History of Solar System formation and evolution hypotheses facts for kids
Have you ever wondered how our Solar System came to be? For hundreds of years, smart thinkers have tried to figure out how the Sun, planets, and moons formed. They also wanted to know how they might change in the future.
The idea of the "Solar System" first appeared in 1704. Since then, many ideas have been shared. One of the earliest ideas came from René Descartes. Over time, more and more scientists joined the discussion.
In the 20th century, a popular idea called the nebular hypothesis became widely accepted. This idea explains how our Solar System likely formed. Scientists also started to understand how stars like our Sun work. This helped them guess how the Sun would change over time.
For the Moon, people had ideas for centuries. But the Apollo space missions in the 1960s and 1970s showed that many of these old ideas were wrong. After Apollo, in 1984, the giant impact hypothesis became the main explanation for how our Moon formed.
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How Did Our Solar System Form?
The most accepted idea for how planets form is called the nebular hypothesis. This model suggests that about 4.6 billion years ago, our Solar System started as a huge cloud of gas and dust. This cloud was many light-years wide.
Gravity caused this giant cloud to slowly pull together and collapse. Many stars, including our Sun, formed inside this collapsing cloud. Most of the cloud's material gathered in the center, creating the Sun.
The remaining gas and dust flattened into a spinning, pancake-like shape. This flat shape is called a protoplanetary disk. All the planets, moons, asteroids, and other space objects we see today formed from this disk.
Early Ideas About Solar System Formation
The French thinker René Descartes was one of the first to suggest how the Solar System began. He wrote about it in his book The World in the 1600s. He thought the universe was full of swirling particles, like giant whirlpools.
Descartes believed the Sun and planets formed from a huge swirling vortex that shrank. He thought this explained why planets move in circles. But this was before Newton's theory of gravity was known. Newton's ideas showed that matter doesn't behave this way.
Later, in 1944, German physicist Carl Friedrich von Weizsäcker had a similar idea. He imagined a spinning disk with swirling patterns, like eddies in a river. He thought these eddies could help planets form. However, this idea had problems. Swirling patterns are usually messy, not organized enough to make planets. It also didn't explain why the Sun spins so slowly compared to the planets.
In 1948, Dirk ter Haar changed this idea. He suggested that random swirling movements, called turbulence, were involved. He thought planets formed by small bits of material sticking together, a process called accretion. He also tried to explain why inner planets are rocky and outer planets are gassy. He said the inner part of the disk was hotter, so only strong materials could form there. But his model didn't allow enough time for planets to form.
The main idea of the nebular hypothesis was first suggested in 1734 by Swedish scientist Emanuel Swedenborg. Later, Immanuel Kant and Pierre-Simon Laplace expanded on it.
In 1749, Georges-Louis Leclerc, Comte de Buffon had a different idea. He thought planets formed when a comet crashed into the Sun. This collision would have sent out material that then formed the planets. But Pierre-Simon Laplace disagreed. He said any planets formed this way would just fall back into the Sun. Also, comets are way too small to have created our entire Solar System.
In 1755, Immanuel Kant thought that fuzzy patches in space, called nebulae, might be places where stars and planets were forming. In 1796, Laplace added to this. He said that a nebula collapsed to form a star. As it collapsed, the leftover material spun out into a flat disk, which then formed the planets.
Why Were Other Ideas Needed?
Even though the nebular hypothesis seemed good, it had a big puzzle: the "angular momentum problem." This refers to how much an object spins. If the Sun formed from a collapsing cloud, it should be spinning much faster. The Sun has almost all the Solar System's mass, but only 1% of its spin. This means the Sun spins very slowly.
Tidal Hypothesis
To solve the spin problem, some scientists looked for new ideas. For decades, many astronomers liked the "tidal hypothesis." James Jeans suggested in 1917 that another star passed very close to our Sun. This close encounter would have pulled out huge amounts of material from both stars due to their strong tidal forces. This material then cooled and formed the planets.
However, in 1929, Harold Jeffreys said such a close call was extremely unlikely. Henry Norris Russell also pointed out that this idea didn't explain the spin of the outer planets. They would have struggled to stay in orbit and not fall back into the Sun.
Planetesimal Hypothesis
In 1900, Forest Moulton showed that the nebular hypothesis didn't fit observations because of the spin problem. So, in 1904, Moulton and Chamberlin came up with the planetesimal hypothesis. They thought that a star passed close to the Sun early on. This caused huge bulges of material to be pulled from both stars.
Most of this material would have fallen back. But some of it stayed in orbit. This material cooled into many tiny solid pieces called planetesimals, and a few larger protoplanets. This idea was popular for about 30 years. But by the 1940s, it was discarded because it couldn't explain the spin of Jupiter. However, the idea that planets grew from planetesimals sticking together was kept.
Interstellar Cloud Idea
In 1943, Soviet astronomer Otto Schmidt suggested that our Sun, already formed, passed through a thick cloud of dust and gas in space. As it passed through, it became wrapped in this cloud. The planets then formed from this captured dust and gas. This idea helped solve the Sun's slow spin problem. It suggested the Sun's slow spin was normal for it, and the planets formed later.
However, Victor Safronov showed a problem with this idea. It would take far too long for planets to form from such a spread-out cloud. The time needed would be much longer than the known age of our Solar System.
Hoyle's Magnetic Idea
In 1955, Fred Hoyle proposed an idea similar to Laplace's. He added that a magnetic force between the disk and the Sun helped transfer spin from the Sun to the disk. This magnetic force would have prevented too much material from being thrown out. It also helped explain the spin of the planets.
Kuiper's Idea
Gerard Kuiper suggested in 1944 that large clumps could form in the solar nebula due to gravity. These clumps would then become planets. He thought the solar nebula could have formed with the Sun or been captured by it.
Protoplanet Idea
In the 1960s, W. H. McCrea proposed the protoplanet hypothesis. He thought the Sun and planets all formed separately from the same cloud of material. The smaller planets were later caught by the Sun's strong gravity.
This idea has some issues. It doesn't easily explain why all planets orbit the Sun in the same direction and in nearly circular paths. If they were all captured individually, their orbits would likely be much more random.
Capture Hypothesis
The capture hypothesis, suggested by Michael Mark Woolfson in 1964, says that the Solar System formed from the Sun interacting with a nearby, less dense, young star. The Sun's gravity would have pulled material from this star's outer layers. This material then collapsed to form the planets.
Solar Fission Idea
In the 1950s, Swiss astronomer Louis Jacot had an idea that planets were pushed out, one by one, from the Sun's equator. He thought the Sun spun so fast that it bulged at its middle, and planets were flung off from there. He even suggested that the asteroid belt formed from a shattered planet that was expelled. This idea also suggested that moons were expelled from their parent planets.
This model tried to explain why inner and outer planets are different. For example, Mercury's odd orbit was explained by it being recently expelled. Venus's slow spin was because it was in a "slow rotation phase." However, this idea is not widely accepted today.
The Nebular Hypothesis Returns!
In 1978, astronomer Andrew J. R. Prentice brought back the nebular model. He suggested that dust in the original disk helped slow down the spin in the center. He also thought the young Sun transferred some of its spin to the disk and planetesimals through powerful bursts of material.
The modern, widely accepted idea of planet formation is called the Solar Nebular Disk Model (SNDM). This model was greatly influenced by Soviet astronomer Victor Safronov. His book, translated into English in 1972, changed how scientists thought about planet formation. He laid out many of the main problems and even solved some of them.
Safronov's ideas were further developed by George Wetherill. He discovered "runaway accretion," where larger objects grow much faster by sweeping up smaller ones.
By the early 1980s, the nebular hypothesis, in the form of SNDM, became popular again. This was thanks to two big discoveries:
- Scientists found that several young stars, like Beta Pictoris, were surrounded by disks of cool dust. This was exactly what the nebular hypothesis predicted.
- The Infrared Astronomical Satellite, launched in 1983, saw that many stars gave off extra infrared light. This could be explained if they had disks of cooler material orbiting them.
What We Still Don't Know
Even though the main idea of the nebular hypothesis is accepted, many small details are still being worked out.
Until the mid-1990s, scientists only knew about our Solar System. So, the nebular model was based only on what we saw here. But since then, we've found many extrasolar planets (planets outside our Solar System). These discoveries have brought many surprises! The nebular model needs to be updated to explain these new planetary systems.
For example, some planets found are as big as Jupiter or even larger. But they orbit their stars in just a few hours! These "hot Jupiters" are so close that their atmospheres would be stripped away. Scientists are still trying to figure out how these planets formed. One idea is "planetary migration." This means planets can move from where they first formed. This might be similar to how Uranus and Neptune moved to their current distant orbits in our Solar System.
Another puzzle is the detailed features of our own planets. The nebular hypothesis predicts that all planets should form exactly in the same flat plane. But the orbits of our classical planets are slightly tilted. Also, the gas giants' spins and moon systems are not perfectly aligned. For example, Uranus is tilted almost 98 degrees!
The Moon is also unusually large compared to Earth. And some moons have odd orbits. Scientists now believe these things happened after the Solar System first formed, perhaps due to big collisions.
How Does the Sun Change Over Time?
Scientists in the 1800s started to wonder where the Sun gets its energy. They wanted to know how long it would last.
Sun's Energy: Early Ideas
In the 19th century, people thought the Sun's heat came from gravity pulling it inward. In the 1840s, J. R. Mayer and J. J. Waterson suggested that the Sun's huge weight caused it to collapse, creating heat. Hermann von Helmholtz and Lord Kelvin added to this idea in 1854. They also thought heat might come from meteors hitting the Sun.
At the time, scientists thought stars started as huge, cool red supergiants. Then they shrank and heated up to become hot, blue stars. Finally, they would become small, cool, dense objects. But the Sun only had enough energy from this shrinking process to last about 30 million years. This is much less than the age of Earth!
In 1905, Albert Einstein developed his theory of relativity. This led to the understanding that nuclear reactions could create new elements and release huge amounts of energy. Arthur Eddington suggested that inside stars, the pressure and temperature were high enough for hydrogen atoms to fuse into helium. This process, called nuclear fusion, could power the Sun for billions of years.
Red Giants: Stars in Their Later Lives
Scientists had known about the unusual light from red giant stars since the 1800s. But in the 1940s, George Gamow realized what they were. He understood that these were stars, like our Sun, that had used up the hydrogen fuel in their cores. So, they started burning hydrogen in their outer layers.
This helped Martin Schwarzschild connect red giants to the idea that stars have a limited lifespan. We now know that red giants are stars in the final stages of their lives.
Fred Hoyle noticed that different stars had different amounts of elements. He thought this meant elements must be made inside the stars themselves. He saw that elements like iron were very common. Iron could only form under extreme pressure and heat. Hoyle concluded that iron must form inside giant stars.
From this, in the 1940s, Hoyle figured out the last stages of a star's life. As a star dies, it collapses under its own weight. This leads to a chain of fusion reactions, making heavier elements up to iron.
White Dwarfs: The End of Some Stars
The first white dwarf star was found in 1783. It was part of a triple star system called 40 Eridani. In 1910, scientists realized that even though 40 Eridani B was dim, it was a very hot, white star.
Soon after, scientists found that white dwarfs were incredibly dense. By studying how they moved in binary systems, scientists could estimate their mass. For example, Sirius B was found to have a mass similar to our Sun. But because it was so dim, it had to be tiny. This meant it was super dense. One scientist in 1916 called a white dwarf's density "impossible" because it was 25,000 times denser than the Sun!
These high densities are possible because white dwarfs are not made of normal atoms. Instead, they are made of a plasma of atomic centers (nuclei) and loose electrons. The electrons are packed so tightly that they can't move freely. This special state, called "degenerate matter," means a white dwarf can cool down but still have a lot of energy.
Planetary Nebulae: Star's Outer Layers
Planetary nebulae are usually faint objects. None can be seen with just your eyes. The first one was discovered in 1764. Early observers with basic telescopes thought they looked like gas giant planets. So, William Herschel called them 'planetary nebulae,' even though we now know they are very different from planets.
The stars at the center of planetary nebulae are very hot but dim. This means they must be very small. A star can only shrink to such a small size after it has used up all its nuclear fuel. So, planetary nebulae are understood to be a final stage in a star's life. Observations show that all planetary nebulae are expanding. This led to the idea that they are caused by a star's outer layers being thrown into space at the end of its life.
How Did Our Moon Form?
For centuries, scientists have wondered how Earth got its Moon. Here are some of the early ideas:
- Binary Accretion Model: This idea suggested that the Moon formed from material orbiting Earth, left over from Earth's own formation. So, Earth and Moon formed together.
- Fission Model: This idea came from George Darwin (son of Charles Darwin). He noticed that the Moon is slowly moving away from Earth. So, he thought that long ago, the Moon was part of Earth. It was flung out by Earth's much faster spin. This idea was supported because the Moon's density is similar to Earth's rocky middle layer, suggesting it doesn't have a heavy iron core like Earth.
- Capture Model: This idea suggested the Moon was a separate object orbiting the Sun. Then, Earth's gravity "snared" it into orbit around our planet.
Apollo Missions Change Everything
The Apollo missions in the late 1960s and early 1970s brought back many Moon rocks. This new evidence changed everything! The rocks showed that the Moon had very little water compared to other rocks in the Solar System. They also showed signs of a huge ocean of melted rock early in its history, meaning its formation created a lot of energy.
Also, the oxygen in Moon rocks was very similar to oxygen on Earth. This suggested they formed in the same general area of the solar nebula.
- The capture model couldn't explain the similar oxygen. If the Moon came from somewhere else, its oxygen would be different.
- The co-accretion model couldn't explain the lack of water. If the Moon formed like Earth, it should have similar amounts of water.
- The fission model could explain the similar chemicals and lack of iron. But it couldn't explain the Moon's tilted orbit or the huge amount of spin in the Earth-Moon system.
The Giant Impact Idea
For years after Apollo, scientists still struggled to find the best idea. Then, at a meeting in Hawaii in 1984, a new idea was put together. It explained all the puzzles! This "giant impact model" suggested that a huge object, about the size of Mars, crashed into Earth very early in its history.
The impact would have melted Earth's outer layer. The other planet's heavy core would have sunk into Earth's core. The superheated vapor from the crash would have shot into orbit around Earth. This vapor then cooled and came together to form the Moon.
This idea explained many things:
- Lack of water: The vapor cloud was too hot for water to condense.
- Similar composition: The Moon formed from part of Earth.
- Lower density: The Moon formed from Earth's outer layers, not its dense core.
- Unusual orbit and spin: An angled collision would have given the Earth-Moon system a huge amount of spin.
What Are the Remaining Questions?
The giant impact model is very good, but it still has some questions. Some critics say it's too flexible, meaning it can be changed to explain anything new. They also point out that if the Moon formed from the impactor, its chemical makeup should be different, but it's not. Also, while water is missing from Moon rocks, other easily evaporated elements are not.
Other Moons in Our Solar System
While the co-accretion and capture models don't explain our Moon, they are used for other moons. For example, Jupiter's large moons are thought to have formed by co-accretion around Jupiter. And many of the Solar System's oddly shaped moons, like Triton (Neptune's largest moon), are believed to have been captured by their planets' gravity.
