Chapter 2
The Solar System and Earth

Since the Big Bang, the Universe has been expanding, recycling itself through the birth and death of stars. Every supernova leaves behind the raw materials for something new.

Watch as Earth evolves from a violent, molten rock into a planet that somehow supports poetry, pizza, and primates. See how astronomers mapped our planet's place in the cosmic neighborhood. And explore Earth's surface, the cause of earthquakes, volcanic eruptions, and continental "surfing."

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2:37
Chapter at a Glance
35 Minutes
1 Threshold
4 Videos
2 Galleries

What Is the Solar System?

Is it daytime? Look up. That bright yellowish thing in the sky is your star. Your star is one of a septillion stars in our Universe. Your planet is one of over 100 billion planets in our galaxy. There are 2 trillion galaxies in our Universe.

You live on a speck of a rock careening around a mid-tier star with a bunch of other rocks and gas.

Don't let that make you feel small. Your home is extraordinary.

Threshold 4: Earth & Solar System

How tasty morsels of gas and rock created our home.
Blue and grey expanse of space with a vertical beam of light, centered is the Earth represented by a tiny white pixel.
Credit: NASA/JPL-Caltech, public domain.

Pale Blue Dot, a photo of Earth from 3.7 billion miles away.

What's in the Solar System?

Earth is part of a group of planets, moons, and smaller objects. All these objects orbit around one star, which we call the Sun. (No! Don't look directly at it... keep reading.) The Sun's gravity pulls all these objects into its orbit. Everything orbiting the Sun is part of the Solar System.

As of 2026, our Solar System contains:

Infographic displaying various compositions of the solar system including: a fiery star, planets, moons, asteroids, comets, dust, and gas.
Credit: One star, by NASA/Goddard/SDO, public domain; Eight planets, Collage by CactiStaccingCrane, CC BY-SA 4.0.; Five dwarf planets, by NASA / JHUAPL / SwRI, public domain; 970 moons, by Lick Observatory (Moon); Io(NASA/JPL/University of Arizona); NASA/JPL-Caltech/DLR (Europa), public domain; Millions of asteroids, comets, and other objects, by NASA/JPL-Caltech/UCAL/MPS/DLR/IDA (Asteroid Vesta); Giotto Project, ESA (Halley's Comet), public domain; Lots of dust and gas, by NASA, ESA, and G. Bacon (STScI), public domain.

How Big Is the Solar System?

Our Solar System is three light-years (18.6 trillion miles) across. Earth is 93 million miles from the Sun. Neptune is 2.8 billion miles out. The edge of the Oort cloud is about 1.5 light-years from the Sun.

If you jumped in NASA's best spaceship, it would take you tens of thousands of years to reach the edge of our Solar System. Don't try this at home.

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How Did the Sun Form?

Wait—"How did the Sun form?" Did you skip Threshold 2? It's a star.

Fine. One last time, for the people in the back:

Eons ago, an ancient supernova exploded across the Milky Way Galaxy, leaving behind a cloud of debris. About 4.6 billion years ago, some of that cloud collapsed under gravity. The center grew so hot and dense that hydrogen nuclei fused into helium. There you go. We got lucky. Our Sun is nice and mediocre. It's not so massive that it will burn through all its fuel in a few million years. And it's not one of those small stars that burns low and slow for trillions of years. It's just-right, allowing the time and heat for interesting stuff to develop around it. The Sun's been burning for 5 billion years, and its lifespan is about 10 billion. Humanity and all our nonsense is sorta like the Sun's midlife crisis. Look out, Universe! This star just turned 40 and bought a bright-red Lamborghini to avoid thoughts of its own mortality.

Activity

The Life Cycle of Our Sun

See how our Sun and stars will evolve

  • Stellar Nursery

    Stellar Nursery

    Our Sun was born within a dense nebula 4.6 billion years ago. Gas and dust contracted into a giant cloud, then floated in one of the spiral arms of the Milky Way.

  • Heating Up

    Heating Up

    As our Sun aged, it grew larger, brighter, and hotter—eventually causing nuclear fusion in the protons at the centers of its atoms. As they exploded, a tiny bit of matter transformed into a great deal of energy, bursting into sunshine. The Sun was born.

  • Shining Bright

    Shining Bright

    The Sun is currently stable, about halfway through its life cycle. It's estimated it will live for about another five billion years before consuming all the hydrogen in its core and transforming into a red giant.

  • Gradually Increasing

    Gradually Increasing

    Our Sun is continually growing. About three billion years from now, our Sun will be 40% larger than its current state and all life on Earth will probably cease to exist.

  • Expansion

    Expansion

    The greater a star's mass, the shorter its lifecycle. High-mass stars live for one million to tens of millions of years. Low-mass stars, like our Sun, live for tens of millions to trillions of years. When the Sun is about nine billion years old, it will begin to expand rapidly.

  • Red Giant

    Red Giant

    Red giants are stars that have exhausted the supply of hydrogen in their cores. Their outer envelope is lower in temperature, giving them a reddish-orange hue.

  • Planetary Nebula

    Planetary Nebula

    As the red giant burns out, it will collapse into a planetary nebula. The outer layers of gas are ejected and the star's core contracts into a white dwarf.

  • White Dwarf

    White Dwarf

    As the planetary nebula collapses further, the star becomes a white dwarf. Then finally, with all its energy exhausted, it—theoretically—expires into a black dwarf. The possibility of the black dwarf stage has not yet been proven, because not enough time has passed since the Big Bang to create one.

Age of star (billions)

How Did the Planets Form?

New elements, combined with the just-right Goldilocks Conditions came together and formed our Solar System.

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14:00

A Tour Across the Solar System

You live in an exceptionally weird neighborhood.


Ultraviolet image of the Sun.
Credit: NASA/GSFC/Solar Dynamics Observatory, public domain.
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The Sun

The Sun is our neighborhood's fusion reactor‐burning 600 million tons of hydrogen a second. It powers every breeze, leaf, and life form on Earth. One day it'll swell and vaporize our planet, but until then, it's doing stellar work.

Side by side images of Mercury, on the left a monochrome view, and on the right a vibrant colorful view.
Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington, public domain.
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Mercury

A tiny metal world cooked by the Sun on one side and frozen solid on the other. Mercury orbits the Sun in just 88 days. This image shows two different views of Mercury. The one on the left is what you'd see with your eyes. On the right, the image is color coded to show the different mineral structures on the planet.

Venus captured against the black expanse of space.
Credit: NASA, public domain.
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Venus

Earth's twin, if Earth lived in a pressure cooker full of acid clouds. Similar size and ingredients, but a totally different outcome, thanks to a runaway greenhouse effect.

Satellite image of the Moon crossing the Earth.
Credit: NASA's Earth Polychromatic Imaging Camera (EPIC) NOAA's Deep Space Climate Observatory (DSCOVR), public domain.
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Earth

The only planet in the whole Solar System with liquid oceans, life, and Wi-Fi. Our home is an improbable cosmic accident. And as far as we know, it's unique.

Wide views of a rocky Mars landscape split into two color versions.
Credit: NASA/JPL-Caltech/ASU/MSSS, public domain.
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Mars

Two images of Mars: The one on top has been color-corrected so you can see the color variation in the rocks. Mars does not actually have a blue sky. Mars is cold, dusty, and a bit tragic. Once, it might have had rivers, oceans, and even life. Today, it's all about dirt, rocks, and robot selfies.

Detailed view of Jupiter, captured against the black expanse of space.
Credit: Enhanced image by Kevin M. Gill (CC-BY) based on images provided courtesy of NASA/JPL-Caltech/SwRI/MSSS.
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Jupiter

A gas giant so big that its mass is twice that of all the other planets, combined. Its Great Red Spot is a hurricane twice as wide as Earth that's been raging for centuries.

Saturn with its prominent gray and black rings against the black expanse of space.
Credit: NASA, ESA, STScI, Amy Simon (NASA-GSFC), public domain.
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Saturn

What if a plant was really into accessories? Saturn's rings are made of billions of icy particles, and it has a weird hexagon-shaped jet stream at its north pole.

A close-up image of Uranus, showing its glowing blue and white atmosphere and its concentric rings.
Credit: NASA, ESA, CSA, STScI; Image Processing: Joseph DePasquale (STScI), public domain.
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Uranus

Uranus is pretty. Cold, pale, and spinning on its side axis, its name still makes scientists giggle, which tells you all you need to know about humanity.

A shimmering and opalescent Neptune and its large moon, which appears as an eight-pointed star.
Credit: NASA, ESA, CSA, STScI; Image Processing: Joseph DePasquale (STScI), Naomi Rowe-Gurney (NASA-GSFC), public domain.
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Neptune

This icy blue giant is the most distant planet. Neptune's winds rip across the planet at 1,200 mph. It takes 165 years for Neptune to orbit the Sun.

Colorful image of Pluto showing its complex surface and set against the black expanse of space.
Credit: ASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute, public domain.
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Pluto—a planet in our hearts

Demoted to a dwarf planet in 2006, Pluto is tiny and icy. According to the International Astronomical Union, "a dwarf planet is an object in orbit around the Sun that is large enough to pull itself into a nearly round shape but has not been able to clear its orbit of debris." Pluto qualifies as a dwarf planet because other objects cross its orbit.

A composite image showing Saturn and its rings, and several moons of Saturn.
Credit: NASA, ESA, CSA, STScI, public domain.
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And a bunch of other stuff

The Solar System is home to more than just planets. Hundreds of moons, like these five orbiting Saturn call the Solar System home. As do millions of asteroids, comets, and other objects. Some astronomers even speculate that there is a ninth, hidden planet orbiting our Sun in the far reaches of the outer Solar System.

Earth: Third Rock from the Sun

Look down. That's Earth. Well, it's probably the floor or sidewalk or something... but under that, that's the Earth! It's a pretty OK place to live right now. But 4.5 billion years ago, no organism could survive where you're sitting. Radiation from a recent supernova kept the planet extremely hot, its surface was molten, and there was no oxygen. Massive meteorites and asteroids kept slamming into the planet, making it even hotter. Cosmic Yelp says: 3/10 stars. Not recommended.

What Did the Young Earth Look Like?

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11:10

History of Astronomy

Ptolemy

Ptolemy
(about 85-165)

Claudius Ptolemy's theory extended the cosmological theories of Aristotle. Earth was at a center of a series of concentric spheres containing the Moon, the planets, the Sun, and a final sphere of fixed stars.

Ibn al-Haytham

Ibn al-Haytham
(965-1040)

An Arab astronomer from Basra, al-Haytham's work in optics enabled future telescope designs, and he published "Doubts Concerning Ptolemy," which prompted later efforts to improve the ancient model.

Copernicus

Copernicus
(1473-1543)

A Catholic, Polish astronomer, Nicolaus Copernicus, synthesized observational data to formulate a Sun-centered cosmology, launching modern astronomy and setting off a scientific revolution.

Galileo

Galileo
(1564-1642)

Galileo Galilei, an Italian Renaissance man, used an improved telescope he designed to collect evidence that supported the Sun-centered model of the Solar System.

Sir Isaac Newton

Isaac Newton
(1643-1727)

By combining physics, mathematics, and astronomy, Newton developed the three basic laws of motion and the theory of universal gravity.

Henrietta Leavit

Henrietta Leavitt
(1868-1921)

By measuring the amount of time between the fluctuating brightness levels of variable stars, Leavitt discovered that it would be possible to estimate their distance away from the Earth, and possible to map the Universe.

Edwin Hubble

Edwin Hubble
(1889-1953)

Hubble drew upon existing ideas and evidence to demonstrate that the Universe was much larger than previously thought and proved that it is expanding—laying the foundations for the Big Bang theory.

Models of the Solar System

A historical map depicting the geocentric model of the universe, split into two circular diagrams on a decorative parchment background.

About 1,900 years ago, the Greek thinker Ptolemy standardized a model of the geocentric Universe, in which the Sun, Moon, and stars all orbit the Earth. Credit: Bartolomeu Velho's illustration of a Ptolemaic geocentric system, public domain.

Geocentric

For most of our history, humans believed the Universe revolved around us—like a whole species of TikTok influencers.

To be fair, if you wanted to study astronomy in the ancient world, you just kinda... looked up at the sky. From that vantage, it would have seemed like the Sun and stars revolved around you—around Earth.

A celestial map depicting the heliocentric system, framed by decorative engravings.

In 1543 CE, Nicolaus Copernicus developed a new model of a heliocentric Universe, in which the Sun sat at the center of everything. Credit: Andreas Cellarius's illustration of the Copernican system, public domain.

Heliocentric

This geocentric view endured for more than 1,400 years until it was toppled by Nicolaus Copernicus, whose work was confirmed by Galileo Galilei.

Through observational evidence, and by using the newly invented telescope, they produced data and logic supporting a heliocentric (Sun-centered) model of the Solar System.

The Atmosphere

How Did Earth's Atmosphere Form?

Early Earth was chaos. See how things settled down enough for the oceans and atmosphere to take shape.


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5:32

Activity

Goldilocks Conditions

Not too hot... Not too cold... Where in our Solar System are the conditions just right to support life?

  • Sun

    Even the coolest sunspots on the surface of the Sun are 5,500 °C. So, yes, way too hot.

  • Mercury

    With no real atmosphere to retain heat, the temperature is a freezing -180 °C at night to a scorching oven of 430 °C during the day.

  • Venus

    Because of a dense atmosphere (over 96% carbon dioxide) it's a runaway greenhouse effect. At 480 °C, it's the hottest planet in the Solar System.

  • Earth

    Our planet contains just the right amount of energy and water to support a diverse variety of life.

  • Mars

    Even though Mars reaches a temperate 20 °C at noon at the equator in summer, it's usually a frozen, arid world. The poles are way too cold to support humans—around -153 °C.

  • Jupiter

    This is a "gas giant"—nothing more than a giant ball of hydrogen, helium, and other gases with little solid surface, with an average temperature of -148 °C.

  • Saturn

    Saturn is too cold and gassy. Life-supporting planets usually posses a heavy-metal core surrounded by a rocky mantle.

  • Uranus

    The surface of Uranus is mostly composed of ices: methane, water, and ammonia. This -216 °C hydrogen and helium atmosphere isn't hospitable.

  • Neptune

    The only energy is lightning, ultraviolet light, and charged particles. Although it's the kind of environment in which scientists believe life began, it's not viable today.

  • Pluto

    Not only does liquid freeze solid on this dwarf planet, but even gases, like methane, will harden when Pluto is at its most distant, 5.9 billion kilometers from the Sun.

Is There Life Beyond Earth?

The textbook version of the Goldilocks Zone says that anything outside a thin band around a star is either too hot or too cold to support the development of life. We're just lucky that Earth sits in that just-right band. That's the textbook version.

But the truth might be...out there.

If you want to find other life in our Solar System, forget Mars. Look to the moons. Jupiter has 95, and Saturn has 274. Some moons, like Europa, Ganymede, and Enceladus hide huge, salty oceans beneath ice. Down there, chemistry and heat might have produced life. Those moons are way outside the "Goldilocks Zone" of our Sun.

Tectonic Plates

A Restless Planet: Plate Tectonics Explained

The Earth's tectonic plates
Credit: By OER Project, CC BY 4.0.

The Earth's crust creeps along at about two centimeters a year, sliding over a layer of molten rock. Where those massive plates meet, things get dramatic—they grind, collide, and occasionally lock up until the tension snaps. When that happens, the planet moves a lot faster than two centimeters a year. The idea that Earth's surface moves wasn't always obvious. Early mapmakers noticed that West Africa looks like it could plug neatly into Brazil. Then, in the early 1900s, German meteorologist Alfred Wegener argued that all the continents had once been part of a supercontinent he called Pangaea.

Why we're all Lava Surfers

Why we're all lava surfers

Journey with our team of Big Historians to Iceland, where the North American and Eurasian plates collide.

How Do We Know About Continental Drift?

Alfred Wegener
Courtesy of the Alfred Wegener Institute for Polar and Marine Research

Alfred Wegener (1880–1930)

Alfred Wegener was the first to gather extensive evidence to support continental drift. He found tropical fossils on Arctic islands and matching mountain ranges on continents now oceans apart. This convinced him that the continents were moving. What he couldn't explain was how whole continents could bulldoze through Earth's crust. That missing piece left his theory adrift and most scientists unconvinced. Wegener died chasing the idea everyone thought was ridiculous... until it wasn't.

Harry Hess
Courtesy of Princeton University Archives

Harry Hess (1906–1969)

During World War II, Harry Hess was hunting enemy submarines in the Pacific using the Navy's new sonar system. But Hess kept the sonar running between battles—mapping the seafloor instead of just scanning for enemies. After the war, he pieced together his data with findings from the Atlantic. The pattern was unmistakable: the ocean floor was growing. By the 1960s, studies of the Mid-Atlantic Ridge confirmed it: the Earth is constantly remaking itself.

Threshold 4

Earth & Solar System

  • Ingredients
  • Goldilocks Conditions
  • New Complexity

  • Which of the following is not a necessary ingredient in planetary formation?

  • Which process best describes how new planets are planets?

  • Why are planets considered to be more complex than stars?

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