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The Proto-Solar System
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This isn't the solar system you know.
Wind the clock back about 4.5 billion years, to the solar system's first roughly 100 million years. The giant impact that made the Moon, Jupiter's inward migration, and the Sun's dim early years didn't all happen in the same breath, but showing them together illustrates how different this place used to be.
- A fainter, angrier Sun is only about 70% as bright as it is now (the Faint Young Sun Paradox), yet throws off far more radiation and solar wind than the calm star we know.
- Theia, Earth's short-lived neighbor, is a Mars-sized world parked near one of Earth's stable Lagrange points, per the giant-impact hypothesis. It's about to collide with proto-Earth and blast out the debris that becomes the Moon.
- Jupiter's Grand Tack, in the Grand Tack hypothesis has Jupiter drifting inward almost to where Mars sits today, then gets yanked back out by Saturn. That inward swing is a big part of why Mars ended up so small.
- No rings, no Triton yet. Saturn hasn't grown its rings, and Neptune's biggest moon, Triton, is still just an unattached world wandering the outer disk.
- A dusty, crowded disk contains dozens of Mars-to-Moon-sized protoplanets, still bumping into each other in a messier inner system than the four planets we ended up with.
Uranus and Neptune's slow migration outward (the "Nice model") is its own can of worms, and one that has aged less cleanly than the Grand Tack, so it's only lightly gestured at here.
Celestial Bodies (17)
The Young Sun
star · system center · 1,391,000 km diameter
Class G (Faint) ~70% Modern Luminosity Hyperactive
Only about 70% as bright as it is today, even though it's roughly the same size. Scientists call that the "Faint Young Sun Paradox," and it's still not fully solved. Something kept early Earth and Mars warm enough for liquid water despite the dimmer star.
Intense Flare Activity
region · orbits The Young Sun · -12,000,000 km–12,000,000 km
The solar flares and outbursts from this era were strong enough to strip away any hydrogen-helium atmospheres the young rocky planets had.
Crowded Protoplanetary Disk
region · orbits The Young Sun · 40,000,000 km–330,000,000 km
Planetesimals Terrestrial Zone Unstable
The four inner planets hadn't sorted themselves out yet. Models suggest dozens of Mars-to-Moon-sized protoplanets crashing into each other, merging, or getting thrown out of the system.
Proto-Mercury
planet · orbits The Young Sun · 57,894,376 km · 4,879 km diameter
Magma Ocean Sub-Earth
Being this small and this close meant its magma ocean had the least heat to shed, so it was probably the first of the rocky worlds to start cooling.
Proto-Venus
planet · orbits The Young Sun · 108,159,261 km · 12,104 km diameter
Magma Ocean Earth-Sized
Already wrapped in a thick, steamy haze of its own outgassed volatiles, yet long before a runaway greenhouse effect turned it into the crushing furnace it is today.
Proto-Earth
planet · orbits The Young Sun · 149,597,870 km · 12,300 km diameter
Magma Ocean Pre-Impact
A ball of molten rock with no Moon yet. Give it tens of millions of years, and a Mars-sized neighbor sharing its orbit is going to change that, violently.
Theia
protoplanet · orbits The Young Sun · 149,597,870 km · 6,102 km diameter
Protoplanet Co-Orbital (L4/L5) Doomed
A Mars-sized protoplanet parked near one of the two stable Lagrange points about 60° ahead of or behind Earth. Sooner or later a gravitational nudge sends it crashing into proto-Earth, and the debris from that collision becomes the Moon.
Planetary Embryo Alpha
protoplanet · orbits The Young Sun · 74,798,935 km · 5,200 km diameter
Protoplanet Illustrative
One of dozens of Mars-to-Moon-sized embryos jostling for space in the young inner disk. Most of them never made it: they got swallowed by something bigger, smashed apart, or thrown clear out of the system.
Planetary Embryo Beta
protoplanet · orbits The Young Sun · 134,638,084 km · 3,800 km diameter
Protoplanet Illustrative
A smaller embryo, somewhere between the Moon and Mercury in size, wedged between the orbits of two much bigger worlds still under construction.
Planetary Embryo Gamma
protoplanet · orbits The Young Sun · 269,276,168 km · 4,400 km diameter
Protoplanet Illustrative
Sitting right in Jupiter's path as the giant planet swings inward. Wrong place, wrong time.
Proto-Mars
planet · orbits The Young Sun · 227,987,155 km · 6,100 km diameter
Stunted Growth Grand Tack Casualty
Already too small for its own orbit. Most formation models predict a bigger Mars unless something cleared out the material it needed to grow, and Jupiter's early migration is the leading suspect.
Jupiter (Grand Tack Position)
planet, gas-giant · orbits The Young Sun · 224,396,806 km · 139,820 km diameter
🪐 Gas Giant Grand Tack Migrating Inward
The Grand Tack hypothesis has Jupiter drifting inward through the gas disk to nearly where Mars orbits today, before Saturn's gravity caught up and towed it back out. That detour reshaped the whole solar system. Its four big moons haven't finished forming yet.
Saturn (Grand Tack Position)
planet, gas-giant · orbits The Young Sun · 299,195,741 km · 116,464 km diameter
🪐 Gas Giant Ringless 2:3 Resonance
Locked into a 2:3 orbital resonance with Jupiter right at this spot, the exact turning point of the "tack" that sends both planets migrating back outward together. Its rings come along much later, probably courtesy of a shattered moon or comet.
Proto-Uranus
planet, gas-giant, ice-giant · orbits The Young Sun · 1,795,174,448 km · 50,724 km diameter
🧊 Ice Giant Compact Configuration Migration Debated
The giant planets likely started out packed closer together than they are now. Nobody agrees on exactly when or how far Uranus later drifted outward, so take this distance as a rough sketch rather than a fixed measurement.
Proto-Neptune
planet, gas-giant, ice-giant · orbits The Young Sun · 2,393,565,931 km · 49,244 km diameter
🧊 Ice Giant Moonless
No moon yet. Triton is still out there on its own, with a long wait ahead before Neptune's gravity finally captures it.
Triton (dwarf planet)
dwarf-planet · orbits The Young Sun · 5,983,914,828 km · 2,706 km diameter
Dwarf Planet Not Yet Captured
An independent dwarf planet drifting through the outer disk, with no idea it will eventually be captured into a backwards orbit around Neptune and turned into that planet's largest moon.
Primordial Planetesimal Disk
region · orbits The Young Sun · 2,540,000,000 km–7,480,000,000 km
Icy Planetesimals Denser Than Today Primordial
A much denser, messier ancestor of today's Kuiper Belt, packed with icy leftovers. Some of this same material sticks around hundreds of millions of years, long enough to eventualy pelt the inner planets with comets.