More than 4.5 billion years ago, the seeds of the Earth were in place, just as the earliest parts of our solar system began to form. Somewhere in the Milky Way, a large expanse of gas collapsed, giving rise to thousands of new stars and star systems, each unique. Some stars have a much greater mass than our Sun; Most are much smaller and of much lower quality. About half of galaxies have multiple stars in them; About half of all stars are formed from their lone stars, just like ours. As these protostars collapse and cool, the temperature and density of their cores rise. After crossing the threshold of nuclear fusion ignition, our home planet, the Sun, was officially "born" as a living star.
But these nascent stars are not alone. Around almost each of them, a large amount of matter is condensed into a disk. These are called protoplanetary disks and will be the starting point for each planet that eventually forms steadily around these stars. With advances in telescope technology over the past few decades, we have begun to image these disks and their details first-hand, and to understand what planetary formation looked like in its early stages. Soon after the birth of the sun, a planetary system formed around it, which eventually led to the modern solar system. For the first time, we have a first-hand look at how planetary systems like ours are formed.
Samples of 20 protoplanetary disks orbiting young young stars, measured by the disc substructure of the High Angular Resolution project: Dsharp. Observations such as these tell us that the protoplanetary disk formed primarily in a single plane and tended to support the core accretion scenario of planetary formation. The disc structure can be seen at both infrared and millimeter wavelengths. We recently learned that after 1-2 million years, gaps begin to form in protoplanetary disks, while young disks do not have this substructure. **s.m.Andrews et al., APJL, 2018.
Theoretically, the process of forming planets around newborn stars is very simple. Whenever you have a large mass like a gas cloud, you can expect the following steps to occur:
Most of the mass is attracted to a central region, and one or more large clumps will grow into protostars, and when the surrounding three-dimensional gas collapses, first a dimension is folded (creating a disk), then the defects in the disk will grow, preferentially attracting matter and forming the seeds of planets, and when energetic particles and wind from the newly formed star interact with this matter, eventually boiling and evaporating volatile compounds that last for some time, leaving only a fully formed set of planets, moons, and other objects.
Since long-wavelength observations are very sensitive to details such as heated dust, we can now observe these protoplanetary disks directly. Notably, we found evidence that these planetary seeds – or defects in protoplanetary disks – began to emerge early in the history of star systems: within 2 million years before the protostars ignited to become stars. However, these protoplanetary disks will not last long, as they will be completely evaporated after only tens of millions of years. After the early matter is blown away, what is left behind will eventually become a full-fledged planetary system.
In a system dominated by a single protostar, there will be major regions defined by multiple lines, including soot and frost lines for each specific molecular species. Despite the defects that grow in the disk, accumulating a large gas envelope that exceeds a certain mass threshold, can well describe the planets that form in our solar system and many other planets, they do not explain the giant planets found beyond the Sun-Neptune distance. **nasa/jpl-caltech/invader xan
For our solar system, a long time ago, the situation was the same. Initially, there was only a primitive cloud of gas, from which all the planets would have formed, consisting of a variety of elements: mainly hydrogen (left over from the Great **), followed by helium (mostly left over from the Great **, but partly formed by previous generations of stars), and all the heavier ones (all formed from previous generations of stars and stellar cataclysms), up and down the periodic table. The closer you get to the newly formed star, the easier it is for the lightest elements to be blown away and evaporated. In a short period of time, each young star system will develop three distinct regions:
A central region where only metals and minerals can condense into planets, an intermediate region where rocks and giant worlds containing carbon compounds can be formed, and an outer region where volatile molecules such as water, ammonia, methane, and nitrogen can persist.
The boundary between the two regions inside is known as the soot line, and inside of it will destroy complex carbon compounds called polycyclic aromatic hydrocarbons. Similarly, the boundary between the two outer areas is known as the frost line, and inside it prevents you from forming stable, solid ice. (In fact, there are multiple frost lines: one for each volatile compound.) Both lines are driven by heat from the star, and as the star heats up and evolves, they migrate outward over time.
Interestingly, JWST discovered a third "line" that may have been unexpected, as the disk of debris found around the nearby stellar Fomalhaut, which was hundreds of millions of years old, showed an asteroid belt analogue, a Kuiper Belt analogue, and a third intermediate zone: an unexpected but exciting discovery.
The structure of the Fomalhaut star system was revealed for the first time in this annotated JWST image. A **inner disk, then a (possibly planetary-induced) gap, a median, more planets (and another gap), and finally a Kuiper belt analogue, as well as something newly formed inside known as the "Great Dust Cloud", were revealed. **nasa、esa、csa、a.Gáspár (University of Arizona) et al., Nature Astronomy, 2023.
As the star continues to fuse light elements into heavier elements in its core, defects in the protoplanetary disk form clumps. Over time, these protoplanetary clumps will grow by accretion of additional material from within their orbits, as well as by inhalation from external material that is slightly closer and farther away from the disk. The more clumps are produced and the closer they are to each other, the greater the chances that they will disturb each other. Over time, various clumps, called protoplanets, can:
Merge and collide, catapult against each other under gravitational pull, or even throw one or two objects into the Sun or a larger protoplanet.
When we run simulations and let the planets grow and evolve, we find a very chaotic history that is unique to each planetary system that is formed.
When it comes to our own solar system, the unfolding cosmic story is not only spectacular, but in many ways unexpectedly violent: filled with many potential planets that are eventually destroyed by bullies on a larger scale nearby. In the interior of our early solar system, it is likely that we had a relatively large world in the early days, which may have been swallowed up by our sun during our cosmic youth. Nothing can prevent a huge world from forming in the inner solar system; The fact that we only have a rocky world close to the sun tells us that there may have been something else in the early days. The largest planets may have been formed from early massive seeds, and there may be more than four. To get the current configuration of the gas giants, the simulations we performed seem to indicate that at least a fifth giant was ejected sometime a long time ago. At the same time, Jupiter begins to move closer to our Sun, clearing out any low-mass worlds that formed there in the early days and migrating outward over time.
It wasn't until millions of years later, when much less of the original material was left behind when our solar system was first formed, that the current inner rocky world began to take shape. This late arrival of their formation explains why the planets now found in our inner solar system have such low masses compared to the larger, outer, gas-rich planets we have discovered.
The asteroid belt between Mars and Jupiter is likely a remnant of our original frost line. Boundaries that could have stable ice should have resulted in massive ice and rock mixtures whose surface ice had largely sublimated over billions of years, resulting in the current rock- and metal-rich asteroid composition.
At the same time, in addition to our last gas giants, asteroids left over from the early stages of the solar system still exist today. Although they may merge together, collide, interact with each other, and occasionally be thrown into the inner solar system by a gravitational slingshot, they remain largely outside of Neptune as a relic of the youngest stage of our solar system. In many ways, these Kuiper Belt and Oort Cloud objects are the oldest and most primitive remnants of the Milky Way Cosmic Corner since its inception.
As the solar system evolves, volatile matter is evaporated, planetary accretion, asteroids merge together, and orbits migrate to a stable configuration. Gas giants may gravitationally dominate the dynamics of our solar system and may migrate outward over time, but in terms of life, the rocky planets inside are where all the interesting biochemistry happens. **astromark/wikimedia commons
But for our purposes, the most interesting place is the inner solar system. Although we have learned that massive planets are common in the interior of exoplanetary systems, with sun-like stars at their cores, they are not mandatory. The easiest way to get rid of any early massive planet is to either be swallowed by the young sun, or have the innermost gas giant move closer to the inner region in the early stages, but then migrate outward. Either way, something delayed the formation of the inner solar system planets, making the four worlds that had already formed – Mercury, Venus, Earth and Mars – much smaller and less massive than the giant outer planets that had formed.
Once the inner solar system was able to form planets steadily, we still had any elements left behind, which we knew were mostly heavy elements in the planetary density measurements we have today, and the rocky planets of our solar system appeared. Each one has a mantle made of heavy metals, accompanied by a less dense mantle made of material that later falls on the core, including volatiles, which would have been boiled off if they had not been anchored to the planet inside the frost line. After millions of years of this evolution and formation, the planets were similar in size and orbit to what they are today.
When two objects in space collide, whether it's an asteroid, a moon, a rocky planet, or even a giant planet, it creates a large cloud of debris that blocks light. When this cloud passes in front of its parent star relative to an outside observer, the star will appear faint and dim. Many of these collisions may have occurred between asteroids in our early solar system, creating a fascinating set of planets and lunar systems. **nasa/jpl-caltech
But there is a huge difference: in these early stages, the Earth does not have our Moon. In fact, Mars does not have moons either. There may be not just four young planets, but six: a young planet as big as Mars but closer to Earth called Theia, and an as-yet-unnamed planet that is closer to Mars and may have a smaller mass than the Red Planet. Before Earth and Mars had moons, they were like Venus and Mercury: isolated worlds that only had to worry about other massive objects colliding with them or gravitational disturbance of them. Only after a major impact will the Earth-Moon system or the satellite system around Mars appear.
How so? Through some type of massive impact, a massive mass hits one of these early worlds, kicking up planetary fragments, some of which will fall back onto the parent planet, but some of which will eventually merge into one or more moons. For Earth, the idea was not particularly taken seriously until we went to the moon and surveyed the rocks we found on the surface of the moon. Surprisingly, the Moon has the same stable isotope ratio as Earth, while all the other planets in the solar system have different isotope ratios. In addition, the rotation of the Earth and the orbit of the Moon around the Earth have a similar direction, the Moon has an iron core, and all the facts point to the common origin of the Earth and the Moon.
The giant impact hypothesis states that a Mars-sized object collided with early Earth and that debris that did not fall back to Earth formed the Moon. This is known as the giant impact hypothesis, and while this is a compelling narrative, it may only have elements of truth rather than a complete story. All rocky planets with large moons could acquire them through such collisions. **nasa/jpl-caltech
Originally, theories about how such collisions occurred were known as the Giant Impact Hypothesis, which theoretically involved an early collision between the primordial Earth and a Mars-sized world known as Theia. The Pluto system has five moons and the Mars system has two moons (and may have once been three), and they all show similar astronomical evidence that they were created by a huge impact long ago.
However, the giant impact hypothesis, at least in its original form, has been shown to pose problems that are difficult to reconcile with actual data. Now it would be better for a smaller (but still very large) impact, from an object with a significantly higher velocity than another planet in adjacent orbit, that could be the creation of our Moon. A high-energy, high-velocity collision with the primordial Earth could form a disk of debris around our world, and instead of the original concept of a gigantic impact, a new type of fluffy ring-like structure around our planet called synesthesia. At the same time, the impactor's core will enter the Earth's interior, where it will not only merge with the Earth's core, but may even remain intact inside, and geophysical evidence will reveal its existence to humanity billions of years later.
Lenovo will consist of a mixture of vaporized material from the larger mass planet Protoplanet and a smaller impactor, which will form one or more large moons inside through the merger of small satellites. It is a general scenario to be able to create a single large moon, with the physical and chemical properties that we observe the Earth-Moon, multiple moons, such as those around Mars or Pluto, or more complex systems around a higher quality world. **s.j. lock et al.， j. geophys.Research, 2018
The Moon has four major properties that must be explained for any successful theory of its origins:
Why is there only one large moon and not many, why are the isotope ratios of elements so similar between the Earth and the Moon, and why the moderately volatile elements in the Moon are depleted.
And why the Moon is tilted relative to the Earth-Sun plane.
Isotope ratios are particularly interesting for the giant impact hypothesis. The similar isotopic properties between the Earth and the Moon suggest that if both the impactor (theia) and the Earth are large, they must form at the same radius as the Sun. This is possible, but the model for the formation of the Moon by this mechanism does not give the correct angular momentum properties. Similarly, a predatory collision with the correct angular momentum produces a different isotope abundance than what we see.
That's why another newer scenario – synesthesia – is so appealing. If you had a fast, high-energy collision between a smaller, less massive celestial body and our original Earth, you would form a huge ring structure around Earth. This structure is called synesthesia and is made of vaporized material from a mixture of the primordial earth and the impacting object. Over time, these materials mix together to form many mini-moons (called mini-satellites) in a short period of time, which can stick together and gravitate to form the Moon as we observe it today. At the same time, most of the matter in synesthesia, especially the interior, will fall back to Earth. We can now talk about it in terms of generalized structures and scenarios that produce large moons like ours, rather than single, man-made mega-impacts, and note that similar things could have happened on Mars in the early stages of the solar system.
About 50 million years after the formation of the Earth, it was hit by a large object the size of Mars called Theia. The consequences of the collision overheated the Earth and triggered a large amount of debris, a significant part of which eventually formed the Moon. The rest either escaped from the Earth-Moon system or fell back into one of the two celestial bodies. While the far side of the Moon cools faster, the near side stays hotter for much longer due to facing the hot Earth. This is one of the main scenarios that explains the difference between the Moon facing the Earth and the opposite hemispheres of the Earth. **mark a. garlick
It is almost certain that in the early stages of the solar system, there was a high-energy collision with an alien extraorbital object that hit our young Earth, and this collision was necessary to produce our Moon. But it was most likely not as massive as Mars today, and it was almost certainly a solid impact rather than a glimpse one. Instead of a cloud of rock fragments, the structure formed is a new type of expanded, vaporized disk called synesthesia, which has quickly become the preferred scenario for the formation of moons around rocky planets in general. Over time, this synesthesia stabilized, forming the Earth and the Moon as we know it today, with the Moon initially very close to the Earth and very hot, cooling over time and spiraling outward.
At the end of the early stages of the solar system, conditions are promising for life, the raw materials of rocky planets with rarefied atmospheres are located at the right distances, and liquid water can stably exist on its surface. With volcanic activity spewing water from the Earth's interior, and being brought to our planet from extraterrestrial during the heavy bombardment of our solar system's early history, we have every reason to think that a young, barren Earth might be habitable. With a central star, three rocky worlds rich in atmospheres (including Venus and Mars), primordial elements needed for life, and gas giants that exist only farther away, all the debris is ready for the emergence of life. We know that we are lucky that humanity has appeared. But with this new understanding, we also think that the possibility of life like ours has happened millions or even billions of times elsewhere across the galaxy.