The Largest Solid Object in the Universe

Published in

Our Space

6 min readJan 27, 2017

The black hole from “Interstellar”, 2014. Source: Syncopy/Paramount Pictures

What is the largest solid object in the universe?

The question is vague. At first glance, most people would immediately jump to the conclusion that some bloated red supergiant star or an extragalactic supermassive black hole holds the title of the largest physical thing in existence within the bounds of our universe. However, there is something fundamentally wrong with that conclusion; these objects aren’t solid.

As most people know, matter comes in four basic states (that we know of and use to study). These states are gas, liquid, solid, and plasma. What most people don’t know is that more than 99% of the matter in the visible universe is made up of that fourth state, plasma. Basically everything we can see in our night sky with the naked eye, like stars and galaxies, are a form of plasma. Because there is a distinct difference between plasma and the other three states of matter, things made up of plasma aren’t really solid. Thus, these objects cannot be included in a computation of the largest solid object in the universe.

A star, which is not a solid object. Source NASA

So, does this assumption basically narrow our search to the largest planet in the universe? Not exactly. Take Jupiter for example, a gas giant. Almost all of it’s mass is in the form of gaseous (or metallic) hydrogen, circling around a small core in a thick layer of atmosphere. Saturn, Uranus, and Neptune also yield similar structures. Gas giants, it would seem, may also be scratched off the proverbial list of largest solid objects in the universe.

The gas giant planets, which too are not entirely solid. Source: Softpedia

The second elementary error with the question posed is that it does not define what “large” is. Something can be larger than something else if it yields more mass, a larger radius, a higher density, or any other number of other celestial factors really. Because humans usually think of “big” as something that is physically larger than something else by visual assessment, this study will focus on an object’s physical volume, which usually depends on its radius.

Okay, so we’re ready to hit the books! Not quite. Because the question has one more formality that needs to be taken into account. The problem statement indicates that we are trying to find the largest solid object in the universe. How can we possibly do that when we only see a tiny fraction of the observable universe even with our most powerful telescopes? We need to narrow our question to only what we can observe. So, the real question is:

What is the largest solid object, by volume, that we have thus far discovered in the observable universe? Let’s begin.

First, let’s start small by analyzing our solar system. At first glance, it would appear that Earth is the blue ribbon holder, as the largest rocky planet orbiting the sun. However, upon closer examination, one would realize that the Earth itself is not completely solid. Compressed by a layer of solid rock kilometers thick, the outer core of the Earth is comprised of molten iron, the liquid which keeps our protective magnetic field continually active. A liquid core has also been postulated to exist on Venus, the second largest rocky planet, as well.

Cutaway of Earth, showing that it is also not completely solid. Source: Wikimedia

The next giant solid candidate is Mars. Mars once yielded a liquid core and hot mantle, which was responsible for all of its volcanic and tectonic features. However, today the core of Mars has since solidified, completely depriving it of a defensive magnetic field from solar radiation. Though pockets of liquid mantle are theorized to still exist, Mars’s structure could remain completely intact if these pockets were to disappear.

Mars has a radius of 3390 kilometers and it is completely solid. Earth has a radius of 6371 kilometers, and it still hosts a liquid interior. So the largest completely solid object by volume in the universe must be somewhere in between these two radii. Well, not exactly…

Could Mars be one of the largest solid objects in the universe? Source: NASA

Though the exteriors of gas giants are, as the name suggests, gaseous, gas giants do possess relatively large solid cores at their centers. Jupiter’s solid core is thought to be comprised completely of rock, metal, and ice, and may be up to 20 times the Earth’s mass. Though the extreme temperatures at its core (~36,000 K) would seem to suggest a molten core structure, the immense pressures from thousands of kilometers of compressed hydrogen and helium likely keep the core completely solid. Jupiter’s core has been theorized to have a radius of up to 0.1 times that of Jupiter’s radius, or about 7,000 km. This gives it a whopping density of over 80,000 kg/m³.

Cutaway of Jupiter, showing it’s completely solid, rock/ice core. Source: NASA

We have discovered exoplanet’s larger than Jupiter, which would have proportionately larger solid cores. However, once a planet hits a certain mass (about 13 times Jupiter’s), the object ceases to be a planet, and becomes a brown dwarf. A brown dwarf is characterized by its ability to perform deuterium fusion, at which point it likely no longer possesses a solid core. So, a planet of 13 Jupiter masses would (potentially) have a solid core with a mass of 260 Earth’s. Assuming that such a planet would have a core that is about the same density as Jupiter’s, the core of this upper mass planet would be about 16,500 km, or 2.58 times the radius of Earth!

So the largest physical solid natural object possible in our universe by volume is the solid core of a hypothetical high-mass planet just on the cusp of declaring its status as a brown dwarf. Cool! Now here’s a list of some really big structures in our universe. Enjoy!