As a group, the eight planets in our Solar System vary greatly in size, mass, and density (i.e. its mass per unit of volume).These planets - those closest to the Sun - are all terrestrial, meaning they are mostly composed of metals and silicate rocks and have solid surfaces.There is a slight variation in density between the surface of these planets and the core, but not considerably.

In contrast, the 4 outer planets are called ice giants (and/or gas giants) and are composed mostly of hydrogen, helium, and water in various forms.While these planets are larger and massier, their density is significantly lower.The outer and inner layers of these materials vary considerably in density, with liquids and rock-solid materials among them.

Density is also necessary to determine a planet's gravity and intrinsic to the understanding of how a planet formed.After the formation of the Sun at the center of our Solar System, the planets were produced from protoplanetary discs.The terrestrial planets in the inner Solar System formed from grains of dust, whereas the outer Solar System formed from holding on to the nebula's leftover gas.

The Solar System. Image Credit: NASA

They grew bigger as they absorbed more gas.The bigger they grew, the more matter accumulated, until a critical point was reached.Saturn and Jupiter grew exponentially, but Uranus and Neptune never reached this critical mass of nebular gas with only a few Earth masses.Density is always measured in grams per cubic cm.

Density of Mercury:

As a terrestrial planet, Mercury is composed of metals and silicate materials.

In addition, these estimates can be used to determine its inner structure.In comparison to Earth, Mercury is much smaller, which contributes to its inner regions being subject to less compression.Thus, its density is attributed to its large, iron-rich core.All told, iron and nickel make up 70 percent of the planet's mass (more than any other planet), while silicate rock makes up just 30 percent.

Internal structure of Mercury: 1. Crust: 100–300 km thick 2. Mantle: 600 km thick 3. Core: 1,800 km radius. Credit: MASA/JPL

Mercury was once believed to have had a thicker silicate crust earlier in its history.During a planetesimal collision, the crust was blown off in large sections.Mercury has a surface gravity of 3.7 m/s2 thanks to its size and mass, which is the equivalent to 0.38 times Earth's gravity (aka. 1 g).

Density of Venus:

Venus, the second planet from our Sun as well as the second closest terrestrial planet, has a mean density of 5.243 g/cm3.Similarly, Earth's density is 5.243 g/cm3.The geology and seismology of Venus remain largely unknown, but astronomers have a general notion of its composition and structure based on analysis of its size, mass, and density.

It is believed that Venus has a mantle and a crust similar to Earth's.Similarly to Earth, the interior is thought to be composed of iron-rich minerals, while the mantle and crust are composed of silicate minerals.As Venus is slightly smaller than Earth, it also has a lower deep interior pressure than the planet.

The internal structure of Venus – the crust (outer layer), the mantle (middle layer) and the core (yellow inner layer). Credit: Public Domain

Venus and Earth have been cooling at the same rate, so it is thought that Venus' core is at least partially liquid. .As far as some have gone, it has no core.

Density of Earth:

In the Solar System, only Earth has a greater density than 5.514 g/cm3.As a result, other planets' densities are measured by this.Due to Earth's size, mass, and density, there is also a 9.8 m/s2 surface gravity.The one g is also considered to be a standard for measuring the surface gravity of other planets.

Throughout Earth's interior are layers that differ chemically and physically (rheologically).Iron and nickel compose the core, and viscous silicates make up the upper and lower mantles, and solid silicates make up the crust.

Artist’s impression of the Earth’s interior, which includes the upper and lower mantle, and the inner and outer core. Credit: Huff Post Science

On the other hand, unlike the other terrestrial planets, Earth's core region is divided into an inner and an outer core.The inner core has a diameter of approximately 1220 km and is composed of iron and nickel, while the outer core has a radius of approximately 3,400 km.Also, the outer core rotates in the opposite direction of the Earth's rotation, which is the source of Earth's magnetosphere.In the inner core of the planet, this density is estimated to reach 12,600-13,000 kg/m3.

Density of Mars:

In addition to having a dense metallic core, a silicate mantle, and a crust, Mars also has layers that can be differentiated by chemical and physical properties.At 3.933 grams per cubic centimeter, the planet's density is lower than that of Earth, and increases as one gets closer to its core.This is due to the fact that the core is mainly composed of iron and nickel, while the mantle is mainly made up of silicates.

.Martian crust is about 50% thicker than Earth's crust, with a maximum thickness of 125 km (78 mi). The average thickness of Earth's crust is 40 km (25 mi).Because of its size, mass, and density, Mars has a surface gravity of about 3.71 m/s2 or 0.38 grams.

Density of Jupiter:

Jupiter is a gas giant (meaning it is largely made up of gaseous/liquid matter). Its density is lower than any terrestrial planet.In addition to its density, 1.326 g/cm3 makes it the second-densest gas giant.In spite of their enormous size and mass, there is less density because they are mostly composed of noble gases, which are stable in states ranging from gaseous to solid.

Jupiter’s internal structure and composition. (Image Credit: Kelvinsong/Wikipedia Commons

In addition, this density ranges considerably between its outer gaseous layers and its core, which is believed to be composed of rock and surrounded by a layer of metallic hydrogen. In the outermost layer, which are made up of elemental hydrogen and helium, the density of the materials is less than that of water – 0.0002 g/cm³ compared to 1 g/cm³ of water.

Beneath that, where the planet’s hydrogen are in a liquid state, the density rises to roughly 0.5 g/cm³ and increases to 1 g/cm³ at the boundary with the layer composed of metallic hydrogen. The layer metallic hydrogen, meanwhile, has an estimated density of 4 g/cm³ – i.e. around the same as Mars. And in the core, who’s composition is still the subject of speculation, the density rises to 25 g/cm³.

Thought its average density is lower than that of the terrestrial planets, Jupiter’s overall size, mass, and the amount of material it packs into it frame makes for some powerful gravity. Measured from its “surface” (which in this case means its cloudtops), Jupiter’s gravity is over two and a hlaf times that of Earth – 24.79 m/s2, or 2.528 g.

Density of Saturn:

At 0.687 g/cm3, Saturn is the least dense of the gas giants. In fact, it’s mean density is actually lower than that of water, which means that if it were possible to place the planet into a tub of water, it would float. But as with Jupiter and the other giants, this density ranges considerably from the plant’s exterior (which is composed of elemental hydrogen and helium) to its core (which is again believed to be rocky, and surrounded by metallic hydrogen).

Diagram of Saturn’s interior. Credit: Kelvinsong/Wikipedia Commons

Owing to its greater size but lower density than terrestrial planets, Saturn’s surface gravity (again, measured from its cloudtops) is just slightly higher than that of Earth’s -10.44 m/s² or 1.065 g.

Density of Uranus:

With a mean density of 1.27 g/cm3, Uranus is the second-least dense of the gas giants, after Saturn. Its slightly higher density is due to its composition, which consists primarily of various volatile ices – such as water, ammonia, and methane – in addition to gases like hydrogen and helium. For this reason, Uranus (and Neptune) are often referred to as “ice giants” to differentiate them from Jupiter and Saturn.

The standard model of Uranus’s structure is that it consists of three layers. Like the other giants, this includes a rocky core and an outer layer of hydrogen and helium. But in Uranus’ case, these layers are connected by an icy mantle in the middle rather that one made up of liquid hydrogen. The presence of methane in its atmosphere is also what gives Uranus’ its particular hue.

Uranus’ overall size, mass, and density also mean that its surface gravity is less than that of Earth’s. All told, it works out to 8.69 m/s², which is equivalent to 0.886 g.

Diagram of the interior of Uranus. Credit: Public Domain

Density of Neptune:

Neptune’s mean density is 1.638 g/cm³, making it the most dense of any of the giants. Like Uranus, its is composed of higher concentrations of volatiles relative to Jupiter and Saturn. Also like Uranus, its interior is differentiated between a dense core consisting of silicates and metals, a mantle consisting of water, ammonia and methane ices, and an atmosphere consisting of hydrogen, helium, and methane gas.

The higher concentrations of methane in Neptune’s atmosphere is why it is darker in hue than Uranus. And between its size, mass and density, Neptune has a surface gravity of 11.15 m/s2 – which is the equivalent of 1.14 g.

As you can see, the densities of the Solar planets varies widely. Whereas those that are closer to the Sun are terrestrial and quite dense, those that inhabit the outer Solar System are largely gaseous and liquid, and are therefore less dense on average.

We have written many interesting articles about the densities of planets here at Universe Today. Here is the Density of Venus, the Density of Earth, the Density of the Moon, the Density of Mars, the Density of Saturn, the Density of Uranus, and the Density of Neptune.

If you are looking for more information, check out NASA’s Solar System exploration page, and here’s a link to NASA’s Solar System Simulator.