I am reading a book where a gas giant is coming into our solar system. It was a rogue planet, traveling between stars.

Given the knowledge we have now, would we expect a gas giant to freeze solid if is far enough away from a sun, e.g. traveling between stars, and/or being in a distant orbit around the Sun?

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    $\begingroup$ Interesting question, but it might be better in the astronomy StackExchange: astronomy.stackexchange.com $\endgroup$ Nov 21 '14 at 12:37
  • $\begingroup$ The book isn't Blindsight, by Peter Watts, is it? If so, that was a great read! $\endgroup$
    – Kirkaiya
    Nov 21 '14 at 22:16
  • $\begingroup$ What timeframe are we talking about? After a long enough time, it would have cooled all the way down to the core to the background radiation temperature at that time. So its just a question of if it contains enough helium to form helium oceans... $\endgroup$
    – PlasmaHH
    Nov 21 '14 at 22:35
  • $\begingroup$ @Kirkaiya the book is When Worlds Collide is a 1933 science fiction novel co-written by Philip Wylie and Edwin Balmer; there is a link in the question to it's Wikipedia entry $\endgroup$ Nov 21 '14 at 22:50
  • $\begingroup$ I don't think this can be a yes/no question because the core of gas giants are already solid. Even if the "surface" was frozen, it would have an atmosphere on top of that. These two things are not qualitatively different, it's only a matter of degree as far as I can tell. $\endgroup$
    – AlanSE
    Nov 22 '14 at 15:11

There is an answer on wikipedia: Rogue planet:

It is calculated that, for an Earth-sized object at a kilobar hydrogen atmospheric pressures in which a convective gas adiabat has formed, geothermal energy from residual core radioisotope decay will be sufficient to heat the surface to temperatures above the melting point of water.[13] Thus, it is proposed that interstellar planetary bodies with extensive liquid-water oceans may exist. It is further suggested that these planets are likely to remain geologically active for long periods, providing a geodynamo-created protective magnetosphere and possible sea floor volcanism which could provide an energy source for life.

It also mentions that if the planet retained a moon, it would have a source of tidal heating.

Note that one such rogue planet, WISE 0855-0714 has a temperature of 225-260 K and is the coldest such object seen so far. It is still far warmer than the 55 K temperature of Neptune, which is mostly fluid mantle.

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    $\begingroup$ That Wikipedia quote is true for Earth-sized planets, but doesn't say much about gas giants. Gas giants contain fewer heavy atoms, so little to no radioactive decay. They just have residual heat from their formation. $\endgroup$
    – Hobbes
    Nov 21 '14 at 18:28
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    $\begingroup$ I think it's presently unknown how many heavy atoms planets like Jupiter have - we still don't really know what the make-up of Jupiter's core is, or even if it has a rocky core or not. Part of the Juno mission (en route to Jupiter right now) is to try to figure that out by orbiting very close to the atmosphere (3,000 miles up I think) and measuring gravity differences, etc. $\endgroup$
    – Kirkaiya
    Nov 21 '14 at 22:19
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    $\begingroup$ Jupiter's is ~71% hydrogen, and ~24% helium (and 5% other). It would take enormous pressure for these to become solid all the way through. See also cold gas giants. $\endgroup$
    – user5892
    Nov 21 '14 at 22:25
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    $\begingroup$ Gas giants are just short of becoming brown dwarves, due to their own gravitational strength. I'd hit up Phil Plait or similar astronomy bloggers for more info on this topic. $\endgroup$ Nov 21 '14 at 22:25
  • $\begingroup$ @CarlWitthoft That really only applies to high-mass gas giants. Jupiter would need to be at least 25 times as massive as it is to be like a brown dwarf. $\endgroup$
    – HDE 226868
    Nov 22 '14 at 0:26

Eventually, yes but I don't think that it has much to do with the sun. Jupiter's magnetosphere shunts all of the solar wind (plasma) from the sun and

Jupiter's atmosphere was also found to be quite turbulent. This indicates that Jupiter's winds are driven in large part by its internal heat rather than from solar input as on Earth. -nineplanets.org

Presumably, Jupiter will freeze some day regardless of sunlight (having assumed the sun is immutable). I would compare the amount of energy Jupiter is receiving from the sun to the heat you add to your coffee by stirring it with a spoon (friction). This is probably a gross understatement but it's one way to look at it.

The magnetosphere of Jupiter is the cavity created in the solar wind by the planet's magnetic field. Extending up to seven million kilometers in the Sun's direction and almost to the orbit of Saturn in the opposite direction, Jupiter's magnetosphere is the largest and most powerful of any planetary magnetosphere in the Solar System, and by volume the largest known continuous structure in the Solar System after the heliosphere.

Jupiter's internal magnetic field is generated by electrical currents in the planet's outer core, which is composed of liquid metallic hydrogen. Volcanic eruptions on Jupiter's moon Io eject large amounts of sulfur dioxide gas into space, forming a large torus around the planet. Jupiter's magnetic field forces the torus to rotate with the same angular velocity and direction as the planet. The torus in turn loads the magnetic field with plasma, in the process stretching it into a pancake-like structure called a magnetodisk. In effect, Jupiter's magnetosphere is shaped by Io's plasma and its own rotation, rather than by the solar wind like Earth's magnetosphere. Strong currents in the magnetosphere generate permanent aurorae around the planet's poles and intense variable radio emissions, which means that Jupiter can be thought of as a very weak radio pulsar. Jupiter's aurorae have been observed in almost all parts of the electromagnetic spectrum, including infrared, visible, ultraviolet and soft X-rays.

The action of the magnetosphere traps and accelerates particles, producing intense belts of radiation similar to Earth's Van Allen belts, but thousands of times stronger. The interaction of energetic particles with the surfaces of Jupiter's largest moons markedly affects their chemical and physical properties. Those same particles also affect and are affected by the motions of the particles within Jupiter's tenuous planetary ring system. Radiation belts present a significant hazard for spacecraft and potentially to human space travelers. -Magnetosphere of Jupiter, Wikipedia

There are a lot of variables; Jupiter is its own mini solar system. Humans have neither the means nor the time to study the effects of it otherwise (I.e, take away its moons, negate the sun or put Jupiter alone in a box for billions of years) and I wouldn't trust any simulation for a definitive answer. The WISE object that Phil mentions could be compared to a rogue planet, and it is surprisingly similar in size (however its mass is 3~10 times larger). Jupiter Mass, Wiki:

If the sun had the capability of keeping Jovian planets from freezing I'd expect Mercury to be completely molten. Given the proximity of Mercury and Jupiter to the sun I believe tidal forces and convection to be the much larger factors at play here than sunlight. The point is moot. You can't live on gas giants anyway and even if it was frozen solid, 2.5x normal gravity would not be pleasant (and a bit nippy). The chances of a rogue planet entering our solar system and achieving Goldilocks status are, wait for it... astronomically [echo] slim.


Actually Mazura is wrong, Callisto the moon of Jupiter has a mean temperature of 134 Kelvin, Jupiter has a cloudtop temperature of 165 Kelvin. So it is warmer, but just by 30 degrees. The Jupiter's internal heat really shows below the top clouds, with the core being 36000 Kelvin hot. The freezing point of hydrogen is 13.99 Kelvin. Any "gut feeling" is wrong in astrophysics, the Sun is not luminous enough to melt Mercury, but even Pluto is way too warm to freeze hydrogen at around 40 Kelvin.

As for this question, I do not really know, but I think there are no gas giants old enough for them to cool to this state in the universe yet. It should be said helium cannot be solid without high pressure, so such frozen gas giants in the far future, billions or even trillions of years in the future would consist of a big core of solid hydrogen surrounded by an envelope of liquid/superfluid and gaseous helium.


No. Helium won't freeze solid even at the temperature of interstellar space. Thus even if there's no source of heat you'll still have liquid helium. Note that hydrogen ice will float on this, the planet could appear solid.

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    $\begingroup$ If there is a core of liquid helium surrounded by an outer layer of hydrogen ice would the increased pressure be enough to solidify the helium? $\endgroup$ Oct 13 '15 at 10:24
  • $\begingroup$ Helium solidifies at temperature of 3 Kelvin if it is under pressure of 100 bar (10 MPa). Helium could be easily under such pressure, if there is enough hydrogen ice on top of it. See the helium phase diagram: bbradu.web.cern.ch/bbradu/cryogenics.php $\endgroup$
    – mpv
    Oct 15 '15 at 9:35
  • $\begingroup$ @mpv Ok, I guess you could just barely have a fully solid planet--not that I think you could get a planet without any warming from radioactivity or anything else. $\endgroup$ Oct 15 '15 at 16:38

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