That's an interesting question! It's not possible in our solar system, or at least, not in the near future surely, it's hard to imagine even mega technology that could move Saturn inwards, get it to change place with Jupiter and set Earth to orbit it, or the other way around.
But in other solar systems - many of the newly discovered giant planets around other stars orbit really close, often so close they have orbits of just a few days.
So in principle, surely you could get a giant gas planet like Saturn that is in the habitable zone.
This is an artist's impression of an exoplanet with rings.
If it is a gas giant like Saturn, made of hydrogen and helium mainly - then it couldn't form in the habitable zone, because it is too hot here, too close to the sun for a hydrogen and helium cloud to collapse to a planet. The hydrogen and helium could never condense to form the planet.
But it could form in the outer solar system and then migrate inwards in the early solar system. And it could retain its hydrogen and helium when it does that, because it is now in a gravity well.
The "hot Jupiters" migrate all the way in, to orbits of just a few days well within the orbit of Mercury, so hot they have clouds of gaseous rock :).
But other ones stop on the way in. In our solar system we are lucky that Jupiter stopped, or there wouldn't be an Earth.
So anyway if the gas giant came in a bit further than Jupiter, but not as far as the hot Jupiters, it could stop in the habitable zone.
There are quite a few gas giants now known in the habitable zone, this is one of them.
HD 28185 b - artist's impression.
So then - it could have large moons, perhaps as large as the Earth. And these could have oceans,
Our best analogue of a moon like that I suppose is Saturn's Titan (moon).
It is tidally locked, but it orbits Saturn with a period of just over 15 Earth days so that means its "day" also is 15 times the length of the Earth day. It's tiny compared with Earth, 0.0225 Earths. It's able to hold onto a thick atmosphere only because it is so cold out at the orbit of Saturn. It has seas of liquid methane, and below the surface may have a liquid subsurface ocean.
It's atmosphere is completely opaque in visible wavelengths but in other wavelengths you can see through to the surface This shows it in ultraviolet and infra red wavelengths.
File:Titan multi spectral overlay.jpg
Anyway it's so small, a little smaller than our Moon, that it would surely lose all its atmosphere, and water too if brought inwards. So, if Saturn had migrated inwards in the inner solar system, Titan would become a planet without any atmosphere surely. And hard to see how it could hold onto its water and ice,
But there's no reason why a gas giant like Saturn couldn't have a moon as big as the Earth, in principle. And if it did - well it could be easily habitable by Earth like life, if Saturn moves inwards to the habitable zone. And a close orbiting moon with an orbit of several days - it could be well habitable. Maybe if we ever encounter ETIs in another solar system, by radio or laser messages or some such, maybe some of them might indeed inhabit moons circling gas giants :).
Another way we could get a ringed planet in the sky is if we had a gas giant like Saturn but with much larger rings. So - not actually orbiting it, it would be in an independent orbit around the sun.
And astronomers have found a gas giant just like that. J1407b.
This is what it would look like in the sky over Berlin if it was in our solar system:
It would be much larger than our full moon and you'd see the rings easily by day with the naked eye.
More about it:
But in other solar systems - many of the newly discovered giant planets around other stars orbit really close, often so close they have orbits of just a few days.
So in principle, surely you could get a giant gas planet like Saturn that is in the habitable zone.
This is an artist's impression of an exoplanet with rings.
If it is a gas giant like Saturn, made of hydrogen and helium mainly - then it couldn't form in the habitable zone, because it is too hot here, too close to the sun for a hydrogen and helium cloud to collapse to a planet. The hydrogen and helium could never condense to form the planet.
But it could form in the outer solar system and then migrate inwards in the early solar system. And it could retain its hydrogen and helium when it does that, because it is now in a gravity well.
The "hot Jupiters" migrate all the way in, to orbits of just a few days well within the orbit of Mercury, so hot they have clouds of gaseous rock :).
But other ones stop on the way in. In our solar system we are lucky that Jupiter stopped, or there wouldn't be an Earth.
So anyway if the gas giant came in a bit further than Jupiter, but not as far as the hot Jupiters, it could stop in the habitable zone.
There are quite a few gas giants now known in the habitable zone, this is one of them.
So then - it could have large moons, perhaps as large as the Earth. And these could have oceans,
Our best analogue of a moon like that I suppose is Saturn's Titan (moon).
It is tidally locked, but it orbits Saturn with a period of just over 15 Earth days so that means its "day" also is 15 times the length of the Earth day. It's tiny compared with Earth, 0.0225 Earths. It's able to hold onto a thick atmosphere only because it is so cold out at the orbit of Saturn. It has seas of liquid methane, and below the surface may have a liquid subsurface ocean.
It's atmosphere is completely opaque in visible wavelengths but in other wavelengths you can see through to the surface This shows it in ultraviolet and infra red wavelengths.
Anyway it's so small, a little smaller than our Moon, that it would surely lose all its atmosphere, and water too if brought inwards. So, if Saturn had migrated inwards in the inner solar system, Titan would become a planet without any atmosphere surely. And hard to see how it could hold onto its water and ice,
But there's no reason why a gas giant like Saturn couldn't have a moon as big as the Earth, in principle. And if it did - well it could be easily habitable by Earth like life, if Saturn moves inwards to the habitable zone. And a close orbiting moon with an orbit of several days - it could be well habitable. Maybe if we ever encounter ETIs in another solar system, by radio or laser messages or some such, maybe some of them might indeed inhabit moons circling gas giants :).
Another way we could get a ringed planet in the sky is if we had a gas giant like Saturn but with much larger rings. So - not actually orbiting it, it would be in an independent orbit around the sun.
And astronomers have found a gas giant just like that. J1407b.
This is what it would look like in the sky over Berlin if it was in our solar system:
It would be much larger than our full moon and you'd see the rings easily by day with the naked eye.
More about it:
About the Author
Robert Walker
Writer of articles on Mars and Space issues - Software Developer of Tune Smithy, Bounce Metronome etc.Studied at Wolfson College, Oxford
Lives in Isle of Mull
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