The Solar System's Predicted Past and Ultimate Future

Everything around you began, originally, in a molecular cloud like the one pictured here.

A molecular cloud is a loose collection of gas and dust, one that is cold enough for molecules to be able to form. They are mostly made up of hydrogen gas. 

Over time, gravity causes parts of these clouds to collapse. The clumps get denser and hotter, attracting even more material from the cloud, until finally their cores reach the conditions required to begin nuclear fusion — the power source of all stars.

We can watch this happening in the Orion Nebula. The bright clumps on the right are regions where gigantic, massive stars are forming. The clump that became our Solar System was about 20,000 times as big as the current distance between the Earth and the Sun.

It was in a place like this that our Sun formed, surrounded by many other stars. Over hundreds of millions of years, these stars drifted apart, leaving little evidence of their original crowding. 

As a star turns on, it continues to attract dust and gas. Some falls onto the star, but other bits collect into a disk that rotates around it instead. At the same time, the star's magnetic field channels some particles through powerful jets from either north/south pole.

Once the cloud clears, some dust and gas remains leftover in the disk. Its continuous orbit keeps it from drifting into the star, like how a satellite is always falling around the Earth. 

Out of this disk, planets form.

Though the dust in this disk is only about a billionth of a meter large to begin with, over thousands of years it coagulates into dust bunnies, then pebbles, then boulders — and eventually, the building blocks of entire planets (called "protoplanets").

This faraway object, also known as "2014 MU69", is thought to be a leftover of the primitive building blocks that glommed together to form the rocky cores of planets.

At this point, the Solar System went through a period of instability that lasted about a hundred million years. Some protoplanets smashed into each other, like the impact that formed the Earth's moon. Others may have been ejected out of the Solar System entirely.

The dwarf planet Haumea, which orbits the Sun from a distance far past Neptune, is thought to have formed through the kind of giant collision that reshaped our Solar System so much.

In this chaotic time, some planets captured others into their orbit, gaining moons. Others had already formed moons early on, from orbiting disks like miniature versions of the one that had surrounded the Sun.

Most scientists agree that about 500 million years after the Solar System formed, Jupiter began migrating closer to the Sun. Its gravitational influence disturbed the asteroid belt and launched many rocks toward the inner planets, including the Earth.

One popular theory says that as Jupiter moved, it eventually interacted with Saturn's orbit, causing a major gravitational disturbance that pushed Uranus and Neptune farther out from the Sun and even switched their order.

That brings us to today.

But the Solar System isn't done evolving.

The orbits of all the planets are stable in the near-term. But there is a small chance that within the next billion years, Mercury's orbit could get more and more oblong until it eventually smashes into Venus or the Earth.

In several billion years, Mars could do the same.

In about 5 billion years, the Sun will exhaust all its fuel and collapse in on itself. The sun isn't massive enough to go supernova or become a black hole, but the collapse will cause an explosion that either destroys any of the remaining planets or ejects them out into space.

The Solar System will then be a "planetary nebula" — a misleading name, since the planets will all be gone. The Sun will have lost about half its mass, and will continue to shine as a small, hot white dwarf. 

But the surrounding nebula will dissipate in mere thousands of years. Most of what used to be the Solar System will be scattered into space. 

Maybe, one day, some of the remains will enrich another star, or another planet, or maybe even another form of life.