Where Did the Solar System Come From? Evidence and Models

A question to start

No one was there 4.6 billion years ago to watch the solar system form. So how can scientists claim to know how it happened? The answer is the same way a detective solves a case: by collecting clues (evidence) and finding the explanation that fits all of them. In this lesson you will also see that our ideas about the solar system have changed over time as new evidence was found.

Part 1: Changing models of the solar system

The geocentric model. For more than a thousand years, many astronomers in Europe, the Middle East and North Africa used the model of Ptolemy, a scholar who worked in Alexandria (Egypt) around 150 CE. Earth sat still at the centre, and the Sun, Moon, planets and stars moved around it. It matched everyday experience (the ground does not feel like it is moving) and could predict planet positions fairly well, but it needed complicated extra loops to explain why planets sometimes seem to move backward across the sky.

The heliocentric model. In 1543, Nicolaus Copernicus published a model with the Sun at the centre and Earth as one of the planets orbiting it. (Ancient Greek astronomer Aristarchus had suggested a Sun-centred idea much earlier, but it was not widely accepted.) The backward motion of planets became easy to explain: Earth simply passes slower outer planets, like a car passing a truck on the highway.

New evidence. In 1610, Galileo Galilei used one of the first telescopes and observed:

• four moons orbiting Jupiter, proving that not everything orbits Earth;
• Venus going through a full set of phases, like the Moon. This can happen only if Venus orbits the Sun.

Around the same time, Johannes Kepler used careful measurements by Tycho Brahe to show that planets move in ellipses (stretched circles), not perfect circles. The heliocentric model with elliptical orbits predicted planet positions much better, and it became the accepted model.

The lesson for science: models are kept as long as they fit the evidence. When better observations appear, models are revised or replaced.

Part 2: The nebular theory

Today's scientific explanation for how the solar system formed is the nebular theory:

1. About 4.6 billion years ago, a huge cloud of gas (mostly hydrogen and helium) and dust began to collapse under its own gravity. Something may have given it a push, such as the shock wave from a nearby exploding star.
2. As it collapsed, it spun faster (like a figure skater pulling in their arms) and flattened into a disk.
3. Most of the material fell to the centre, became very hot and dense, and fusion began: the Sun was born.
4. In the disk, dust grains stuck together to form pebbles, then boulders, then planetesimals kilometres across. These collided and grew into planets.
5. Close to the hot young Sun, only rock and metal could stay solid, so the inner planets became small and rocky. Beyond the frost line (roughly between the orbits of Mars and Jupiter), it was cold enough for ices to form too. There was much more solid material, so planets grew big enough to pull in huge amounts of hydrogen and helium gas, becoming the giants.
6. Leftover pieces became asteroids (in the asteroid belt) and comets (in the Kuiper belt and Oort cloud).

Part 3: The evidence

The nebular theory is accepted because it explains many different observations at once. Each row of the table is one clue.

Exceptions that make sense

Some things don't follow the pattern. Venus spins slowly backward, and Uranus is tipped on its side. Scientists think giant collisions in the early solar system could explain these. Earth's Moon is thought to have formed when a body about the size of Mars crashed into the young Earth; Moon rocks brought back by the Apollo astronauts are very similar in composition to Earth's rocky outer layers, which supports this idea.

Canadian clue: some of the oldest rocks known on Earth, the Acasta Gneiss, are found in the Northwest Territories. They are about 4 billion years old, which fits with Earth forming about 4.5 billion years ago.