Planet formation begins in the protoplanetary disks through nuclear accretion, where gravity causes particles in the disk to stick together. The process eventually leads to the formation of large solid bodies such as asteroids or planets. After the planet’s birth, it begins to cut holes in the protoplanetary disk, like grooves on a vinyl record.
In addition to the grooves, ALMA observations have revealed even more peculiar structures in protoplanetary disks, including clusters and arcs with shapes that resemble bananas or peanuts. It was believed that planets were also responsible for powering at least some of these structures.
There must be something that causes these structures to form. One of the possible mechanisms for producing these structures – and certainly the most intriguing – is those dust particles that we see as arcs and clumps concentrated in the centers of fluid vortices: essentially small hurricanes that can be caused by some instability to the edges of the openings carved into protoplanetary disks by planets.
Researchers at the University of Cambridge and the Institute for Advanced Study have developed a technique that uses observations of these ‘hurricanes’ by the Atacama Large Millimeter/submillimeter Array (ALMA) to set limits on the mass and age of planets in a young star system. According to researchers, these small “hurricanes” can be used to study certain aspects of planet formation, even for smaller planets that orbit their star at great distances and are beyond the reach of most telescopes.
The two researchers first theorized how long it would take a planet to create a vortex in its disk to develop their technique. By placing lower limits on the planet’s mass or age, they used these calculations to constrain the parameters of planets in vortexed disks. They refer to these methods as “vortex dating” and “vortex weighting” of planets.
The telltale gap in the disk is the result of a growing planet that begins to push material away from the disk once it has grown large enough. As a result, the material on the outside of the opening becomes denser than the stuff inside the opening. An instability can develop as the gap widens and the density differences widen. This instability disrupts the intervertebral disc, which can eventually result in a vortex.
Ph.D. student Nicolas Cimerman said: “Over time, multiple vortices can merge and evolve into one large structure similar to the arches we observed with ALMA. Because the eddies take time to form, the researchers say their method is like a clock that can help determine the planet’s mass and age.”
Lead author Professor Roman Rafikov from Cambridge’s Department of Applied Mathematics and Theoretical Physics said: “Larger planets produce vortices earlier in their development because of their stronger gravity, so we can use the vortices to put some constraints on the planet’s mass, even if we can’t see the planet directly.”
Astronomers can estimate a star’s age using numerous data points such as luminosity, velocity and spectra. With this knowledge, the Cambridge researchers calculated the smallest planet mass that may have been in orbit since the protoplanetary disk formed and were able to generate an ALMA-observable vortex. This allowed them to estimate the mass of the planet without directly observing it.
The scientists found that the potential planets responsible for these vortices must have masses of at least several tens of Earth masses, in the super-Neptune range, by applying this technique to several known protoplanetary disks with significant arcs suggesting vortices.
Cimerman said, “In my day-to-day work, I often focus on the technical aspects of running the simulations. It’s exciting when things come together, and we can use our theoretical findings to learn more about real systems.”
Rafikov said, “Our limitations can be combined with the limits of other methods to improve our understanding of planetary features and planet formation pathways in these systems. By studying planet formation in other galaxies, we can learn more about how our own solar system has evolved.”
- Roman R. Rafikov and Nicolas P. Cimerman. “Vortex Weighing and Dating of Planets in Protoplanetary Discs.” Monthly Notices of the Royal Astronomical Society (2022). DOI: 10.1093/mnras/stac3692 or DOI: 10.48550/arXiv.2301.01789
- Nicolas P. Cimerman and Roman R. Rafikov. “Emergence of Vortices at the Edges of Planetary Gaps in Protoplanetary Discs.” Monthly Notices of the Royal Astronomical Society (2022). DOI: 10.1093/mnras/stac3507