Formation and evolution of the Solar System

The Solar System began forming about 4.6 billion years ago when a part of a large cloud of gas and dust collapsed under its own gravity. Most of this collapsing material gathered in the center to create the Sun, while the rest spread out into a flat, spinning disk. From this disk, the planets, moons, asteroids, and other smaller objects formed.

The idea that the Solar System formed from a spinning cloud of material was first suggested in the 1700s by scientists like Emanuel Swedenborg, Immanuel Kant, and Pierre-Simon Laplace. Since then, many areas of science, such as astronomy, chemistry, geology, physics, and planetary science, have helped us better understand this process. With new discoveries, like the existence of planets around other stars called exoplanets, scientists continue to improve our understanding.

Artist's conception of a protoplanetary disk

The Solar System has changed a lot since it first formed. Some moons came from material around their planets, while others may have been caught by a planet's gravity after forming elsewhere. Big crashes between objects have happened many times and helped shape the system we see today. Far from the Sun, many icy objects orbit in strange paths, called trans-Neptunian objects. The positions of the planets may have moved over time due to the pull of gravity.

In the far future, the Sun will change. In about five billion years, it will grow huge and bright, then shrink down into a small, dim remnant called a white dwarf. Over even longer periods, the Sun may lose all its planets to the pull of other stars, ending up alone in space.

History

Main article: History of Solar System formation and evolution hypotheses

Pierre-Simon Laplace, one of the originators of the nebular hypothesis

People have had ideas about how the world began for a very long time. But they didn’t think about the Solar System as we know it today until much later. The idea that the Sun is at the center, with the Earth orbiting around it, began with Nicolaus Copernicus in 1543 during a time of big changes in science.

The main idea today about how the Solar System formed is called the nebular hypothesis. It was first talked about in the 1700s by Emanuel Swedenborg, Immanuel Kant, and Pierre-Simon Laplace. Some people were not sure about this idea because it seemed to not explain why the Sun doesn’t spin as much as the planets do. But since the 1980s, we’ve seen young stars with disks of dust and gas around them, just like this idea says.

We also learned that the Sun’s energy comes from special reactions in its center, turning hydrogen into helium. This discovery was made possible by work on a big scientific idea called theory of relativity. Later, scientists suggested that bigger stars can make many other elements, which can then become part of new star systems when those stars change.

Formation

Subsequent evolution

The planets might not have formed exactly where they are today. Scientists now think that the early Solar System could have looked very different. Some big objects, maybe as large as Mercury, might have been in the inner Solar System. The outer parts might have been closer together, and the area where objects like Pluto live might have been nearer to the Sun.

Terrestrial planets

Simulation showing outer planets and Kuiper belt:a) Before Jupiter/Saturn 2:1 resonanceb) Scattering of Kuiper belt objects into the Solar System after the orbital shift of Neptunec) After ejection of Kuiper belt bodies by Jupiter Orbit of Jupiter Orbit of Saturn Orbit of Uranus Orbit of Neptune

In the beginning, the inner Solar System had many small worlds about the size of the Moon or Mars. Over time, these worlds crashed into each other and merged, growing bigger until they became the four planets we know today. One big crash is thought to have created our Moon, and another might have made Mercury smaller.

Asteroid belt

Between the inner planets and Jupiter is the asteroid belt. It once had enough material to make two or three Earth-sized planets. But Jupiter’s strong gravity changed everything. It pulled and pushed these small worlds, breaking many apart and scattering them. This left the asteroid belt as we see it today, filled with smaller pieces instead of big planets.

Meteor Crater in Arizona. Created 50,000 years ago by an impactor about 50 metres (160 ft) across, it shows that the accretion of the Solar System is not over.

Planetary migration

Uranus and Neptune, the outer planets, might not have formed where they are now. They could have started closer to Jupiter and Saturn and then moved outward over many millions of years. This movement helped shape the distant parts of our Solar System, like the areas where comets come from.

Late Heavy Bombardment and after

As the planets moved, they sent many asteroids into the inner Solar System. This might have caused a time long ago when lots of space rocks hit the planets and moons. After this, comets were sent far out to form a distant cloud, and some later came back into the inner Solar System. The asteroid belt continued to change with collisions, breaking some objects apart and sometimes forming new ones.

Moons

Future

Astronomers believe the Solar System will not change much until the Sun runs out of hydrogen fuel. At that point, the Sun will grow into a red giant, possibly swallowing the inner planets like Mercury and Venus. Eventually, it will shrink down to a white dwarf, and the outer planets will continue to orbit this small remnant.

Neptune and its moon Triton, taken by Voyager 2. Triton's orbit will eventually take it within Neptune's Roche limit, tearing it apart and possibly forming a new ring system.

The Solar System can be unpredictable over very long periods. The orbits of planets can change, making it hard to know exactly where they will be far in the future. For example, Mars might eventually move into an orbit that could bring it close to Earth.

Moons around planets can also change due to gravitational forces. Some moons move farther away from their planets while others spiral inward and might crash into the planet or break apart. The Earth and its Moon are an example where the Moon is slowly moving away from Earth.

As the Sun ages, it will grow brighter and hotter. In about 600 million years, it will become too hot for trees and forests to survive on Earth. Eventually, in about 1.1 billion years, the Sun will be so bright that Earth will become too hot for any complex life, leaving only simple single-celled organisms. Mars might become warmer and possibly support life.

In about 5.4 billion years, the Sun will expand into a red giant, possibly swallowing Mercury and Venus. Earth’s fate is uncertain—it might be vaporized by the Sun’s expansion. Later, the Sun will shrink to a white dwarf, and the remaining planets will continue to orbit this dim remnant, eventually becoming cold and lifeless.

Galactic interaction

The Solar System travels around the center of the Milky Way galaxy in a big circle. It is about 30,000 light years from the center and moves at a speed of about 220 kilometers per second. It takes between 220 to 250 million years to complete one trip around the center.

Some scientists think the Solar System’s path through the galaxy might be connected to times when many plants and animals on Earth died out long ago. One idea is that as the Sun moves through the galaxy, it sometimes passes through a crowded area filled with stars. These areas might send more objects toward our Solar System, increasing the chance of big impacts.

Galactic collision and planetary disruption

Main article: Andromeda–Milky Way collision

The Andromeda Galaxy is moving toward the Milky Way and will collide with it in about 4 billion years. When this happens, the shape of both galaxies will change, but the Solar System is not likely to be greatly affected. Even though the galaxies will merge, the Sun and its planets should stay in their orbits. Over a very long time, however, passing stars might eventually pull the planets away from the Sun. This would happen in about 1 quadrillion (10¹⁵) years, marking the end of the Solar System as we know it.

Chronology

Scientists use special tools to figure out how old things are. They believe the Solar System is about 4.6 billion years old. The oldest bits of rock on Earth are about 4.4 billion years old, but rocks this old are hard to find because Earth’s surface keeps changing. To learn more, scientists look at rocks from space called meteorites. These meteorites are also about 4.6 billion years old, showing that the Solar System is at least that old.

Looking at other stars, scientists see that stars with discs of gas around them are usually one to three million years old. Stars older than 10 million years often don’t have much gas left, which helps us understand how the Solar System grew and changed over time.

Chronology of the formation and evolution of the Solar System
Phase	Time since formation of the Sun	Time from present (approximate)	Event
Pre-Solar System	Billions of years before the formation of the Solar System	Over 4.6 billion years ago (bya)	Previous generations of stars live and die, injecting heavy elements into the interstellar medium out of which the Solar System formed.
Pre-Solar System	~ 50 million years before formation of the Solar System	4.6 bya	If the Solar System formed in an Orion Nebula-like star-forming region, the most massive stars are formed, live their lives, die, and explode in supernova. One particular supernova, called the primal supernova, nicknamed Coatlicue, possibly triggers the formation of the Solar System.
Formation of Sun	0–100,000 years	4.6 bya	Pre-solar nebula forms and begins to collapse. Sun begins to form.
	100,000 – 50 million years	4.6 bya	Sun is a T Tauri protostar.
	100,000 – 10 million years	4.6 bya	By 10 million years, gas in the protoplanetary disc has been blown away, and outer planet formation is likely complete.
	10 million – 100 million years	4.5–4.6 bya	Terrestrial planets form. Giant impact occur and the Moon form. Giant impacts phase; water delivered to Earth.
Main sequence	50 million years	4.5 bya	Sun becomes a main-sequence star.
	200 million years	4.4 bya	Oldest known rocks on the Earth formed.
	500 million – 600 million years	4.0–4.1 bya	Orbital resonance between Jupiter and Saturn moves Neptune out into the Kuiper belt. Late Heavy Bombardment occurs in the inner Solar System.
	800 million years	3.8 bya	Oldest known life on Earth. Oort cloud reaches maximum mass.
	4.6 billion years	Today	Sun remains a main-sequence star.
	6 billion years	1.4 billion years in the future	Sun's habitable zone moves outside of the Earth's orbit, possibly shifting onto Mars's orbit.
	7 billion years	2.4 billion years in the future	The Milky Way and Andromeda Galaxy begin to collide. Slight chance the Solar System could be captured by Andromeda before the two galaxies fuse completely.
Post–main sequence	10 billion – 12 billion years	5–7 billion years in the future	Sun has fused all of the hydrogen in the core and starts to burn hydrogen in a shell surrounding its core, thus ending its main sequence life. Sun begins to ascend the red-giant branch of the Hertzsprung–Russell diagram, growing dramatically more luminous (by a factor of up to 2,700), larger (by a factor of up to 250 in radius), and cooler (down to 2600 K): Sun is now a red giant. Mercury, Venus and possibly Earth are swallowed. During this time Saturn's moon Titan may become habitable.
Post–main sequence	~ 12 billion years	~ 7 billion years in the future	Sun passes through helium-burning horizontal-branch and asymptotic-giant-branch phases, losing a total of ~30% of its mass in all post-main-sequence phases. The asymptotic-giant-branch phase ends with the ejection of its outer layers as a planetary nebula, leaving the dense core of the Sun behind as a white dwarf.
Remnant Sun	~ 1 quadrillion years (10¹⁵ years)	~ 1 quadrillion years in the future	Sun cools to 5 K. Gravity of passing stars detaches planets from orbits. Solar System ceases to exist.