stable orbit in the inner system without deliberate intent is unbelievably improbable.

Which is not to say that you should not use the idea, plenty of people are willing to overlook scientific plausibility in a well-written story. (Far too many have no consideration for science in real life!) Probably best not to try to go into details of how it happened though, just get on with the story.

$\endgroup$
1
$\begingroup$

In you scenario the planet's recapture looks much more questionable to me. First, the space is really big. So the probability of such a massive planet to be so close to a star so that it would capture it is extremely low. Even if it happens, the most likely orbit would be very elongated, which makes life challenging by going in and out of the habitability zone. And if the new host star had a planetary system, the arrival of a new gas giant to the relatively close orbit would wreak total havoc on it with a heavy bombardment of the planet with the system's small bodies.

$\endgroup$
0
$\begingroup$

First - in the context of a fictional world and narrative, even if the math/physics dictate that such an occurrence (resulting in a stable Gas-planet/Moon system which can hold together through the disengagement from the host star, and then again when being trapped by the gravity of the new star) is extremely unlikely and statistically rare - it could still happen and thus - why not go ahead with it :)

Second - there are 2 scenarios which can influence the world you are building and the narrative around it:

1) the disengagement from the host star is a surprising, maybe catastrophic event which was not predicted before - life went on as usual on the moon, until some cosmic event happened and the gas giant starts drifting away. Hysteria all around the moon-based society, all sorts of solutions are considered etc, maybe even the actual result of being able to maintain a stable system with the gas giant through the drift is a direct successful result of an invention created by the inhabitants / your protagonist / etc so someone "saved" the day

2) the drift was always a known situation, and our moon-based society knows that they were living in the system for a limited window of time until the drift will advance. thjey have prepared for that future, the entire society was shaped (economically, value sets, myths, community structure etc.) as a temporary environment until "the drift" arrives... so it's all a planned journey.

From the physics point of view - sure, you are describing an extremely improbable situation. but it sure opens up interesting areas for a world/story.

Good Luck!

$\endgroup$
1
  • $\begingroup$ If the society has enough knowledge of astronomy to predict being ejected a few centuries before it happens, they should be advanced enough when it happens to have thermonuclear fusion as their power source and thus to build fusion powered arcologies for themselves, or even replace sunlight over the whole moon with artificial sun satellites. $\endgroup$ – M. A. Golding May 25 '20 at 21:18

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.