The Shadow Play of Forming Planets

Planets are thought to form inside disks of gas and dust, called protoplanetary or circumstellar disks, orbiting young stars. In recent years, large telescopes such as ALMA have delighted us with detailed images of these disks. However, it has been difficult to detect planets in their formation process, although we can deduce that they are present by the traces they leave in the protoplanetary disks. 

Such traces are variations in the structure of the disks and can appear as spirals in the gas, gaps, cavities, and even shadows. A recent research led by Matías Montesinos, collaborator Millenium Nucleus of Planetary Formation (NPF) and research associate of the Max Planck Tandem Group, modeled a protoplanetary disk to understand how a planet in formation can cast a shadow on the disk that hosts it and what is the observational evidence of these shadows. This is because the difficulties of studying planets in formation directly can be minimized by investigating such indirect effects.

The research, published in the prestigious scientific journal Astrophysical Journal, also involved NPF research associates Johan Olofsson (leader of the Max Planck Tandem Group) and Jorge Cuadra, the center’s director Amelia Bayo and postdoctoral researcher Clément Perrot.

To analyze the structure of a circumstellar disk, it is important to determine its height, which indicates how “inflated” it is in the vertical direction. This height, the astrophysicists explain, is produced by a constant struggle between the pressure of the gas in the disk and the gravity of the star and the gas. As a rule of thumb, the disk is expected to be higher the farther away from the star you are. 

In this research, hydrodynamic numerical simulations were used to model the effects that a forming planet embedded in a protoplanetary disk has on it. “When a planet is forming, the gas around it is very hot (above 1000K), which greatly increases the radiation pressure of the gas in the region near the planet. As the gas pressure increases locally, it is able to lift a column of gas and dust above the vertical of the planet, generating a real bulge on the surface of the disk”, explains Montesinos.

The protuberance that is generated on the surface of the disk, the astrophysicist points out, casts a shadow when illuminated by the central star. 

“This work gives us clues about the process of planetary formation, and how indirect effects of this process can be useful for discovering new worlds. From a more philosophical point of view, it gives us a look at how our own Earth was once assembled, leaving all kinds of traces and clues of this process around it”, the researcher emphasizes.

The next stage of the research is to include more detailed physics by calculating the vertical structure of the circumplanetary disk in regions where a planet is found. “Hydrogen is a very abundant element in space and, of course, present in the transfer of material from the disk to the planet, which we call accretion. With simulations, you can predict, depending on how this transfer of material is, how we might observe different spectral lines of hydrogen (kind of like its fingerprint). We are no longer so focused on the shadows of a planet, but on how to observe them directly”, concludes Montesinos.

Scientific publication


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