For decades, astronomers have had a rough picture of how planets around stars (such as the Sun) (such as Earth or Mars) formed: around a young star, a protoplanetary disk of gas and dust appeared, in which small objects appeared and grew larger and larger, eventually reaching a diameter of several thousand kilometers, in other words, they became planets.
But the problem is that there are still many details in this simple picture that remain unexplained. Recently, in a new study, astronomers used a series of simulations to explore different possibilities for the evolution of planets within the band. They found that the inner solar system is actually a rare but possible evolutionary result. The study was recently published in Nature astronomy.
Image credit: pixabay
The swirling discs and rings changed everything
In recent years, due to the improvement of observation methods, the modern planet formation picture has been improved and improved in a very specific direction.
The most striking change came from the protoplanetary disk around the young star Taurus HL released by ALMA (Atacama Large Millimeter Wave/Submillimeter Wave Array) in 2014. It reveals in unprecedented detail the nested structure of the rings and ring slits that are clearly visible in this dish.
In 2014, ALMA revealed for the first time the ring-like structures in protoplanetary disks. | Image credit: ALMA (ESO/NAOJ/NRAO), S. Andrews et al.; NRAO/AUI/NSF, S. Dagnello
The researchers believe that this ring and ring slit is often associated with a kind of "pressure bump", that is, the local pressure in some places is lower than in the surrounding area. These local changes are usually inseparable from the changes in the composition of the dish, mainly the difference in the size of the dust particles.
In particular, certain pressure bumps are associated with three important shifts in the session.
Very close to the star, at temperatures above 1400 Kelvin, silicate compounds are gaseous because the temperature there is so high that they cannot exist in other states. Of course, this also means that planets cannot form in such a high temperature region. At this temperature, the silicate compound will sublimate, that is, any silicate gas will be directly transformed into a solid state. This pressure bump defines an integral inner boundary for the formation of the planet.
Farther away, at 170 Kelvin conditions, a transition from water vapor to water ice occurs, which is known as the water snow line.
At lower temperatures, at 30 Kelvin, there is a carbon monoxide snow line, and below this temperature, carbon monoxide forms solid ice.
Image credit: Rajdeep Dasgupta
What does this mean for the formation of planetary systems? Many early simulations have shown how this pressure bump contributes to the formation of starlets.
The starting point of the planet formation process is dust particles. These particles tend to congregate in low-pressure regions of pressure bumps, and dust particles of a certain size drift inward (in the direction of the star) until they are blocked by the high pressure at the inner boundary of the bump.
As the concentration of particles increases at pressure bumps, especially the ratio of solid matter (tending to aggregate) to gas (tending to push particles away), these particles are more likely to form "gravel" between a few millimeters and a few centimeters in size, that is, a collection of solids, and then these gravel are also more likely to continue to accumulate into larger objects.
But the problem that remains unresolved is the role of these substructures in the overall shape of planetary galaxies, such as our solar system, which has the distribution of rocky, in-band terrestrial planets, and out-of-band gaseous planets. That's what the new institute is trying to solve.
Simulates the inner solar system
In the search for answers, the scientists combined several simulations that covered different aspects and stages of planet formation.
Specifically, astronomers have modeled a gas disk with three pressure bumps at the boundary where silicates become gases and at the snow lines of water and carbon monoxide. They then simulated how dust particles grow and break in the gas disk and the formation of starlets.
An interesting question in the simulation is this: If the initial setup is slightly different, is the final result still similar? Understanding such variations is important for understanding which components are key to the simulation results. The team thus analyzed a series of different scenarios with different compositional properties and temperature curves of protoplanetary disks.
These results suggest a very direct link between the emergence of our solar system and the ring-like structure of its protoplanetary disk. As expected, in these models, starlets naturally form near pressure bumps.
For the simulated portion of the inner planetary galaxy, the researchers determined the correct conditions to form similar to the solar system. If the region beyond the innermost (silicate) pressure bump contains about 2.5 Earth-mass starbets, those planets will grow into Mars-sized objects, which is consistent with the in-band planets of the Solar System.
Disks with greater masses, or more efficient star-forming, can lead to the formation of super-Earths, i.e., more massive rocky planets. These super-Earths will orbit the host star closer, just above the innermost pressure bump boundary.
The existence of this boundary may also explain why there is no planet closer to the Sun than Mercury, because the necessary material is evaporated so close to the star.
The simulations here can also explain the slightly different chemical compositions of Mars and Earth and Venus. In the model, Earth and Venus collect most of the material from regions closer to the Sun than Earth's current orbit, which will form their main body. Unlike this, the "Mars-like" in the simulation is mainly composed of materials from regions slightly farther away from the Sun.
Asteroid belts, giant planets and Kuiper belts
Beyond the orbit of Mars, the simulated areas began to become scarcer and, in some cases, even completely devoid-free of starons, the precursors of today's asteroid belt in our solar system.
However , some astrologers from nearby regions in or out of the band will later intrude into regions of the asteroid belt and become trapped there. When these starlets collide, the smaller debris produced will form the asteroids seen today.
These simulations can even explain the emergence of different asteroid groups. Astronomers call S-type asteroids, or bodies made primarily of silica, are remnants of stray bodies originating in the region around Mars, while C-type asteroids, which are predominantly carbon-containing, are remnants of stray bodies from outside the asteroid belt.
And further afield, in addition to the pressure bumps that mark the limits of water ice, the simulations also show the starting point of giant planet formation. Planetary margins near this boundary typically have a total mass of 40 to 100 times the mass of Earth, which is consistent with the total mass estimates of the nuclei of the giant planets in our solar system (Jupiter, Saturn, Uranus, and Neptune).
Finally, the simulations could also explain the last class of objects and their properties, the so-called Kuiper Belt objects, which form outside the outermost pressure bumps. It can also tell us about the subtle differences in composition between known Kuiper Belt objects.
Relatively rare solar system
Overall, extensive simulations bring two basic results.
The first is that pressure bumps at the water and ice lines form very early, in which case the inner and outer regions of the planetary system have parted ways within the first 100,000 years. This has led to the formation of low-mass land planets in the inner-band region, which is very similar to what happens in the solar system.
The second is that if the pressure bumps in the ice and water line form later than this, or are not so obvious, more mass can drift into the in-band region, which in turn leads to the formation of super-Earths or ultra-small Neptunes in the introgeners.
From the current observational evidence of exoplanet galaxies, the second scenario is more likely, while our solar system is a relatively rare result of planet formation.
Explore the wider area
In this study, astronomers focused on the inner solar system and terrestrial planets. Next, they hope to run simulations that include more details about the outer solar system, such as Jupiter, Saturn, Uranus, and Neptune. The ultimate goal of the study is to come up with a complete explanation of the properties of the solar system and other planetary systems.
At least for the inner solar system, we now know that the key properties of Earth and its nearest neighboring planets can be traced back to some fairly basic physics, namely the boundary between water ice and water vapor and associated pressure bumps in the swirling disk of gas and dust surrounding the young sun.
#创作团队:
Compile: M ka
Typography: Wenwen
#参考来源:
https://www.mpia.de/5793300/news_publication_18027576_transferred
https://nautil.us/blog/planets-are-born-from-dust-trap-rings
#图片来源:
Cover: Pixabay
Caption: Pixabay