Earth sits squarely in the middle of our solar system’s size range, larger than the four rocky inner planets on average but dwarfed by the four outer giants. With an equatorial diameter of about 12,756 kilometers, Earth is the biggest of the rocky worlds but still small enough that roughly 1,300 of them could fit inside Jupiter. That contrast between Earth and its neighbors turns out to matter for everything from geology to the ability to hold onto an atmosphere, and it becomes even more interesting when you factor in the thousands of planets discovered orbiting other stars.
Earth Versus the Other Rocky Planets
The inner solar system contains four rocky, or terrestrial, planets: Mercury, Venus, Earth, and Mars. Among them, Earth is the largest and most massive, though Venus comes remarkably close. Venus has a diameter of about 12,104 kilometers, roughly 95 percent of Earth’s. The two planets are sometimes called near-twins in terms of physical size, even though their surface conditions could hardly be more different.
Mars is considerably smaller, with a diameter of about 6,779 kilometers, a little over half of Earth’s. That makes Mars roughly one-tenth Earth’s mass. Mercury is smaller still, at about 4,880 kilometers across, barely larger than our Moon. Despite its small size, Mercury is surprisingly dense for its stature. Its average density of about 5.43 grams per cubic centimeter implies an unusually iron-rich interior, suggesting that a huge fraction of the planet is metallic core rather than rocky mantle.1Journal of Geophysical Research: Planets. Internal structure of Mercury: Implications of a molten core That is a useful reminder that a planet’s diameter alone does not tell you everything about what it is made of or how much it weighs.
How Earth Stacks Up Against the Gas and Ice Giants
Once you step past Mars and the asteroid belt, the scale changes dramatically. Jupiter and Saturn are classified as gas giants, while Uranus and Neptune are often called ice giants because a larger fraction of their mass is in heavier compounds like water, ammonia, and methane ices. All four are vastly bigger than Earth by every measure.
Jupiter is the heavyweight. Its mass is about 318 times Earth’s, and Saturn weighs in at roughly 95 Earth masses.2Oxford Research Encyclopedia of Planetary Science. The Interiors of Jupiter and Saturn In terms of diameter, Jupiter stretches about 139,820 kilometers across its equator, nearly 11 times Earth’s width. Saturn is not far behind in diameter at around 116,460 kilometers, though it is much less dense because of its composition. Saturn is famously the least dense planet in the solar system, with an average density lower than water.
Uranus and Neptune are smaller than the gas giants but still enormous compared to Earth. Uranus has a diameter of about 51,118 kilometers, roughly four times Earth’s, and Neptune comes in at about 49,528 kilometers. Each is around 14 to 17 times Earth’s mass. To give a sense of the volume difference, you could pack about 63 Earths inside Uranus and around 58 inside Neptune. Jupiter, by contrast, could swallow more than 1,300 Earths.
These numbers make Earth look puny, but context matters. Most of that giant-planet volume is gas and compressed fluid, not rock. If you stripped Jupiter down to its rocky and metallic core, many models suggest it would be closer to a few Earth masses of heavy elements, though the exact size and even the sharpness of the core boundary are still debated.
Earth Is Not Quite a Perfect Sphere
When people picture Earth, they usually imagine a smooth marble. In reality, Earth’s rotation causes it to bulge slightly at the equator and flatten at the poles, making it an oblate spheroid. The equatorial diameter is about 12,756 kilometers while the polar diameter is about 12,714 kilometers, a difference of roughly 42 kilometers, or about 26 miles.3Research Starter. Earth’s Shape4How to Free Your Inner Mathematician. Pay attention to details, as Earth is an oblate spheroid That is small relative to the overall size, roughly one part in 298, but it is measurable and has real consequences for satellite orbits and gravity mapping.
Earth is not unique in this respect. All the planets in the solar system are oblate spheroids to some degree, and the faster a planet spins, the more pronounced the bulge. Saturn, which completes a rotation in just over ten hours despite being enormously wide, has the most extreme oblateness of any planet in our system. Its equatorial diameter is about 10 percent larger than its polar diameter. Jupiter is similarly flattened, though not quite as dramatically. Even slow-spinning Venus, which takes 243 Earth days to rotate once, has measurable oblateness, though the effect is tiny.
Why Planet Size Matters for Geology
One of the most consequential things about a planet’s size is how it shapes the planet’s geological activity over billions of years. Earth is the only planet in our solar system with unambiguous, ongoing plate tectonics, where large slabs of the outer crust slide over, under, and past each other. This process recycles carbon, builds mountains, drives volcanism, and plays a major role in regulating the climate over geological timescales. Many scientists consider plate tectonics an important ingredient for long-term habitability.
You might assume that a bigger planet would cool faster and lose its geological activity sooner, or that a smaller planet would go cold quickly while a big one stays hot. The actual relationship is more nuanced. Thermal models show that smaller terrestrial planets do not necessarily cool faster than larger ones, provided their interiors convect, meaning heat is transported by slow churning of rock.5Comptes Rendus. Géoscience. Styles of mantle convection and their influence on planetary evolution In some scenarios, smaller planets may even cool more slowly than larger ones, because the forces resisting plate motion and associated cooling scale in unintuitive ways with planet size.6Icarus. A note on planet size and cooling rate
That said, smaller planets do tend to have somewhat lower internal temperatures at any given age, which means they may stop being volcanically active at an earlier point in their history. Mars is a good illustration. It was geologically active billions of years ago but appears to have lost most or all of its tectonic and volcanic activity, though there are hints of geologically recent lava flows in a few regions. Mercury, being even smaller, seems to have gone geologically quiet even sooner.
For planets larger than Earth, the picture actually looks favorable for plate tectonics. Modeling suggests that so-called super-Earths, rocky planets up to several times Earth’s mass, are at least as likely to have plate tectonics as Earth itself. As planetary mass increases, the stresses available to drive plate subduction tend to increase while the lithosphere (the rigid outer layer) gets thinner, both of which make it easier for plates to move.7The Astrophysical Journal. Inevitability of Plate Tectonics on Super-Earths Some analyses go further, concluding that plate tectonics becomes increasingly likely with increasing planet size across multiple convection scenarios.8Elsevier (Earth and Planetary Science Letters). Plate tectonics on super-Earths: Equally or more likely than on Earth Earth’s size, in other words, may be close to the lower end of what comfortably sustains plate tectonics over billions of years.
Size and the Ability to Hold an Atmosphere
A planet’s size and mass determine how tightly it grips its atmosphere. More massive planets have stronger gravity, which makes it harder for atmospheric gases to escape into space. This is one reason why the giant planets have such thick, extensive atmospheres while Mars, at one-tenth Earth’s mass, has an atmosphere so thin that liquid water cannot persist on the surface.
Earth sits in a productive sweet spot. It is massive enough to retain a substantial atmosphere of nitrogen and oxygen, but not so massive that it has captured and held onto enormous envelopes of hydrogen and helium the way Jupiter and Saturn did. The gas giants had enough gravitational pull to sweep up light gases directly from the disk of material surrounding the young Sun, a process that snowballs once enough gas accumulates. Earth-sized planets, forming closer to the Sun where it was hotter, never crossed that threshold.
For planets in between, things get interesting. Low-mass planets with hydrogen-rich atmospheres tend to lose gas through a process researchers sometimes call “boil-off,” where intense stellar radiation heats the upper atmosphere enough for it to escape. This effect is more severe for planets with lower core masses and those orbiting close to their stars, while planets above roughly ten Earth masses are much less vulnerable.9The Astrophysical Journal. ATMOSPHERES OF LOW-MASS PLANETS: THE “BOIL-OFF” One factor that helps intermediate-sized worlds fight atmospheric stripping is dissolved gas in their interiors. Hydrogen that dissolves into the planet’s magma ocean during formation can later outgas as the interior cools, replenishing the atmosphere over time and boosting the planet’s ability to hang onto a gaseous envelope.10The Astrophysical Journal. On the Role of Dissolved Gases in the Atmosphere Retention of Low-mass Low-density Planets
Earth’s atmospheric history reflects some of these dynamics. Early Earth likely had a much different atmosphere, richer in hydrogen compounds, that was shaped by both outgassing from the interior and losses to space. What we breathe today is the product of billions of years of geological and biological reshaping, something only possible because Earth was large enough to hold onto a substantial atmosphere in the first place.
Earth-Sized Worlds Around Other Stars
The question of how Earth compares to other planets took on a much broader meaning after the Kepler space telescope began discovering planets outside our solar system by the thousands. It turns out that Earth-sized planets are not rare at all. Based on Kepler data, roughly one in nine Sun-like stars harbors an Earth-sized planet receiving between one and four times the stellar energy that Earth gets from the Sun.11PubMed Central. Prevalence of Earth-size planets orbiting Sun-like stars Around smaller, cooler red dwarf stars, which are the most common type in the galaxy, the frequency may be even higher, with estimates suggesting that roughly 40 to 50 percent of late K and M dwarf stars could have a potentially Earth-like planet in their habitable zone.12PubMed Central. Remote life-detection criteria, habitable zone boundaries, and the frequency of Earth-like planets around M and late K stars
“Earth-sized” in this context usually means a radius between about half and 1.4 times Earth’s. That is a useful bin for astronomers but covers a wide range of actual planets. A world at the small end of that range, half Earth’s radius, would have a tiny fraction of Earth’s mass and much weaker gravity. A world at 1.4 Earth radii could be substantially more massive and might hold onto a thicker atmosphere. The label “Earth-sized” tells you something about physical dimensions but very little about what the planet’s surface is actually like.
Even so, the sheer prevalence of Earth-sized worlds was one of Kepler’s most striking findings. Before the mission, it was an open question whether planets our size were cosmically common or unusual. The answer, that they are everywhere, reframed the search for habitable environments from “are there places like Earth” to “which of the many places like Earth should we look at first.”
When Size and Density Tell Different Stories
Comparing planets by diameter or mass alone can be misleading, because two planets of very different sizes can have surprisingly similar densities, and two planets of similar sizes can be made of completely different stuff. Earth’s average density is about 5.51 grams per cubic centimeter, making it the densest planet in the solar system. Venus is close behind. Saturn, despite being vastly larger, has an average density of only about 0.69 grams per cubic centimeter.
Among newly discovered exoplanets, density has become one of the most revealing measurements. One dramatic example is GJ 367b, a planet orbiting a nearby red dwarf star. Its radius is only about 72 percent of Earth’s and its mass is roughly 55 percent of Earth’s, making it a genuine sub-Earth, smaller and lighter than our planet. Yet its density is around 8.1 grams per cubic centimeter, close to that of pure iron.13Science. GJ 367b: A dense, ultrashort-period sub-Earth planet transiting a nearby red dwarf star That suggests GJ 367b is mostly metallic core with very little rocky mantle, somewhat like a scaled-up version of Mercury’s unusual composition. A planet smaller than Earth can still be denser than Earth if its makeup is different enough.
This diversity means that “bigger” does not automatically translate to “heavier” or “denser” or “more Earth-like.” A planet twice Earth’s diameter could be a low-density water world, a rocky super-Earth, or something with no analog in our solar system. Size is one dimension of comparison, but without knowing what a planet is made of, it only tells part of the story.
The “Sub-Neptune Gap” and What It Means for Earth’s Place
One of the more puzzling patterns to emerge from exoplanet surveys is the so-called radius gap, sometimes called the Fulton gap after the lead author of the paper that established it statistically. Planets between about 1.5 and 2 times Earth’s radius are surprisingly rare. Below that gap, you find lots of rocky, Earth-like and super-Earth-sized planets. Above it, you find mini-Neptunes with thick gaseous envelopes. But the gap itself is relatively empty.
The leading explanation ties back to the atmospheric physics discussed earlier. Planets just above the gap formed with thin hydrogen-helium atmospheres, but many of them were not massive enough to hold onto that gas permanently. Stellar radiation stripped those atmospheres over hundreds of millions of years, shrinking the planets down to their bare rocky cores and depositing them below the gap. Planets that started with slightly more mass or slightly thicker envelopes managed to keep their gas and stayed above the gap as mini-Neptunes.
Earth, with no primordial hydrogen envelope, sits well below this gap. Its secondary atmosphere, built from volcanic outgassing and later transformed by life, is a different kind of atmospheric story entirely. The radius gap underscores that Earth occupies a distinctive niche: large enough to be geologically active and retain a hefty atmosphere, but small enough that it never accumulated the kind of thick gas blanket that would have made it a fundamentally different type of world.
How Gravity Shapes the Surface You Would Stand On
For anyone imagining what it would be like to visit another planet, surface gravity is the comparison that hits closest to home. Surface gravity depends on both a planet’s mass and its radius, so it does not simply scale with size. Mars has about 38 percent of Earth’s surface gravity, meaning a person who weighs 150 pounds on Earth would feel like they weighed about 57 pounds on Mars. Jupiter’s surface gravity, calculated at the cloud tops since there is no solid surface, is roughly 2.4 times Earth’s. You would feel crushingly heavy.
Venus, despite being slightly smaller and less massive than Earth, has a surface gravity about 91 percent of ours. If you could somehow survive the 460-degree Celsius temperatures and crushing atmospheric pressure, walking around would feel almost normal from a gravitational standpoint. Mercury, at about 38 percent of Earth’s gravity, is coincidentally almost identical to Mars in this regard, even though Mercury is much smaller in diameter. The reason is Mercury’s high density from its oversized iron core, which compensates partially for its smaller size.
Among the ice giants, Uranus has a surface gravity only slightly lower than Earth’s, around 89 percent, even though the planet is over 14 times Earth’s mass. That seems counterintuitive until you remember that Uranus is also much larger in radius, and surface gravity at the cloud tops reflects the distance from the center of mass. Neptune, a bit more massive and slightly smaller in radius than Uranus, has a surface gravity of about 1.14 times Earth’s. These comparisons highlight that the relationship between a planet’s size and what you would actually feel standing on (or floating in) it is not straightforward.