Artist's concept of early solar nebulae. This illustrates material in the disk as the disk cools and coalesces. Eventually, it becomes rocky planets. The composition of the rocky and meteorites in our Solar system is different from that of the Sun. This is a puzzle as both come from the same molecular cloud material. Astronomers have created complex simulations of the temperature evolution of the young planet disk. They conclude that the Solar Nebula formed under unusual conditions. This could have been due to the formation from a parent molecular center with unusually high temperatures or some other energy source heating it. Credit: USRA/LPI
The same reservoir of nebular materials is used to form planets. Their chemical compositions should be closely related. However, the observed compositions for planets are not identical to their central stars. For example, the majority of the rocky planets, and planetesimals in our Solar system have near-solar amounts of refractory element (elements such as aluminum that condense from gas when temperature drops below 1500 kelvin), but they are also depleted with volatile elements (elements that evaporate quickly, such as nitrogen). Astronomers believe this is due to planets being formed by the coalescence or condensed minerals.
The initial, cold molecular cloud core will collapse and form a disc. Heating from the new star, plus the viscosity, can vaporize some primordial condensed matter. This allows the condensation sequence to start over again, but under much higher pressure and temperature conditions. Astronomers also study meteorites of different types to determine their chemical makeup. The properties of the molefular cloud core, and the disc may have affected the ability to vaporize most of the pre-existing material. The complex nature of the planetesimals' chemistry makes it difficult to understand how different minerals condense in different places and times.
Michail Petaev, a CfA geologist, and his colleagues created a simulation of the collapse of a molecular clouds core and the formation star, disk, and planets. They also analyzed the changing distribution of temperatures across the disk in order to determine the mineral condensation sequence. The properties of the initial cloud center have a significant impact on the maximum temperatures in the disk and the resulting compositions of planets and asteroids. The maximum temperature is reached at the end of the collapse phase after a few hundred of thousands of years. The composition of the star is very similar to the molecular cloud center's, but the star may be slightly less refractory than the core. This could mean that the stellar composition might not accurately reflect the original composition. Refractory-rich planets will only be produced by cloud cores that have high initial temperatures or low disk rotation. They conclude that to reproduce the composition of the Solar system meteorites as well as the terrestrial planets, either the initial core must have rare properties such temperatures close to 2000 kelvin (well beyond the expected median value 1250 kelvin) or some other source of heating.
The Monthly Notices of the Royal Astronomical Society published the research.
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More information: Min Li and al, Maximum temperatures for the formation of protoplanetary discs, and the composition of planetary buildings, Monthly Notices of the Royal Astronomical Society (2021). Information for the Journal: Monthly Notices from Royal Astronomical Society Min Li and al, Maximum temperatures to evolve protoplanetary discs, and composition of planetary-building blocks, (2021). DOI: 10.1093/mnras/stab837
