Maximum and minimum masses and sizes of giant planets?

Question

What are the minimum & maximum masses and diameters of giant planets?

Minimum end of the scale

• Earth has mass of 1 Earth mass and a mean radius of 6,371.0 kilometers, and thus a mean diameter of 12,742 Kilometers,

• Uranus has a masss of 14.536 Earth mass and a mean radius of about 25,362 kilometers, and thus a mean diameter of about 50,724 Kilometers.

• Neptune has a masss of 17.147 Earth mass and a mean radius of about 24,622 kilometers, and thus a mean diameter of about 49,244 Kilometers.

So the minimum mass for a giant planet shouuld be somewhere between 1 Earth mass and 14.536 Earth mass, while the minimum diameter of a giant planet should be somewhere between 12,742 kilometers and about 49,244 kilometers. And I am sure some people here can find more precise values for the smallest possible masses and diameters of giant planets.

Upper end

• Jupiter has a mass of 317.8 Earth mass, and a mean radius of about 69,911 kilometers, and thus a mean diameter of about 139,822 kilometers.
• Planets can have several times the mass of Jupiter, the dividing line between giant planets and brown dwarfs is about 13 times the mass of Jupiter, or about 4,131.4 Earth mass.

From what I have read, giant planets do not have diameters much greater than that of Jupiter, since with increassing mass they become more and more compressed and dense. But I don't know if the upper limit for the diameter of giant planet would be 150,000 kilometers, 160,000 kilometers, or any other specific figure. I expect that some people here know the maximum diameter of a giant planet which is not very close to its star and very hot and swollen.

Answer 1

Gas giant or ice giant?

A gas giant planet is predominantly hydrogen and helium.

An ice giant, predominantly what astronomers call "ices", such as water, CO2, CH4, NH3 and others. On Earth, those are liquids or gases, but in space, when a solar-system is forming and beyond the frost line, they're usually frozen as ices.

As you stated, Gas giants, even as they add mass, don't add much size. Heavy jupiters, with 2, 4, 8 or 10 times the mass of Jupiter remain roughly the size of Jupiter, though they can grow larger if they are near their star and are hotter.

Saturn is a little less than 1/3rd the mass of Jupiter but almost the same size and if Saturn moved into Jupiter's orbit, the additional heat would expand it somewhat.

There is a minimum mass for gas giant planets, below which, they are unlikely to accrue hydrogen, and as a result, the minimum size is likely not too much smaller than Saturn, though there might be exceptions to the rule, for example, a giant impact might blow much of the hydrogen and helium off, so you might get a wierd gas giant of smaller size. But with more traditional formation, the upper and lower limits in size for a hydrogen-helium gas giant is probably not too much smaller than Saturn and only not much larger than Jupiter unless heated up considerably.

Ice giants, made up icy debris with considerably lower percentages of hydrogen and helium are probably more variable in size, as we have icy moons and asteroids and comets, so they can get quite small, though obviously, they're not called "ice giants" when they're moon sized or smaller, but it's generally the same material.

Likewise, how large an ice giant planet can get depends on how much ice it takes on to form, but there are limitations. If it's too large, it's likely to start accruing hydrogen and helium as well and begin to resemble a gas giant, perhaps a more dense one.

An theoretical ice-giant planet, if one existed, with Jupiter mass, would be smaller and denser than the hydrogen-helium ice giant, but a planet like that might be unlikely to exist given gravity's natural tendency to take on whatever it can hold and the relative abundance of hydrogen in solar-systems.

This site (it's just Wikipedia) suggests a possible maximum mass for an ice giant of maybe 80 Earth's. That's pretty ballpark though and our telescopes can't get a very good look at exoplanets, so a lot of these size estimates are inexact.

On the small scale, I see no reason why an "ice-planet" couldn't be the size of Earth or even smaller, but would it still be called an ice giant or just an ice planet? It's more a problem of terminology than astronomy. The really interesting stuff happens when you look closer. The category a planet falls under is just the beginning.

So, there's no exact determination where ice giant ends and gas giant begins. These are terms of convenience and not exact. There can be ranges and kind of hybrid planets with some features like an ice giant and some features like a gas giant. In our solar-system, terrestrial, gas giant, ice giant and dwarf planet are all fairly clear cut, but in other solar systems that might not be the case.