I have heard many times that this planet is gaseous like jupiter this planet is like super earth like rocky planet etc. so is there any planet which is mix of gase and rock ?


2 Answers 2


Probably. Relatively little is known about exoplanets because they're very hard to get a good look at, but there's no reason why a rocky world couldn't accumulate enough ices and/or gas to also resemble a gas giant. Now half rocky half gas giant (hydrogen/helium) might be rare. Half rocky half "ices" is very possible and those have likely already been observed. It helps to understand planet formation and elemental abundance.

There's likely a size limit to rocky planets with thin atmospheres because mass tends to accumulate and retain atmosphere and with hydrogen and helium being the most abundant gases in the universe, once a planet gets massive enough, it should collect and retain hydrogen and helium and begin to resemble Jupiter or Saturn, even if it started out as a rocky world.

Rocky worlds, need elements like Magnesium, Silicon, and Iron, often bound with oxygen. close to 90% of the mass of rocky worlds in our solar system comes from those 4 elements and 90% isn't a bad estimate for rocky exoplanets.

Based on our solar-system:

Iron makes up about 0.117%, Silicon 0.065% and and Magnesium 0.051%, for about .234% of the mass of the solarsystem. Oxygen bound to those elements, using Earth's composition as an estimate adds about another 50%, up to about 0.35%. Hydrogen and Helium make up about 97.6%, which leaves about 2% of the mass of the solar system in the form of ices and heavier gases, not hydrogen and helium. Those are primarily water, ammonia, CO2 and Methane, with smaller amounts of other gases/ices.

These numbers are rough but good enough for an estimate. In our solarsystem there's about 6 times as much ice and heavier gases by mass than there is rocky material and there's close to 40 times as much hydrogen and helium as the other elements.

Planets need to be quite large to hold hydrogen and helium. Jupiter and Saturn are the only planets in our solar-system that are hydrogen and helium abundant. Even the smaller gas giants, Uranus and Neptune have comparatively little hydrogen and helium.

So there's probably no such thing as a small gas giant. Gas giants need to be large or they don't exist at all. (I should make a footnote that as a gas giant loses atmosphere, you can get a small gas giant, but that would be in a transition phase).

I don't like the term "Ice giants", though it's often used to define Neptune and Pluto because they are primarily ices (water, methane, CO2, NH3), not primarily hydrogen/helium. I don't like calling them "ice giants" when they can also be hot, so I prefer the term Neptune like planets or Neptunes.

Uranus (a Neptune type planet) is mostly made up of ices and heavier gases with an estimate of just 3%-10% hydrogen and helium. Estimates of about 70%-90% of ice/not hydrogen/helium gas and the remaining, about 7%-20% more dense material. That would be some other elements, perhaps some solidified carbon (diamonds), higher sulfur and lower Iron by percentage than Earth as well as some Earth like silicates. magnesium-oxides, Iron and Iron-oxides, though the high internal temperature might not leave much in the way of chemical bonds towards the center of the planet where temperatures reach 9,000 degrees C.

Neptune is similar to Uranus but more massive and more dense. It's thought to have more water and perhaps a larger internal mantle, though like Uranus, the majority of it's mass is ices and heavy gases (not hydrogen/helium).

Neptune and Uranus have hydrogen in the form of Methane, Ammonia and Water, but I'm counting that hydrogen as part of their "heavy gases/ices". They both have a comparatively low percentage of free hydrogen (3%-10%) making a clear distinction between Neptune like planets and gas giants like Jupiter and Saturn with which the majority of their mass is hydrogen.

It helps to think of how planets form. Smaller planets have too little gravity to retain hydrogen and helium gas and solar-systems are mostly too warm for hydrogen to freeze, so planets are made up of solid material for the most part, either rocky or ices that clump and stick together during formation. When they have enough gravity, then they can begin to hold onto an atmosphere and further from the star makes this easier.

The Moon Titan, as an example, is small to have an atmosphere, but it's quite far from the sun and largely made up of ices on it's surface, so as the ice on it's surface melts, it basically replenishes it's atmosphere. Titan is still out-gassing, and it losing atmosphere, but it loses atmosphere slowly enough that it retains an atmosphere, but in a sense, it's in a transition, where it's atmosphere is outgassed. When it runs low on surface ice that readily thaws, it should begin to resemble the icy moons of Jupiter.

So, back to your original question. Many combinations are possible, some are not. Baby Jupiters (the mass of Neptune) are probably unlikely, though a gas giant close to it's star that's lost a lot of it's atmosphere could resemble a baby Jupiter, but like Titan, that would be transitional.

Rocky Jupiters are unlikely because hydrogen outnumbers rocky material by over so much, over 200 to 1 in most of the milky-way. Now, some solarsystems are likely more "metalic" than others, so that ratio will have some variation, but it's once there's enough mass for a gas giant to form, hydrogen is likely to be the abundant element and Helium #2 and any rocky core would be dwarfed by the hydrogen and helium.

A rocky Neptune however, no reason why not. A planet with 8 earth masses of Earth like material and 8 Earth masses of ices (and 5%-10% hydrogen) would basically be a rocky Neptune. It would likely be a little smaller and certainly denser, but not all that different in outward appearance.

Water worlds is a common term that might qualify as a "rocky-neptune". We don't know exactly what they are made of, but density estimates suggest a high percentage of water (and presumably CO2, CH4, NH3), which is the majority of Uranus and Neptune.

Here's a chart of exoplanets of less than 20 Earth masses. I suspect there's a considerable margin of error in these estimates, but it more or less agrees with the rocky-neptune argument. Planets with 2 or 3 grams per cc would be in that range.

See chart: www.hpcf.upr.edu/~abel/phl/hec_plots/exoplanet_df.png


Baby Neptunes of just 1 or 2 Earth masses might be possible too, but they'd probably need to be quite cold and far from their star or they'd be in danger of losing their atmosphere.

Iceworlds like Pluto can get quite small, but Pluto has very little atmosphere. Planets generally need to be fairly large to retain their atmosphere. If they are too cold, that atmosphere freezes. Titan, as I mentioned above, is in what could be called a slow transition where it's atmosphere comes from it's surface and it loses it slowly. Ceres, based on it's density, is an icy moon like object too, though it's lost nearly all it's surface ice, but it probably has a lot of water/"ices" below it's surface.

  • $\begingroup$ Shouldn't it be Uranus instead of Pluto in the ice giants paragraph? Otherwise excellent answer $\endgroup$ Commented Dec 22, 2021 at 17:24

I think @userLTK answer is pretty complete, but I would like to add one from a different perspective.

Astronomers, as all humans, tend to categorize things we see using words. This is necessary for fast communication but sometimes there are objects that are at the edge of one category and another (think in a brown dwarf).

Reality is pretty analogic at big scales and we tend to cut it, "digitalize" it in order to understand what are we talking about, but almost always there will be examples in the limits from one categorize to the next, or even superposition, having a same object being in two categories.

As pointed, the amount of gas a planet has depends primarily on its mass. At the same time, there is one category of planets -by mass- missing in our solar system: super-earths (and probably mini-neptunes... again, this is about what granularity we want to use in definitions to a proper communication, and the bounds of each category. I won't get into that). So if Earth has 1 earth mass and Neptune-Uranus about 17-15 earth masses, you can easily imagine a planet having the right mass in the middle of those for being half rock and half gas. Maybe 3, 5 or 8 Earth masses, no idea about the exact mass for the exact 50%. It also depends on its formation and evolution.


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