Of all the planets, dwarfs, moons, asteroids and others in the solar system, only one element can be the densest. You might think that the most massive elements like Jupiter or even the Sun are the densest, but they actually represent less than a quarter of the density of the Earth. Science explains why.
You might follow a different path and think that the worlds that are made of the largest proportion of the heaviest elements would also be the densest. But if that were the case, Mercury would be the densest world, and it is not. Instead, of all the great known objects in the solar system, the Earth is the densest of all.
Density is one of the simplest non-fundamental properties of matter imaginable. Each existing object, from the microscopic to the astronomical, has a certain amount of energy at rest that is intrinsic to it: what we commonly call mass. These objects also occupy a certain amount of space in three dimensions: this is what we call volume. Density is only the ratio of these two properties: the mass of an object divided by its volume.
Our solar system itself formed about 4.5 billion years ago in the same way as all solar systems: from a gas cloud in a star-forming region that contracted and collapsed under its own gravity. Recently, thanks to observatories such as ALMA (Atacama Large Millimetre/submillimetre Array), we were able for the first time to directly image and analyze the protoplanetary discs that form around these emerging stars.
Some of the features of such an image are striking. You can see a large disk spread around a star in formation: the material that will give birth to planets, moons, asteroids, an outer belt (Kuiper type), etc. You can see holes in the disk: places where massive objects like planets are already forming. You can see a color-coded temperature gradient, where the interior regions are warmer and the outer regions are colder.
But what you can’t see visually on an image like this is the presence and abundance of different types of materials. Although complex molecules and even organic compounds are found in systems like this, three important effects combine to determine which elements are found in which parts of the solar system.
The first factor is gravitation, which is always a force of attraction. In a disk of matter composed of tiny particles, those closest to the inside of the disk will rotate around the center of the solar system at slightly higher speeds than those that are a little further away, causing collisions between particles as they intersect in this orbital dance.
Where slightly larger particles have already formed, or where smaller particles stick together to form larger particles, the gravitational force becomes slightly more important because having an overdense region preferentially attracts more and more of the surrounding mass. Over thousands, millions or even tens of millions of years, this will lead to the formation of fleeing planets at the place where mass has accumulated most rapidly.
The second factor is the temperature of the central star, which evolves from its pre-birth as a molecular cloud, through its proto-star phase, to its long life as a star in its own right. In the inner region closest to the star, only the heaviest elements can survive, as everything else is too light to be separated by intense heat and radiation. The inner worlds will be made entirely of metals.
Apart from that, there is a frost line (without volatile ice inside but with volatile ice beyond), where our terrestrial planets have all formed within the frost line. While these lines are interesting, they also tell us that there is a gradient of matter forming in the solar system: the heaviest elements are found in greater proportion near the central star, while heavier elements are less abundant further away.
And the third and final element is that there is a complex gravitational dance that takes place over time. Planets are migrating. Stars warm up and ice breaks off where they were previously allowed. Planets that may have orbited our star at an earlier stage can be ejected, projected into the Sun, or triggered to collide and/or merge with other worlds.
And if you get too close to the star that anchors your solar system, the outer layers of the star’s atmosphere can provide enough friction to destabilize your orbit, spiraling into the central star itself. By observing our solar system today, 4.5 billion years after its formation, we can easily imagine how things were at the beginning. We can paint a general picture of what happened to create things as they are today.
But all we have left are the survivors. What we see follows a general pattern that is very consistent with the idea that our eight planets formed in roughly the order they are today: Mercury as the inner world, followed by Venus, Earth, Mars, the asteroid belt , then four gas giants each with their own lunar system, the Kuiper Belt, and finally the Oort cloud.
If everything were based solely on the elements that make up them, Mercury would be the densest planet. Compared to any other known world in the solar system, Mercury has a higher proportion of elements in the periodic table. Even asteroids that have boiled their volatile ice are not as dense as Mercury is from the elements alone. Venus is in 2nd position, the Earth is 3rd, followed by Mars, a few asteroids, then Jupiter’s innerst moon: Io.
But it is not only the composition of the raw materials of a world that determines its density. There is also the issue of gravitational compression, which has an even greater effect on the worlds as their mass is large. This is a subject on which we have learned a lot by studying the planets beyond our own solar system, because they have taught us what the different categories of exoplanets are. This allowed us to deduce what physical processes are involved that lead to the worlds we observe.
If you are below two earthly masses, you will be a rocky planet, similar to Earth, with larger mass planets undergoing greater gravitational compression. Above, you start clinging to an envelope of gaseous matter, which “inflates” your world and decreases its density enormously as you increase in mass, which is why Saturn is the least dense planet. Beyond another threshold, gravitational compression regains the upper hand; Saturn has 85% of Jupiter’s physical size, but only a third of its mass. And beyond another threshold, nuclear fusion lights up, turning a planet into a star.
If we had a world like Jupiter that was close enough to the Sun, its atmosphere would be stripped, revealing a nucleus that would certainly be denser than all the planets in our current solar system. The densest and heaviest elements always sink into the nucleus as planets form, and gravitation compresses this nucleus to be even denser than it would otherwise have been. But we do not have such a world in our backyard.
Instead, we just have a relatively heavy and rocky Earth planet: Earth, the heaviest world in our solar system without a large gaseous envelope. Because of the power of its own gravitational pull, the Earth is compressed by a few percent compared to what its density would have been without such a mass. The difference is sufficient to overcome the fact that it is made up of elements generally lighter than Mercury (in the range of 2 to 5%) to make it about 2% denser than Mercury as a whole.
If the elements you are made of were the only ones that matter for density, then Mercury would undoubtedly be the densest planet in the solar system. Without a low-density ocean or atmosphere, and made of heavier elements on the periodic table (on average) than any other object in our neighbourhood, it would be the most remarkable. And yet, the Earth, almost three times farther from the Sun, made of lighter materials, and endowed with a substantial atmosphere, squeaks forward with a density greater than 2%.
The explanation? The Earth has enough mass for its gravitational self-pressure to be significant: almost as large as you can get before you start clinging to a large, volatile envelope of gas. The Earth is closer to this limit than any other object in our solar system, and the combination of its relatively dense composition and its enormous self-gravity, since we are 18 times more massive than Mercury, places us alone as the most important object dense of our solar system.