How Does the Solar System Work? A Tour of Our Cosmic Neighborhood
The solar system is a gravitationally bound system of the Sun and everything that orbits it. Learn what keeps planets in orbit, why each planet is radically different, and the surprising facts about our cosmic neighborhood that most people get wrong.
Explain It Simply Editorial Team
Published May 17, 2026
What Holds It All Together: Gravity and Orbits
The solar system exists because of a balance between two forces: gravity pulling everything toward the Sun, and the sideways momentum (orbital velocity) of each object trying to fly away in a straight line. An orbit is essentially a continuous state of falling toward the Sun and missing.
Isaac Newton proved in 1687 that the same force making an apple fall from a tree keeps the Moon orbiting Earth and planets orbiting the Sun. The strength of gravity between two objects depends on their masses and the distance between them — double the distance, and gravity drops to one-quarter strength (inverse square law).
The solar system formed approximately 4.6 billion years ago from a giant molecular cloud — a vast region of gas (mostly hydrogen and helium) and dust. Something disturbed this cloud (possibly a nearby supernova), causing it to collapse under its own gravity. As it collapsed, it began spinning faster (like a figure skater pulling in their arms), flattening into a disk. The dense center became the Sun. Remaining material in the disk gradually clumped together through collisions, forming planetesimals, then protoplanets, then the planets we see today.
This process — called accretion — explains why all planets orbit in roughly the same plane (the ecliptic) and in the same direction (counterclockwise when viewed from above the North Pole). They inherited the rotation of the original disk.
Kepler's laws of planetary motion (1609-1619) describe orbits mathematically. Planets travel in ellipses (not perfect circles) with the Sun at one focus. A planet moves faster when closer to the Sun and slower when farther away. More distant planets take longer to complete an orbit — Mercury orbits in 88 Earth days; Neptune takes 165 Earth years.
The Sun contains 99.86% of the solar system's mass. The four inner rocky planets are tiny compared to the four outer gas and ice giants.
The Inner Planets: Rocky Worlds
The four inner planets — Mercury, Venus, Earth, and Mars — are small, dense, rocky worlds. They formed close to the young Sun, where temperatures were too high for lightweight gases (hydrogen, helium) to condense, leaving only heavier elements (iron, silicon, oxygen) to form solid planets.
Mercury is the smallest planet (slightly larger than our Moon) and closest to the Sun. It has essentially no atmosphere, resulting in extreme temperature swings: 430°C (800°F) on the sun-facing side to -180°C (-290°F) on the dark side. Despite being closest to the Sun, Mercury is NOT the hottest planet — that distinction belongs to Venus.
Venus is Earth's 'evil twin' — nearly identical in size but with a runaway greenhouse effect that makes its surface temperature 465°C (870°F), hot enough to melt lead. Its thick atmosphere of CO2 (96.5%) creates surface pressure 92 times greater than Earth's — equivalent to being 900 meters underwater. Venus rotates backwards (east to west) so slowly that a Venusian day (243 Earth days) is longer than its year (225 Earth days).
Earth is the only known planet with liquid surface water, plate tectonics, and life. Our relatively large Moon (formed from a giant impact 4.5 billion years ago) stabilizes Earth's axial tilt, preventing extreme seasonal variations that could make complex life impossible. Earth's magnetic field, generated by its liquid iron core, deflects solar wind that would otherwise strip away our atmosphere.
Mars once had liquid water, a thicker atmosphere, and possibly conditions suitable for life. Today it's a cold desert (-60°C average) with an atmosphere less than 1% as dense as Earth's. Mars has the solar system's tallest volcano (Olympus Mons, 21.9 km — nearly 3x Mount Everest's height) and longest canyon system (Valles Marineris, 4,000 km long).
The Outer Planets: Gas and Ice Giants
Beyond the asteroid belt, the four outer planets are massive worlds composed primarily of gas and ice. They formed far enough from the Sun that lightweight gases could condense and accumulate.
Jupiter is the solar system's heavyweight — more massive than all other planets combined (318 Earth masses). It's primarily hydrogen and helium, with no solid surface. Its Great Red Spot is a storm larger than Earth that has been raging for at least 350 years. Jupiter has at least 95 known moons, including Europa (which likely has a liquid ocean beneath its ice crust that could harbor life) and Ganymede (the solar system's largest moon, bigger than Mercury).
Saturn is famous for its spectacular ring system — billions of particles of ice and rock ranging from grain-sized to house-sized, orbiting in a disk that's 282,000 km wide but only about 10 meters thick. Saturn is the least dense planet — it would float in a bathtub large enough to hold it (density 0.687 g/cm³, less than water). Its moon Titan has a thick atmosphere and liquid methane lakes on its surface.
Uranus and Neptune are 'ice giants' — smaller than Jupiter and Saturn and composed more of water, ammonia, and methane ices than hydrogen gas. Uranus is tilted 98° on its axis, essentially rolling on its side as it orbits — likely the result of a massive collision billions of years ago. Neptune, despite receiving far less solar energy, has the fastest winds in the solar system (2,100 km/h) and generates more internal heat than it receives from the Sun.
Beyond Neptune lies the Kuiper Belt — a vast region of icy bodies including Pluto (reclassified as a dwarf planet in 2006) and billions of comets. Even farther out, the theoretical Oort Cloud extends nearly halfway to the nearest star and is the source of long-period comets.
Scale, Exploration, and What We're Still Discovering
Human exploration of the solar system is one of our species' greatest achievements. Every planet has been visited by at least one spacecraft. Highlights include the Voyager missions (launched 1977, now in interstellar space — still transmitting data from over 24 billion kilometers away), the Mars rovers (Curiosity and Perseverance, actively exploring), the Cassini mission (13 years orbiting Saturn), and New Horizons (which revealed Pluto in stunning detail in 2015).
The James Webb Space Telescope (launched 2021) is transforming our understanding of planetary formation by observing protoplanetary disks around young stars — essentially watching other solar systems being born.
Current mysteries include: Is there microbial life in the subsurface oceans of Europa or Enceladus? What is the hypothetical 'Planet Nine' — a massive undiscovered planet potentially lurking in the outer solar system, whose gravitational influence seems to explain unusual orbits of distant Kuiper Belt objects? How did the solar system's architecture emerge from the chaos of its formation?
Our solar system is just one of billions. As of 2024, over 5,500 exoplanets have been confirmed orbiting other stars, with many more candidates awaiting verification. Some orbit in their star's 'habitable zone' — the range of distances where liquid water could exist on a rocky surface. The question of whether we're alone in the universe may be answered in our lifetime.
Sources: NASA Solar System Exploration (solarsystem.nasa.gov), European Space Agency (esa.int), JPL (Jet Propulsion Laboratory), IAU (International Astronomical Union), de Pater & Lissauer 'Planetary Sciences' (Cambridge University Press).
💡 AHA Moment
Here's the insight about the solar system that completely resets your sense of scale: the solar system is almost entirely empty space.
If you shrank the Sun to the size of a basketball, Earth would be a small peppercorn 26 meters (85 feet) away. Jupiter would be a walnut 135 meters away. Neptune would be a small plum 0.77 kilometers (half a mile) away. The nearest star, Proxima Centauri, would be another basketball 6,900 kilometers (4,300 miles) away — roughly the distance from New York to Berlin.
Now here's the truly humbling part: if you collected all the matter in the entire solar system — every planet, moon, asteroid, comet, and grain of dust — the Sun contains 99.86% of it. Everything else — all eight planets, 200+ moons, billions of asteroids and comets, all the rings of Saturn, all the spacecraft humanity has ever launched — is a 0.14% rounding error.
You don't live in a solar 'system.' You live in a star, with some debris.
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