Planets on a collision course
The binary star system XZ Tauri, 450 light-years from Earth, could be an interesting sight in a few billion years. As researchers have discovered with the help of the Atacama Large Millimeter/submillimeter Array (ALMA), the protoplanetary disks of the two stars are perpendicular to each other, see animation. The first planets are apparently being formed in these disks, which consist of gas and dust. When this process is complete, it should have a fascinating (but possibly dangerous) effect for the system’s inhabitants: Again and again, planets belonging to another star approach and move away again – depending on the rotation of the two stars around each other and on the rotation of their planetary systems.
But the insight is also relevant for us today and here. Indeed, researchers have proposed two formation mechanisms for binary star systems: first, the breakup of a single large disk of gas, and second, the fragmentation of the larger molecular cloud due to violent turbulence. In the former case, astronomers assume that the orbits of the binary stars and the individual disks should lie in the same plane. In the latter case, however, the orbital plane of the binary stars and the plane of the disks are expected to be different. This is an important aspect that will affect the final orbits of planets in binary star systems.
The two authors of the research paper from Kagoshima University, Japan, did not have to make new observations on this. They accessed the ALMA data archive and obtained data for the young XZ Tauri system from 2015, 2016, and 2017, and carefully analyzed the data to produce, for the first time, an animation of the binary stars’ orbital motion, showing that XZ Tau B moved 3.4 astronomical units (3.4 times the radius of Earth’s orbit) around XZ Tau A during those three years.
The team determined the three-dimensional structure of the orbit. Analyzing the Doppler effect and the distribution of radio waves from the disks around each star in the XZ Tauri system, they also found that these disks are significantly offset from each other and are also not in the same plane as the orbit of the stars around each other. Previous observations with ALMA had already found examples of young binary stars with protoplanetary disks inclined with respect to each other. However, this is the first time that the orbital motion of a binary star system has been clarified, and the inclination was found to be different from that of circumstellar disks. These results support the idea that the XZ Tau system was formed by the fragmentation of molecular clouds.