The Cosmic Birth of Our Solar System

What transpired 4.56 billion years ago marks a pivotal event in the cosmic narrative of our existence. If you could have observed the Universe during the era of our Solar System’s formation, it would have seemed rather ordinary. The Milky Way, relatively solitary, would have appeared as the second-largest member of a minor group of galaxies. Dwarf galaxies would be observed merging with larger ones, just as they do throughout the Universe. At that time, our galaxy was already home to hundreds of billions of stars, with gas clumps contracting along its spiral arms, leading to fresh waves of star formation. Numerous regions throughout the Milky Way, tens to hundreds, were actively forming new stars.

In one of these star-forming areas, 9.2 billion years post-Big Bang, our Sun, planets, and Solar System began to take shape. Let’s delve into what this formative period was like.

Stars have been forming from gas clouds for over 99% of the Universe's lifespan, but the creation of systems like ours required a specific set of circumstances. The Universe had to undergo several generations of stars, which lived and died, releasing heavy elements through supernovae and collisions between white dwarfs and neutron stars. This process enriched our galaxy with the essential elements needed for life.

In order for our Solar System to form with the attributes it possesses today, a series of precise events had to unfold. Spiral galaxies, shaped somewhat like pancakes, have denser gas at their centers, with decreasing density toward the edges. As they spin, the inner sections complete more rotations than the outer sections, resulting in differential rotation.

The densest regions gather heavier elements, while lighter elements are pushed toward the outer areas. Our Solar System emerged from a gas cloud situated about 25,000 light-years from the galaxy’s center, roughly halfway to the edge of the galactic disk. At the moment of our formation, the composition was roughly 70% hydrogen, 28% helium, and only about 2% comprised of heavier elements. This was a significant shift from the conditions right after the Big Bang, which consisted of 75% hydrogen and 25% helium with almost no other elements.

In evolved spiral galaxies like ours, most stars form when gas clouds traverse the spiral arms. This movement causes material to funnel into these clouds, increasing their density and often triggering gravitational collapse. When this collapse occurs, vast clouds of gas—ranging from thousands to millions of solar masses—begin to fragment into smaller clumps. The largest clumps attract more matter and grow into massive stars, while smaller clumps evolve at a slower pace, often merging to accelerate their growth.

The competition between gravity, which seeks to form and grow stars, and radiation emitted by the hottest new stars, results in a dynamic environment.

Over time, it's evident which stars will dominate: the most massive stars can be tens or even hundreds of times heavier than our Sun, radiating energy thousands to millions of times more luminous than our star. These colossal stars can obliterate the star-forming regions by dispersing the gas. Yet gravity remains relentless, drawing gas into various regions. While a large star-forming nebula may produce numerous high-mass stars, it will generate hundreds of times more low-mass stars. The massive, bright stars that capture our attention are brief flashes in the cosmic timeline, often gone within a few million years.

The saying goes that a flame burning twice as bright lasts half as long, and for stars, this is even more pronounced. A star twice the mass of another will consume its fuel roughly eight times faster. For a star like our Sun, which has a lifespan of about 10–12 billion years, a star many times its mass may only endure for a few million years.

While our early Solar System continued to accumulate matter and develop into a central star orbited by planets, the most massive stars nearby were rapidly exhausting their fuel, culminating in supernova explosions, which halted star formation in the surrounding regions. The Universe is a tumultuous environment, and star-forming regions are among the most chaotic.

Interestingly, our Solar System isn’t situated at the lower end of the stellar spectrum. The clump of matter that would evolve into our Sun was initially larger and grew faster than most clumps around it. In fact, if we were to analyze our Sun today and compare it with other stars across the Universe, we would find that it surpasses 95% of them in mass. Remarkably, between 75% and 80% of all stars are red dwarf (M-class) stars, which are the smallest and coolest. Additionally, more than half of the remaining stars belong to the next smaller class, K-class. Thus, the matter that coalesced to form our Solar System was above average in mass and unique in one significant aspect: we formed alone.

In massive star-forming regions within Milky Way-sized galaxies, thousands of new stars are born. Many of these stars will be part of multi-star systems, while around half will exist as solitary stars. Recent studies by the REsearch Consortium On Nearby Stars (RECONS) have shown that among 2,959 stars surveyed within 25 parsecs (approximately 81 light-years) of Earth, 1,533 are single stars, while 1,426 are part of binary or trinary systems.

What determined that our Sun would be a solitary star rather than part of a multi-star system? Simply luck.

As time progressed, the fragment of gas that gave rise to our Solar System accumulated matter onto a central clump. This process led to radiative heat loss, allowing the clump to develop into our Sun, while gravitational collapse raised the temperature in the center. Once a critical temperature of 4 million K was achieved, nuclear fusion could commence, fusing protons into heavier elements.

This marks the moment when a star is considered to be “alive.” Based on our current understanding, this event occurred 4.56 billion years ago, when the Universe was approximately two-thirds of its current age. This instant signifies the official formation of our Solar System.

In recent years, we’ve been able to observe solar systems in the early stages of formation, discovering central stars and proto-stars enveloped in gas, dust, and protoplanetary disks that exhibit gaps. These are the precursors to what will eventually become both giant and rocky planets, leading to the formation of solar systems akin to our own. Although most stars—including likely our own—formed in massive star clusters, there are exceptions that emerged in relative isolation.

While the cosmic history may have separated us from our stellar and planetary counterparts formed billions of years ago, scattering them across the galaxy, our shared origins endure. Whenever we identify a star with a similar age and abundance of heavy elements as our Sun, we can’t help but speculate: could this be one of our long-lost siblings? The galaxy is likely filled with such stars.

Further reading on the Universe's history includes:

• What was it like when the Universe was inflating?
• What was it like during the Big Bang?
• What was it like when the Universe was at its hottest?
• What was it like when matter first outnumbered antimatter?
• What was it like when the Higgs field gave mass to the Universe?
• What was it like during the formation of protons and neutrons?
• What was it like when the last of the antimatter vanished?
• What was it like when the Universe formed its first elements?
• What was it like when atoms first emerged?
• What was it like when the Universe was devoid of stars?
• What was it like during the first stars' illumination?
• What was it like when the first stars perished?
• What was it like during the Universe's second generation of stars?
• What was it like during the formation of the earliest galaxies?
• What was it like when starlight pierced the Universe’s neutral atoms?
• What was it like when supermassive black holes were formed?
• What was it like when life became feasible in the Universe?
• What was it like when galaxies underwent peak star formation?
• What was it like when habitable planets first emerged?
• What was it like when the cosmic web formed?
• What was it like when the Milky Way took shape?
• What was it like when dark energy began to dominate the Universe?

Starts With A Bang is now featured on Forbes and republished on Medium, thanks to our supporters on Patreon. Ethan has authored two books, "Beyond The Galaxy" and "Treknology: The Science of Star Trek from Tricorders to Warp Drive."