Is a Jupiter-sized planet plausible in a habitable zone?

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.

posted about 10 years ago

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