Top 10 Differences Between Exoplanets and Planets in Our Solar System

#1: Size Extremes (Less than 1,000 miles to over 180,000 miles in diameter)

Planets in our solar system range in size from Mercury’s 3,030-mile diameter to Jupiter’s massive 88,850-mile spread. But exoplanets shatter this range. The smallest known exoplanets, such as Kepler-37b, measure less than 1,860 miles across—smaller than Earth’s Moon—while the largest, like HD 100546 b, may exceed 180,000 miles in diameter. Some of these super-sized worlds are so massive and hot they blur the line between planet and brown dwarf, the failed stars of the universe. On the small end, there are planets that might be nothing more than exposed planetary cores, stripped bare by radiation from their stars. These size extremes defy neat classification and suggest that planet formation in the cosmos produces far more variety than we see locally.

#2: Atmospheric Diversity (Sulfuric acid clouds to metallic vapor)

Earth’s nitrogen-oxygen atmosphere and Jupiter’s ammonia clouds seem exotic enough, but exoplanets take atmospheric diversity to wild extremes. WASP-76b is a gas giant so hot—over 4,000°F—that iron vapor condenses and falls as molten metal rain on its night side. Venus’s thick sulfuric acid clouds are tame by comparison. Other exoplanets have hazes of carbon monoxide, titanium oxide, or even clouds made of silicate glass particles. The hot Neptune HAT-P-11b stunned researchers when it revealed water vapor in its upper atmosphere, while GJ 1214b appears to be a “steam world,” with a thick water-rich envelope. These alien atmospheres not only demonstrate chemical diversity but also offer clues about planet evolution and even potential biosignatures.

#3: Orbital Weirdness (Orbits less than 1 day to thousands of years)

While Earth’s one-year orbit seems normal, many exoplanets have orbital periods far more extreme. Some “ultra-short-period” planets like Kepler-78b complete a full orbit in just 8.5 hours. Others, such as HR 5183 b, follow wildly elongated paths, taking hundreds or thousands of Earth years to complete an orbit. Our solar system’s orbits are mostly circular, but many exoplanets have eccentric orbits resembling comets. These orbital oddities hint at chaotic pasts—possibly involving gravitational interactions, near-collisions, or planetary migration. Such dynamics suggest that stable, circular orbits like ours may be the exception rather than the rule in planetary systems across the galaxy.

#4: Temperatures Beyond Extremes (Below –400°F to over 4,000°F)

The hottest exoplanets, such as KELT-9b, reach surface temperatures over 7,800°F—hotter than some stars—while rogue planets drifting through space with no star to warm them may dip below –400°F. By comparison, the hottest planet in our solar system, Venus, barely reaches 880°F. These extreme temperatures dramatically affect atmospheric chemistry, surface conditions, and habitability. A planet’s proximity to its star, the star’s type, and the planet’s own atmosphere all play a role. Some of the hottest exoplanets glow with thermal radiation like miniature stars. The coldest might have frozen atmospheres and oceans made of liquid hydrogen. It’s a climate spectrum no one expected until we began seeing it with our own telescopes.

#5: Composition Variety (Carbon planets to water worlds)

Planets in our solar system are generally silicate-based or gas-dominated, but exoplanets offer exotic materials. Some might be made primarily of carbon, forming diamonds under immense pressure. Others, like Kepler-22b, may be ocean planets, covered entirely in deep, global seas. Researchers have even proposed “lava planets,” such as 55 Cancri e, where temperatures are so high that molten rock covers the surface. The detection of different elements through spectroscopy has hinted at planets made of graphite, ice, gas, or unknown materials entirely. These worlds may have formed in star systems with unusual elemental ratios, altering the fundamental makeup of the planets that arise.

#6: Host Star Variety (From red dwarfs to pulsars)

All eight planets in our solar system orbit the same type of star: a yellow G-type main-sequence star. But exoplanets orbit an astonishing variety of stars. Many have been found around red dwarfs—cool, long-lived stars that make up the majority of stars in the Milky Way. Others orbit blue giants, subgiants, and even white dwarfs. Most surprisingly, some planets, like those in the PSR B1257+12 system, orbit pulsars—spinning neutron stars that emit beams of radiation. These planets likely formed from the wreckage of stellar explosions. The diversity of host stars influences planetary temperature, radiation levels, and the potential for life in complex and unpredictable ways.

#7: Stellar Proximity (Less than 1 million miles to over 10 billion miles)

The closest planet to our Sun, Mercury, orbits at about 36 million miles. But exoplanets break this mold entirely. Some hug their stars at less than 1 million miles, resulting in tidal locking, extreme heating, and atmospheric evaporation. Others lie tens of billions of miles out, perhaps orbiting in the far reaches of their star’s gravitational influence. This wide range of distances affects orbital periods, climate, and even survival. Close-in exoplanets often face intense radiation and can lose their atmospheres, while far-out planets may never experience daylight or surface warmth. Some exoplanets even orbit binary or trinary star systems, experiencing wildly complex light and gravitational cycles.

#8: Migration and Chaos (Planets that change orbits)

In our solar system, planetary orbits have been relatively stable for billions of years. However, many exoplanetary systems show evidence of violent pasts. Planets can migrate inward or outward due to gravitational interactions with other planets or their protoplanetary disks. These movements can lead to planets switching places, colliding, or being ejected entirely. Some hot Jupiters likely formed far from their stars and moved inward, disrupting any smaller worlds in their path. In other systems, gravitational resonances lock planets into complex, rhythmic orbits. The result is a level of dynamic evolution rarely seen in our own backyard.

#9: Moon Scarcity (Very few confirmed exomoons)

Our solar system is rich in moons—Jupiter and Saturn each host dozens, and even Earth has its faithful satellite. Yet among the thousands of exoplanets discovered, very few confirmed exomoons exist. Part of this is due to observational difficulty—moons are smaller and harder to detect. But it raises intriguing questions: Are moons less common in other systems, or are we simply not equipped to find them yet? One tantalizing candidate, Kepler-1625b-i, may be a Neptune-sized moon orbiting a Jupiter-sized planet. If confirmed, it would suggest that large moons can exist far from our solar system, potentially even with their own atmospheres and habitable conditions.

#10: Discovery and Technology (Radial velocity, transit method, direct imaging)

The planets of our solar system were discovered over centuries through direct observation. In contrast, exoplanets are mostly found using indirect methods. The two main techniques are the radial velocity method, which detects star wobbles due to planetary gravity, and the transit method, which measures dips in starlight as a planet crosses in front of its star. Other tools include gravitational microlensing and direct imaging using advanced telescopes. The technological revolution behind exoplanet discovery has driven innovation in optics, data analysis, and space instrumentation. Projects like NASA’s Kepler and TESS missions have revolutionized our understanding of the universe, and the James Webb Space Telescope is poised to further transform our ability to characterize distant worlds.

Expanding the Planetary Frontier

The discovery of exoplanets has redefined our understanding of what a planet can be. The neat, orderly model provided by our solar system now looks like just one example in a vast, chaotic, and dazzlingly diverse universe. Exoplanets come in every size, temperature, and composition imaginable, orbiting all types of stars and sometimes none at all. They migrate, collide, and evolve in ways that challenge long-held theories of planetary formation and stability. As our tools and techniques improve, the next decades will likely uncover even more bizarre examples of planetary behavior, perhaps including Earth-like worlds that hint at life beyond our solar system. What began as a curiosity has now become one of the most exciting frontiers in astronomy—one that’s expanding our view of the cosmos, and our place within it.