Anyone growing up in the 1990s or earlier would recollect that our solar system had nine planets, but did you ever wonder if planets exist outside the solar system? Planets found outside of our solar system are called extrasolar planets or exoplanets. Approximately 5,600 exoplanet candidates have been discovered since 1993, and nearly 2,000 exoplanets have been confirmed since 1995. Now we know that billions of stars in our galaxy have one or more planets orbiting them. Since the universe has billions of galaxies and trillions of stars, there are at least trillions of planets in our universe.
Astronomers have long wondered how these planets form. According to the “solar nebula” theory, the gravitational collapse of a cloud of gas leads to the formation of a star and planets orbiting it. When such a gas-cloud collapses under its own gravity, most of its mass accretes into the central region of the cloud. Then nuclear fusion begins producing energy in this very high-density and high-pressure central region, and thus, a star is born.
The initial collapse of the gas cloud creates a circumstellar disk, and from this disk, planets are formed. Once planet formation begins, the planet-forming disk is called a protoplanetary disk. As the gas is spinning, the angular momentum is transported outward, and the mass is accreted onto the central star in a few million years. Astrophysicists have considered several physical processes that could be responsible for the angular momentum transport. Gravitational Instabilities provide one such mechanism. At Indiana University, researchers have been studying gravitational instabilities in disks for over two decades.
The study of gravitational instabilities helps us understand the angular momentum transport in a protoplanetary disk, especially when a disk is in very early evolutionary stages. Understanding gravitational instabilities thus helps us understand the overall process of planet formation. In a recently finished work, I compared gravitational instabilities in a disk around one star and another disk around two stars. A disk around two stars can be seen in Figure 1. More than 60 percent of the stars in our galaxy are found in multi-stellar systems which consist of two or more stars. Approximately 20 exoplanets have been discovered in about 15 disks around two stars.
My research group performed numerical simulations of a disk around one star and a disk around two stars to understand the evolution and different physical processes in the disks. No significant differences were found between the disks, but we found an anticyclonic vortex in the inner region of both disks. Basically, the anticyclonic vortex is a relatively smaller region of gas (see red oval-shaped banana in Figure 2) spinning around a central point (center of the banana). This small region spins in the opposite direction as the gas in the disk, which is why it is called anticyclonic. This was an interesting discovery because such vortices can be a birthplace of small planetary cores when the vortices accrete particles in their pressure-maxima centers. Another example of an anticyclonic vortex that you may be familiar with is the giant red spot on Jupiter.
Recent observations of protoplanetary disks from the state-of-the-art observatory ALMA have shown the formation of gaps in such disks. The inner edges adjacent to a gap are known to be a site for the formation and growth of a vortex. Our simulations show the formation of a vortex near the inner edge of the disk as shown in Figure 2, and thus explain some of the protoplanetary disk-observations.
Although more sophisticated codes that can simulate particle accumulation at such sites are required to better understand the process of vortex formation and its relation to the formation of a planetary core, our study is a small step in the right direction. Understanding how small structures of matter form inside a disk will ultimately lead us to elucidate how the planets form in any solar system. Future work related to gravitational instabilities will include studying the migration of a planet in a protoplanetary disk. Stay tuned for more updates on this front.