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We lived through loads of questions regarding aquatic races, so buckle up, I am going to gather some ideas around insectoids.

The homeworld and race

  • This race lives on a planet which has lower gravity than Earth (0.8 g)
  • The planet is hostile in environment, so an exoskeleton is the best evolutionary advantage
  • Oxygen level is on oxygen level of Paleozoic Era
  • So the race living here either evolved from insect, or at least looks like insect
  • The race definitely lives in a hive and is hive-minded
  • And I want them to "shoot for the Moon"

The question

When I set the definition of their home world and their mindset, I have to explain "why would they want to shoot for the Moon?"

If their home world was a stand alone planet with a moon, I could be hoping for a Phobos-sized moon, which makes no sense whatsoever to go there.

So, the different idea is: The moon is a different moon of the same planet, and we are already on a moon.

But is it plausible?

Is it plausible to have a gas giant in the habitable zone of a Sun-sized star? Would there be any drawbacks for life to start on one of the moons of such a planet?

Edit: To clarify: I am thinking of a setup, where the main planet is a gas giant the size of Jupiter or bigger. And the homeworld planet is orbiting around it as one of its moons.

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    $\begingroup$ Wikipedia has an entire category for actual gas giants in habitable zones of their stars: en.wikipedia.org/wiki/Category:Gas_giants_in_the_habitable_zone $\endgroup$ – mic_e Nov 28 '14 at 16:25
  • $\begingroup$ Size of Jupiter? 0.8g? What's this made of, exactly? Because Jupiter is 88%+ Hydrogen, and has a surface gravity of over 2.5g... $\endgroup$ – corsiKa Nov 28 '14 at 17:31
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    $\begingroup$ I am thinkig of setup, where main planet is gas giant of size of Jupiter or bigger. And the homeworld planet is orbiting around it as one of its moons. $\endgroup$ – Pavel Janicek Nov 28 '14 at 17:38
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We have already found exo-planets matching this criteria. For example HD_100777_b has a mass just slightly higher than Jupiter and orbits its star at the same distance from the sun that our earth does. (The star is a similar size to our sun but I didn't check the brightness so I don't know for sure if it's in the habitable zone).

You can explore the known exo-planets using: http://exoplanets.org/plots

A plot of mass vs distance for known exo-planets

This makes it clear that there are a lot of planets matching the size and distance you need.

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    $\begingroup$ +1 for a definitive answer. I looked for empirical evidence but couldn't find one. Nice. $\endgroup$ – HDE 226868 Nov 28 '14 at 17:08
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    $\begingroup$ Wait, are the y-axis points factors of Jupiter's mass? Or am I misunderstanding the graph? I don't want to even imagine something 22 times the mass of Jupiter! $\endgroup$ – Crabgor Nov 28 '14 at 18:36
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    $\begingroup$ @cragor Multiple if I am reading it right, so yes by that point they are in Brown dwarf territory. $\endgroup$ – Tim B Nov 28 '14 at 19:32
  • $\begingroup$ While I agree that your answer, "yes," is correct, isn't it true that 1AU doesn't necessarily mean habitable zone (depends on the star, how far the habitable zone is located)? $\endgroup$ – Mikey Apr 27 '15 at 18:43
  • $\begingroup$ @Mikey Yes, that is correct. In fact I commented on that in the answer by mentioning the size of the star as being similar to our own. $\endgroup$ – Tim B Apr 27 '15 at 23:09
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Absolutely.

This would only take a few simply steps, and a small bit of luck. Here's how it could happen:

  1. A protostar forms from a collapsing gas cloud. A giant sphere of gas and dust collapses upon itself. The pressure is so great that the sphere begins nuclear fusion, and it beings to emit light.
  2. An accretion disk forms. The protostar begins to collect matter around it. Heavy elements formed form supernovae, along with gas, dust, and surrounding hydrogen and helium begin to coalesce into a disk around the star.
  3. Bodies begin to form in the disk. The disk is really a protoplanetary disk by now. Small dust grains begin to grow larger through collisions. They eventually become planetesimals, which group together into large spheres. By now, the star has entered the main sequence.
  4. A gas giant forms. One of the larger spheres gathers an envelope of gas around it. It accretes material in a similar way to the star, although it isn't nearly as massive as the star. It is now a gas planet. It may collect moons, or form a ring system. Other planets might form around it.
  5. The gas giant migrates. Chances are, the gas giant won't form in the habitable zone. However, through interactions with other bodies (such as other gas giants), it may change its orbit, moving further out or closer in to the star. The Nice model says that this happened in our Solar System, moving Jupiter and Saturn closer in while Uranus and Neptune moved further out.

    Migration is also possible via tidal interactions between the disk and the planet. This may account for why Hot Jupiters are extremely close to their parent stars.

And you're good!


Drawbacks:

  • Tidal forces on the moon from the planet it's orbiting could be a problem. This is the case on Io, a moon of Jupiter. Too much stress could have some dramatic effects.
  • Possible orbital instabilities could result from the planet's formation. Chances are, the moon didn't form around the gas giant if the moon is so massive. This means that I'd bet that the moon was captured by the gas giant's gravity. This happens occasionally on a small scale, but it's plausible here. What does this have to do with the orbit? Well, I'd assume that the planet didn't scoot in to a very circular orbit. It would most likely have a high eccentricity initially, although it could become more circular as time goes on, and it might be fine by the time life pops up.

  • During the time that the orbit is very eccentric, there's a small - very small - chance that the moon will go near Jupiter's Roche limit - the sphere inside which a small body may be torn to bits. This would be the death knoll for life on the planet forming in the future.


Could life form here?

This is tricky. Given the choice, I'd pick a terrestrial planet orbiting by itself over a moon like this to harbor life. In the scenario I just described, there are a lot of factors that could make things tough for life, the eccentricity of the orbit being one of them. I'd say that life here could develop, but probably in a more shielded environment. Underground would be my choice. Extremophiles could be the first to spring up, and over time, perhaps they could evolve into the insectoids you want.

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    $\begingroup$ High tidal forces only occur if the moon is very close to its planet. The sun is blocked half the day only if the moon is very, very close to the planet. If the moon is too cold, simply move the planet closer to the star, or move the moon further away from the planet. $\endgroup$ – mic_e Nov 28 '14 at 16:12
  • $\begingroup$ @mic_e All of those are good points (and true, to the best of my knowledge), but I'm assuming that the moon has a highly eccentric orbit, which probably takes it close to the planet at times. I think this takes care of the two points, although I could be completely wrong. $\endgroup$ – HDE 226868 Nov 28 '14 at 16:18
  • $\begingroup$ Depending on the moon's orbital period, the high eccentricity will cause daylight, temperature and tidal forces (-> earthquakes) to be very irregular, which may or may not be beneficial to evolution, and is probably not beneficial to survival of an existing race. $\endgroup$ – mic_e Nov 28 '14 at 16:21
  • $\begingroup$ I'm also pretty sure that highly eccentric orbits will be circularized by tidal forces, over time. $\endgroup$ – mic_e Nov 28 '14 at 16:22
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    $\begingroup$ Gas giants collect or form an awful lot of material around them - 67 confirmed moons of Jupiter, Saturn's rings, etc. If we had a superjovian primary with even more material around it, I'd think an early captured eccentric would collide with a lot of this material, which has two nice effects: one, accreting additional mass, which could help bulk it up to the 0.8g surface gravity we're looking for, and two, tending to circularize its orbit. $\endgroup$ – Russell Borogove Nov 28 '14 at 17:37
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How attached are you to the 0.8g figure for homeworld surface gravity?

Smaller moons are generally more plausible; Ganymede (Jupiter's biggest) is about 0.15g. Depending on your assumptions for density, your homeworld would be about 30 Ganymede masses, which is something like 10 times the mass of all Jupiter's moons together.

A larger primary could possibly support that, though. Here's a paper describing simulations of gas giants up to 12 Jupiter masses forming moons larger than Mars.

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    $\begingroup$ As long as the homeworld moon orbiting around the gas giant has atmosphere and atmosphere of oxygen level higher than on Earth, then the gravity level if fair game $\endgroup$ – Pavel Janicek Nov 28 '14 at 18:12
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A very recient SETI weekly seminar (posted on youtube) was about planet formation and migration. Link

It is well worth watching to see how gas giants form and move around the system and why ours is the strange one.

From those details, figure how a gas giant moves in and then out, stopping at the right distance; meanwhile it captures a sizable terrestrial planet rather than scattering it or rolling right over it.

Basically, add a second smaller giant (like ours), adjust the composition of the protosolar disk and fiddle with the timing by clearing the remaining material at the right time, and add in a dash of luck. Perhaps one of the original close-in rocky worlds is tossed into a long commet-like orbit and it comes back near the same place in a few million years as orbits do, and by then the construction has wound down and it gets captued in a retrograde orbit by the giant occupying its former slot.

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