And the earth was without form and void, and darkness was upon the face of the deep.
Sometimes, if the night is dark and clear enough, you can look up and see the Milky Way in its arc across the sky. That's our galaxy, and so are most of the other stars visible to the unaided eye. Many, if not most, stars are thought to have planets—some like ours, some fascinating and strange, and a few subject to such extremes they could never harbor anything we recognize as life. Does it ever make you wonder how all those different systems could come together, from dust and cold gas floating in the void? How our own Solar System came to be? How we could know?
Telescopes, microscopes, spectrometers, and gravitational wave detectors all help to piece together the deep history of our Solar System. Thanks to new measurements of very old meteorites, the current consensus on the Solar System's age is 4.5684 billion years old, with an uncertainty of 240,000 years—a tiny fraction of a percent. To learn the age of our home in the cosmos is to learn its story.
How Did the Solar System Form?
The tale of our sun may begin with another star: a predecessor whose fiery death brought about the birth of our solar system. According to the nebular hypothesis, our star system formed from the gravitational collapse of part of a giant molecular cloud, most likely at the edge of a Wolf-Rayet bubble. (More on these in a moment.) Evidence of our solar system's history comes from its chemical composition and physical organization, along with observations of light sources in the night sky.
Like most stars, the Sun likely formed as part of a young star cluster, in our case including other nearby stars such as Alpha Centauri. Within half a billion years of its birth, it had drifted well away from its stellar nursery, moving some three kiloparsecs to its current home on the Orion Arm of our barred spiral galaxy. (A parsec is about 3.26 light-years, so 3 kiloparsecs or kpc is about 9,800 light-years.)
Wolf-Rayet Bubbles
Stars give off a constant stream of ions and plasma known as their stellar wind. This creates a low-density radius of stellar influence mostly cleared of debris, surrounded by a denser shell of material like the surface of a soap bubble. Invisible to the eye, it still marks a star's position in the sky.
Long before quantum physics explained the 21 cm line*, astronomers had spotted colorful points of light in the sky that were big enough to be planets but didn't move like planets, dubbing them planetary nebulae. With modern telescopes, these points of light often resolve themselves into bubbles of color surrounding a brilliant central star—or the place where a star ought to be. The Voyager I and II probes established that our own star has just such a bubble. What lies within it, our Sun's material radius of influence, is called the heliosphere.
* The 21 cm line is the spectral evidence of infrared light with a wavelength w ≈ 21 cm, which is emitted by cold molecular hydrogen. Its existence confirms aspects of quantum physics, and its presence or absence in a given region helps astronomers understand the 3D structure of the visible cosmos.
Sometimes, when a bubble is in want of a star, the star is absent because it has exploded in a supernova. However, sometimes it's because the star is just throwing off light we can't see. For example, Wolf-Rayet stars are enormous, surprisingly dim stars that burn so hot they mostly radiate in the ultraviolet band—invisible to the unaided eye. Their powerful stellar wind blows a larger bubble around them. These stars have lost all the hydrogen in their outer layer and begun fusing more massive elements such as helium, nitrogen, and carbon. Wolf-Rayet stars are marked by their abundance of heavy elements, including a rare isotope of aluminum (²⁶Al) mostly found where a Wolf-Rayet star is or has recently been. And when they die, they also happen to explode in a supernova.
Wolf-Rayet stars may play a key role as the source of the explosive shockwave that created regions of greater density out of the diffuse interstellar fluff: a supernova that left behind stellar ashes from which our solar system was born. That shockwave flung away the material the star had already shed as solar wind. Evidence for this hypothesis lies in rare fragments of very old meteorites. Traces of ²⁶Al found in inclusions within ancient meteorites, thought to be some of the first solid matter to condense in the solar system, may represent specific evidence of a Wolf-Rayet star.
If the nebular hypothesis holds true, the star that went supernova and caused our nebula to begin its collapse probably lived about 3 kpcs closer to the galactic center.
Coalescence
Based on the distribution of mass in and around our local interstellar cloud, the original giant molecular cloud was thousands of times the mass of our sun and about 20 parsecs (65 light-years) in diameter, with numerous satellite "fragments" roughly a parsec (a little more than three light-years) from edge to edge. Gravitational attraction within the fragments led to their collapse into denser "cores" just 0.01–0.1 parsecs (2,000–20,000 AU) wide. The presolar nebula that would one day become the solar system condensed out of one such fragment over about 100,000 years.
Tidal forces from the greater mass of the galaxy tend to drag matter inward toward Sagittarius A*, the supermassive black hole at the center of the Milky Way. Meanwhile matter within the galaxy is subject to its rotational forces. The difference in direction between those forces is enough to create an axis of rotation: an eddy, transient on cosmological time scales, with a vortex at its center. Heavier elements fell in and concentrated toward the center of mass, while lighter elements, such as hydrogen gas, lingered around the edges. Conservation of momentum made the collapsing cloud spin faster, and it heated up as it collapsed.
Within 500,000 to a million years, the presolar nebula had differentiated into a hot, dense, spheroid protostar core of some three solar masses and an accretion disc of perhaps twenty percent of the protostar's mass.
Wandering Planets
Just a few hundred thousand years after the protostar had distinguished itself from its disc, well before it had driven off or ingested enough matter to clear its environment, the inner Solar System was populated by perhaps a hundred protoplanets, each about five percent of the Earth's mass and sized between the Moon and Mars. Then, a phase of collision and merging began, lasting perhaps a hundred million years and ending with four surviving terrestrial (rocky) inner planets.
The largest celestial bodies in our solar system that still exist—massive asteroids and minor planets—are thought to be the last vestiges of this era of planetary formation. This is the period during which many scientists believe the young Earth collided with another nascent planet called Theia, forming the Moon. Closer to the Sun, another such giant impact is thought to have torn away the outer layers of the young Mercury, leaving a small planet dominated by its iron core.
Radiation from the Sun warms everything around it, and this effect is key to how the planets formed. Terrestrial planets formed nearest to the Sun, devoid of volatile gases and ice because only rocky material could withstand the heat. However, the nebula contained very little such material, which limited the size of the rocky planets. Between the orbits of Mars and Jupiter, there is a band where liquid water can persist. Most, if not all, stars have such a band, sometimes called the "Goldilocks zone." The outward boundary of that band is known as the snow line.
Earth, between sweltering Venus and frozen Mars, formed and orbits within a band where water could persist in solid, liquid, and gaseous phases. Mars had surface water, but not as much as Earth, and not for very long. Instead, Mars is so cold its poles are covered in drifts of dry ice: frozen carbon dioxide that falls out of the atmosphere as snow.
Meanwhile, the volatiles (gases, liquids, and ice) driven off from the inner planets condensed in a ring or disc beyond the snow line in the outer regions of the young solar system. Gravity pulled these materials together, and that is where we find gas giants Jupiter and Saturn, and the ice giants Uranus and Neptune.
One popular hypothesis of Jupiter's origin holds that it formed at more than twice the distance at which it currently orbits from the Sun, then swooped inward, trading places with Saturn due to an orbital resonance. However, that model challenges the current narrative of a Late Heavy Bombardment with a calamitous peak in impact frequency toward the end. In either case, Jupiter's gravity played and still plays a major role in clearing out the inner Solar System of the largest (and most dangerous) debris.
The Sun accounts for some 99.86% of the mass in our Solar System; of the remaining fraction of a percent, fully two-thirds is embodied in Jupiter, which itself contains more than 70% of the total combined mass of all the planets in the Solar System. After the planets, the last scrum of remaining matter—rocky material, ice, dust, and cold gas—resides in the asteroid belt between Mars and Jupiter, the Kuiper belt beyond the orbit of Neptune, and the Oort cloud.
Where No One Has Gone Before
Our Solar System is full of mysteries, but if the inner planets are a puzzle, the boundary between the Solar System and interstellar space is a secondhand puzzle with half the pieces missing. Here, the solar wind finally exhausts its momentum and comes to a turbulent halt at a place called the termination shock. A little farther out, the Sun's magnetic field tails off and starts to blend in with the interstellar medium. Still another, more distant boundary is the outer edge of the Oort cloud: the place where the Sun's gravity yields to the galactic tide.
The Oort cloud is a gigantic spherical shell of debris surrounding our Solar System, the most distant objects that the Sun can keep in its gravity well. Every star should have one. Based on their orbits, some of our comets and asteroids must have originated in the Oort cloud as debris that was never incorporated into a planet. No human has ever gone there, but two NASA spacecraft launched in 1977 have crossed the termination shock: Voyager 1 in 2004 and Voyager 2 in 2007. Voyager 1 officially reached interstellar space in 2012, and Voyager 2 did so in 2018. But it will be many thousands of years before the two Voyagers exit the Oort Cloud. Sadly, their instruments will have long since shut down for good.
As the boundary between our stellar bubble and the larger galaxy, the Oort cloud also plays a surprising role as our cosmic threshold. In about 1.3 million years, a nearby star called Gliese 710 is likely to make a close pass by our Solar System: so close that it will go right through our Oort cloud, dragging its own along for the ride. If it does, we may have to get ready for a new Late Heavy Bombardment, as asteroids and other rocks of unpleasant size come pelting in toward the inner Solar System. Happily, we've got a while to figure it out.
For more on where Earth and our Solar System fit into the deep history of the cosmos, check out the other posts in this series: How Old is the Universe? and How Old is the Earth?
