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This is a hypothetical. If a gas giant with the approximate mass and density of Uranus existed within the goldilocks range to support life around a star, what would be the conditions on the service. Let's assume the same general composition as Uranus for a control. Would a gas giant still be mostly ice at this distance? And if there is a change in the state of the composition on the gas giant, how might this effect moons with water/liquid (i.e., would the tidal shifts be able to support like on land on a moon around such a planet?)

As a follow up, would different types of stars change this state, assuming stars ranging from Class A, F, G, or K?

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    $\begingroup$ What is your thought on it? What is the result of your research for an answer? $\endgroup$ Jun 23, 2023 at 19:58
  • $\begingroup$ I am attempted to build to a book series that is conceptually possible in the universe as we know it. Additionally, this will drive flora and fauna and minor adaptive/evolutionary changes in the humans there. For the record, I am attempting validate if a planet could have an annual/frequent eclipse lasting 2-3 days, either by means of it being a moon to a much larger planet (i.e. the Uranus question) or some other means. $\endgroup$ Jun 23, 2023 at 23:51
  • $\begingroup$ I'm also considering a multi-"star" system with two massive brown dwarf twins orbiting each other countered by a Class G or K star, orbital times as such that the brown dwarves would block the light/heat from the star to a tidally locked (to the barycenter of the system) goldilocks planet periodically. $\endgroup$ Jun 23, 2023 at 23:51
  • $\begingroup$ You can check WorldBuilding.SE for hypothetical questions. $\endgroup$ Jun 24, 2023 at 2:49
  • $\begingroup$ I don't see your brown dwarf system possible. Or it is a close triple system of start and brown dwarfs and the planet so far out that it definitely is not habitable $\endgroup$ Jun 24, 2023 at 7:33

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Hot Neptunes are known, beginning with an exoplanet orbiting the star Gliese 436. They may be formed either at greater distances from the star and then migrate inwards (ex-situ) or at close distances initially (in-situ).

Either way, these exoplanets would not be Earth-like. Their composition depends on how they are formed, but in general they must contain predominantly hydrogen and helium, overlying heavier ices or perhaps more refractory compounds:

The first theoretical study of how hot Neptunes could form was carried out in 2004.[1] If these planets formed ex situ, i.e., by migrating to their current locations while growing, they may contain large quantities of frozen volatiles and amorphous ices. Otherwise, if they formed in situ, their inventory of heavy elements should be made entirely of refractory materials.[2] Yet, regardless of the mode of formation, hot Neptunes should contain large fractions (by mass) of gases, primarily hydrogen and helium, which also account for most of their volume.[3][4]

On this basis a "composition similar to Uranus" would require an ex-situ formation process,csince Uranus itself has more ices than refractory material. With the hydrogen-helium gas overlay and a heat input similar to Earth the exoplanet would then look much like Uranus itself.

Cited References

  1. Adrián Brunini and Rodolfo G. Cionco (September 2005). "The origin and nature of Neptune-like planets orbiting close to solar type stars". Icarus. 177 (1): 264–68. arXiv:astro-ph/0511051. Bibcode:2005Icar..177..264B. doi:10.1016/j.icarus.2005.02.015. S2CID 18979082.

  2. D'Angelo, G.; Bodenheimer, P. (2016). "In Situ and Ex Situ Formation Models of Kepler 11 Planets". The Astrophysical Journal. 828 (1): id. 33. arXiv:1606.08088. Bibcode:2016ApJ...828...33D. doi:10.3847/0004-637X/828/1/33. S2CID 119203398.

  3. D'Angelo, G.; Durisen, R. H.; Lissauer, J. J. (2011). "Giant Planet Formation". In S. Seager. (ed.). Exoplanets. University of Arizona Press, Tucson, AZ. pp. 319–346. arXiv:1006.5486. Bibcode:2010exop.book..319D.

  4. D'Angelo, G.; Lissauer, J. J. (2018). "Formation of Giant Planets". In Deeg H., Belmonte J. (ed.). Handbook of Exoplanets. Springer International Publishing AG, part of Springer Nature. pp. 2319–2343. arXiv:1806.05649. Bibcode:2018haex.bookE.140D. doi:10.1007/978-3-319-55333-7_140. ISBN 978-3-319-55332-0. S2CID 116913980.

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  • $\begingroup$ So, to be clear, if Uranus, as it is composed, move into Earth's orbit, the thermal gain would not be sufficient for it to develop an atmosphere or melt? $\endgroup$ Jun 23, 2023 at 23:56
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    $\begingroup$ Probably not. Uranus is already hot in its interior, so solar heat gain would have little impact there. Uranus would be warmer at the surface, but likely not hot enough to give hydrogen loss given its large escape velocity. $\endgroup$ Jun 24, 2023 at 0:20
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    $\begingroup$ @curious You assume that uranus or neptune are frozen. They are not. The visible 'surface' is composed of gas, they have no (visible) surface. And the solid core is warm or hot as is any planet core as result of its formation by accretion. $\endgroup$ Jun 24, 2023 at 0:25
  • $\begingroup$ Yeah I was wondering why the asker referred to the planet melting. This on an astronomy board. Yeesh! $\endgroup$ Jun 24, 2023 at 0:27
  • $\begingroup$ Apologies for the layman's terms, but I appreciate the clarification. $\endgroup$ Jun 24, 2023 at 0:30
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In my recollection, until the late 1990s, all four outer planets, including Uranus and Neptune, were generally referred to (in lay media) as gas giants. They all have very deep atmospheres and may be wholly fluid, although pressures and temperatures are so high in the interior that scientists cannot be sure if the matter is crystalline and solid because the pressure is so high, or fluid because the temperature is so high. But none of them are known to have solid surfaces.

But Jupiter and Saturn are mostly hydrogen (with some helium), while Uranus and Neptune are mostly methane, ammonia, and water (with some hydrogen and helium). Astronomers call all of those substances "ices" when talking about the outer solar system, because that's the form they would normally take at those temperatures. So Uranus and Neptune are now called ice giants and the Wikipedia article says, for instance, that Uranus is mostly ice. But that does not mean that it is mostly frozen solid, just that it's mostly materials that would be liquid or gaseous on earth and would be frozen solid on a moon at that distance.

So, all of the above was just background. If you moved Uranus to Earth's orbit, it would be very similar to the way it is now, just much warmer at the cloud tops.

Note that Oscar Lanczi's answer is much more detailed, and based on stronger knowledge, but I thought this would provide helpful background.

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