How did the solar system get its typical structure? And why are the inner planets the way they are? Astronomers may have found answers to these questions by reconstructing the conditions in the solar system’s protoplanetary disk. According to this, three ring-shaped border zones were characteristic for the formation of planets around the young sun: an inner temperature limit above which silicates evaporate, and two snow limits – zones above which water or carbon dioxide freezes to form ice. Because dust and planetesimals accumulated at these transition zones, the earth-like planets formed on the first ring, the gas giants on the middle one and the Kuiper belt on the outside.
Young stars are usually surrounded by rotating clouds of gas and dust. In this protoplanetary disc, the gradual agglomeration of material can then form celestial bodies – planets and their moons. In the meantime, astronomers have observed many planetary systems around other stars in addition to the solar system. But there are still many open questions. This includes how the planets form in such systems and why, for example, neither super-Earths nor mini-Neptunes exist around the sun, although they are common around other stars. However, model simulations suggest that the position and presence of certain phase transitions in the protoplanetary disk play a role – zones determined by temperature, from which, for example, rock no longer evaporates or water and other molecules freeze.
The secret of the pressure thresholds
Andre Izidoro from Rice University in Houston and his colleagues have now examined in more detail how such transition zones specifically influence planet formation. The starting point for this was the observation that many protoplanetary disks are divided into an almost regular sequence of rings and gaps. “Such ring-like structures suggest that dust and chunks are concentrated there in pressure thresholds of the disk,” the astronomers explain. The material that tends to migrate inward toward the star in the rotating cloud appears to be trapped and pooled at these ridges. The researchers tested the consequences of this for planet formation with a simulation in which they reconstructed the development of the young sun and its protoplanetary disk – sometimes with and sometimes without the pressure thresholds that were present at the time.
There were three such transition zones in the young solar system: In the immediate vicinity of our star, at temperatures of more than 1400 Kelvin, rock-forming silicate compounds only occur as gas. Planets can therefore only form outside of this inner boundary, because only there do dust and small pieces of rock remain. Further out are the so-called snow lines, the transition zones where water vapor and carbon monoxide freeze to form ice. These thresholds are at temperatures of minus 100 and minus 240 degrees Celsius. As the protoplanetary disk gradually cools, these transition zones also slowly migrate inward over time.
Transitional zones determine planetary size and number
The simulations confirmed that the pressure thresholds at the three transition zones lead to an accumulation of dust and planetary building blocks. The dust grains and lumps drifting inwards accumulate at the border regions and are increasingly found there to form planetesimals and thus to planetary building blocks. The inner planets form just outside the silicate boundary. How big they get and how many planets there are depends on how much material is available between the innermost pressure threshold and the next transition zone, the water-snow line. “Only the dust that occurs within the snow line can contribute to planetesimal formation in the inner disc,” explain Izidoro and his colleagues. If the snow line forms early and thus prevents the replenishment from the outer regions of the primordial cloud from migrating inwards, the material is only sufficient for the formation of low-mass, Mars- to Earth-sized planets – as was the case in the solar system. On the other hand, if the water-snow line forms later or only weakly, more material can drift into the inner area and super-Earths or mini-Neptunes can form in the inner part of the planetary system.
In the outer solar system, just beyond the water-snow line, material also collects and forms the building blocks of the later gas giants. In the simulation, an accumulation of 40 to 100 Earth masses of dust and debris led to the formation of cores of giant planets corresponding to Jupiter, Saturn, Uranus and Neptune. The orbits of these giants are initially very close together. Uranus and Neptune move outward to their current distances from the Sun only later. Farther out, beyond the carbon monoxide snowline, the icy bodies of the Kuiper Belt formed in a similar manner. “It came as a complete surprise to me how well our models were able to depict the evolution of a planetary system like our solar system – right down to the slightly different masses and chemical compositions of Venus, Earth and Mars,” says co-author Bertram Bitsch from the Max Planck Institute for Astronomy in Heidelberg. The results of the simulation confirm that basic physical parameters in the form of phase transitions and pressure thresholds in protoplanetary disks first allow rings and gaps to form, and then the planets.
Source: Andre Izidoro (Rice University, Houston) et al., Nature Astronomy, doi: 10.1038/s41550-021-01557-z