Eight planets, billions of smaller rocky and icy bodies, and vast clouds of debris all orbit the sun. The solar system contains far more variety than most people realize, from house-sized rocks tumbling through the inner solar system to frozen worlds so distant they take millions of years to complete a single orbit.
The Eight Planets
The four planets closest to the sun, Mercury, Venus, Earth, and Mars, are known as terrestrial planets because they have solid, rocky surfaces. Beyond them lie four much larger worlds without hard surfaces at all. Jupiter and Saturn are gas giants, made mostly of swirling hydrogen and helium above a dense core. Uranus and Neptune are ice giants, with interiors rich in heavier compounds like water, ammonia, and methane.
To qualify as a planet under the International Astronomical Union’s definition, an object must orbit the sun, have enough mass to pull itself into a roughly round shape, and be large enough that its gravity has cleared away other objects of similar size near its orbit. That third requirement is what separates planets from everything else on this list.
Dwarf Planets
Dwarf planets meet two of those three criteria. They orbit the sun and are nearly round, but they haven’t cleared the neighborhood around their orbits. Pluto is the most famous example, reclassified from planet to dwarf planet in 2006. It shares its orbital region with thousands of other icy objects and simply isn’t massive enough to dominate that space the way Neptune dominates its own orbit.
The IAU recognizes five dwarf planets: Pluto, Eris, Haumea, Makemake, and Ceres. Ceres sits in the asteroid belt between Mars and Jupiter, while the other four occupy the outer solar system. All five except Ceres have at least one moon. Dozens of additional objects are strong candidates for dwarf planet status but haven’t been formally classified yet.
The Asteroid Belt
Between the orbits of Mars and Jupiter lies the main asteroid belt, a broad ring of rocky and metallic objects left over from the solar system’s formation. These chunks of rock never coalesced into a planet, largely because Jupiter’s powerful gravity stirred them up and prevented them from merging. Estimates suggest the belt contains roughly 1.9 million asteroids larger than 1 kilometer in diameter, plus countless smaller fragments.
Most asteroids are irregularly shaped, too small for gravity to pull them into spheres. They range from boulders a few meters across to Ceres, which at about 940 kilometers wide is large enough to be round and classified as a dwarf planet. Despite what movies suggest, the belt is mostly empty space. Spacecraft have passed through it many times without incident.
Comets
Comets are balls of ice, rock, and dust that develop glowing tails when they approach the sun and their surface material vaporizes. They come from two distinct reservoirs. Short-period comets, which orbit the sun in less than about 200 years, typically originate in the Kuiper Belt, a region just beyond Neptune’s orbit. Jupiter-family comets, a subset of these, complete an orbit roughly every 9 to 30 years.
Long-period comets come from the Oort Cloud, an enormous shell of icy objects surrounding the solar system at vast distances. These comets can take hundreds of thousands or even millions of years to complete a single orbit. A passing star’s gravity occasionally nudges one of these objects inward, sending it on a long fall toward the sun. Short-period comets tend to orbit in roughly the same plane as the planets, while Oort Cloud comets approach from random angles.
Kuiper Belt Objects
The Kuiper Belt extends beyond Neptune’s orbit and contains hundreds of thousands of icy objects larger than 100 kilometers wide. So far, astronomers have cataloged more than 2,000 of these trans-Neptunian objects, representing only a tiny fraction of the estimated total population.
Pluto, Eris, Haumea, and Makemake are all Kuiper Belt objects large enough to be classified as dwarf planets. Eris, the largest known member of the “scattered disk” (a more distant, spread-out extension of the Kuiper Belt), is slightly more massive than Pluto. Sedna is another notable object in this region, following an extremely elongated orbit that takes it far beyond the main belt. These frozen worlds preserve material from the earliest days of the solar system, essentially unchanged for over four billion years.
Trojans and Centaurs
Trojan asteroids share a planet’s orbit, locked into stable gravitational sweet spots about 60 degrees ahead of and behind the planet. Jupiter has the largest known collection, with thousands of Trojans clustered in two swarms along its orbital path. Their compositions look similar to Kuiper Belt objects, and many scientists believe they were originally outer solar system bodies trapped in their current positions during a period of planetary migration early in the solar system’s history. NASA’s Lucy mission is currently visiting several of Jupiter’s Trojans to learn more about their origins.
Centaurs occupy orbits between Jupiter and Neptune, crossing the paths of one or more giant planets. This makes their orbits unstable over millions of years. They’re thought to be a transitional population: objects that escaped the Kuiper Belt and are gradually being pulled inward, where they may eventually become short-period comets. Some Centaurs already show comet-like activity, developing faint tails as solar heating reaches their icy surfaces.
The Oort Cloud
The most distant objects still gravitationally bound to the sun reside in the Oort Cloud, a theoretical shell of icy bodies stretching from roughly 2,000 to 100,000 astronomical units from the sun. For perspective, one astronomical unit is the distance from Earth to the sun, so the outer edge of the Oort Cloud is nearly two light-years away, a significant fraction of the distance to the nearest star.
No Oort Cloud object has ever been directly observed in place. The cloud’s existence is inferred from the orbits of long-period comets that fall inward from that region. Scientists estimate it may contain trillions of icy bodies, most of which formed much closer to the sun, near the orbits of Uranus and Neptune, before being flung outward by gravitational interactions with Jupiter billions of years ago.
Interstellar Visitors
Occasionally, objects from other star systems pass through. The first confirmed interstellar visitor, ‘Oumuamua, was detected in October 2017 by the Pan-STARRS1 telescope in Hawaii. It was a small, oddly shaped object moving too fast to be captured by the sun’s gravity, passing through on a one-way trip. The second, a comet named 2I/Borisov, was discovered by amateur astronomer Gennadiy Borisov in August 2019 using a telescope he built himself. Unlike ‘Oumuamua, Borisov looked like a conventional comet, trailing gas and dust as it swung past the sun.
These objects don’t orbit the sun. They follow open, hyperbolic paths that carry them back out into interstellar space. But their brief passages through the solar system offer rare glimpses of material formed around distant stars, and astronomers expect to find more of them as telescope technology improves.