Planet formation begins in the protoplanetary discs through core accretion, in which gravity causes particles in the disc to stick to each other. The process eventually leads to forming large solid bodies like asteroids or planets. After the planet’s birth, it starts carving gaps in the protoplanetary disc, like grooves on a vinyl record.
In addition to the grooves, ALMA observations have revealed additional peculiar structures in protoplanetary discs, including clusters and arcs with shapes resembling bananas or peanuts. It had been assumed that planets were also responsible for powering at least some of these structures.
Something must be causing these structures to form. One of the possible mechanisms for producing these structures – and certainly the most intriguing one – is those dust particles that we see as arcs and clumps are concentrated in the centers of fluid vortices: essentially little hurricanes that can be triggered by a particular instability at the edges of the gaps carved in protoplanetary discs by planets.
Researchers from 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/submillimetre Array (ALMA) to place some limits on the mass and age of planets in a young star system. According to researchers, these little ‘hurricanes’ can be used to study certain aspects of planet formation, even for smaller planets that orbit their star at large distances and are out of reach for most telescopes.
The two researchers first theorized how long a planet would need to create a vortex in the disc to develop their technique. Then, by putting lower limitations on the planet’s mass or age, they used these calculations to constrain the parameters of planets in discs with vortices. They refer to these methods as “vortex dating” and “vortex weighing” of planets.
The tell-tale gap in the disc results from a growing planet beginning to push material from the disc away once it has grown large enough. As a result, the material on the gap’s outside grows denser than the stuff inside the gap. An instability may be created as the gap widens and the density differences increase. This instability disturbs the disc, which may eventually result in a vortex.
Ph.D. student Nicolas Cimerman said, “Over time, multiple vortices can merge, evolving into one big structure that looks like the arcs we’ve observed with ALMA. Since the vortices need time to form, the researchers say their method is like a clock that can help determine the mass and age of the planet.”
Lead author Professor Roman Rafikov from Cambridge’s Department of Applied Mathematics and Theoretical Physics said, “More massive planets produce vortices earlier in their development due to their stronger gravity, so we can use the vortices to place some constraints on the mass of the planet, 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’s mass that might have been in its orbit since the protoplanetary disc originated and was able to generate an ALMA-observable vortex. This enabled them to estimate the planet’s mass 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 discs with significant arcs that are suggestive of vortices.
Cimerman said, “I often focus on the technical aspects of performing the simulations in my daily work. It’s exciting when things come together, and we can use our theoretical findings to learn about real systems.”
Rafikov said, “Our constraints can be combined with the limits provided by other methods to improve our understanding of planetary characteristics and planet formation pathways in these systems. By studying planet formation in other star systems, we may learn more about how our own Solar System 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 planet-driven gaps in protoplanetary discs.’ Monthly Notices of the Royal Astronomical Society (2022). DOI: 10.1093/mnras/stac3507