Planetary Gravitational Pull

I have read that Jupiter and Saturn apparently have no surface. I have a hard time believing that a ball of gas can have such a strong gravitational pull without any kind of a solid surface. Can someone explain to me how a planet with no surface can have a gravitational pull? Wouldn't there have to be SOMETHING solid in a rotational state to cause a pull?

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• Anything with mass produces gravity. The sun is another massive object with no solid surface, Sep 4, 2018 at 3:33
• In General Relativity any form of energy can produce a gravitational field - no matter is even required. Sep 4, 2018 at 3:35

3 Answers

I have read that Jupiter and Saturn apparently have no surface.

The surfaces are liquid, much like most of Earth's surface is liquid.

I have a hard time believing that a ball of gas can have such a strong gravitational pull without any kind of a solid surface.

It only needs mass to have gravity, a single hydrogen atom has gravity and there's a lot of hydrogen in space, the Sun, Jupiter and Saturn - thus large planets and the Sun have a large gravitational pull (bending of space).

Can someone explain to me how a planet with no surface can have a gravitational pull?

NASA's webpage on Jupiter and Saturn (from which the information below was derived) explains that while it's not a certainty that there is much rock at the core of these planets there's a lot of compressed hydrogen, along with heavier elements. With enough gravity and pressure the electrons can be stripped from the atoms leaving a core consisting of protons (source for that statement below), metallic hydrogen and metallic helium.

Jupiter has a radius of 43,440.7 miles, Saturn 36,183.7 miles, and Earth is only 3,959 miles. Jupiter's gravity is 24.79 m/s$^2$, Saturn's 10.4 m/s$^2$ and Earth's 9.80665 m/s$^2$. Jupiter's density is 1.326 g/cm$^3$, Saturn's 0.687 g/cm$^3$ and Earth's 5.513 g/cm$^3$.

• Mass:

$$\begin{array}{lr} \text{Jupiter:} & 1,898,130,000,000,000,000,000,000,000 \text{ kg} \\ \text{Saturn:} & 568,319,000,000,000,000,000,000,000 \text{ kg} \\ \text{Earth:} & 5,972,190,000,000,000,000,000,000 \text{ kg} \\ \end{array}$$

• Jupiter

"Structure
The composition of Jupiter is similar to that of the Sun—mostly hydrogen and helium. Deep in the atmosphere, pressure and temperature increase, compressing the hydrogen gas into a liquid. This gives Jupiter the largest ocean in the solar system—an ocean made of hydrogen instead of water. Scientists think that, at depths perhaps halfway to the planet's center, the pressure becomes so great that electrons are squeezed off the hydrogen atoms, making the liquid electrically conducting like metal. Jupiter's fast rotation is thought to drive electrical currents in this region, generating the planet's powerful magnetic field. It is still unclear if, deeper down, Jupiter has a central core of solid material or if it may be a thick, super-hot and dense soup. It could be up to 90,032 degrees Fahrenheit (50,000 degrees Celsius) down there, made mostly of iron and silicate minerals (similar to quartz).".

• Saturn

"Structure
Like Jupiter, Saturn is made mostly of hydrogen and helium. At Saturn's center is a dense core of metals like iron and nickel surrounded by rocky material and other compounds solidified by the intense pressure and heat. It is enveloped by liquid metallic hydrogen inside a layer of liquid hydrogen—similar to Jupiter's core but considerably smaller.".

Wouldn't there have to be SOMETHING solid in a rotational state to cause a pull?

There is something and it doesn't have to rotate for there to be gravity, rotation causes frame dragging. Also important: Gravity doesn't pull. Gravity is not a force, but a consequence of the curvature of spacetime caused by the uneven distribution of mass.

My suggestion is to provide yourself with intuitive evidence of the weight of matter that is in a gaseous state. If you can see the weight of a gas act, it may reassure you that it is substantial enough for it to present a gravitatational field.

There are a range of videos on you-tube showing the effects of the weight of the approximately 100mile high atmosphere of the earth, such as it crushing an empty oil barrel, or forcing two metal hemisphere's together etc.

100miles is a small portion of planetary radius of earth, let alone of the gas giants in our system.

Both those planets contain huge amounts of mass, which is all a planet needs to generate gravity. Think of it this way:

Get you a bunch of really big boxes, all of the same size. Inside #1, you place a gas giant with the mass of Jupiter. Inside #2, a white dwarf with the same mass. #3 gets a neutron star, same mass. #4 gets a black hole, same mass.

Knowing how much you weigh and how far you are from box #1, you measure your orbital period and calculate how much mass is inside box #1. Repeat with each of the other boxes. You get the same answer in each case, because each contains the same amount of mass. What state it's in doesn't affect the outcome.