When you hear the phrase “spiral arms,” ​​you probably think of galaxies. Many galaxies have bright arcs of stars receding from their center, including our Milky Way. But not all galaxies have spiral arms, and galaxies aren’t the only celestial objects with spiral arms. About a third of protoplanetary disks around young stars have spiral arms, and now we think we know why. In galaxies, spiral arms are caused by density waves within the galactic disk. The density waves create a sort of traffic jam effect, where individual stars move in and out of spiral arms, but the overall spiral structure is maintained. The spiral structure is further maintained by the increased density of gas and dust in the arms, which triggers the production of stars in the arms.

Protoplanetary disks have a structure similar to young galaxies. Both are a flat disk of gas and dust that orbit around a huge central bulge. But the difference in scale and age means that we can’t just say that the spiral arms of galaxies and planetary disks have the same cause.

One model for the spirals of planetary disks is that they form similar spiral galaxies. Essentially, gravitational instabilities within the disk cause density fluctuations that rapidly evolve into a spiral structure. The problem with this idea is that, unlike galaxies where stars only interact gravitationally, the gas within a disk exerts pressure that would break the spiral structure.

Another idea is that the spiral structure is triggered by the presence of a large protoplanet. A Jupiter-sized object in a planetary disk would generate turbulence and a gravitational tug that could cause spiral arms to form, like ripples in a pond. The only problem with this idea is that large protoplanets have never been seen in a spiral protoplanetary disk. That is, until now.

Astronomers have discovered a Jovian protoplanet orbiting a young star known as MWC 758, located about 500 light-years from Earth. The planet, named MWC 758c, is about twice the mass of Jupiter and orbits its young star at a distance of about 100 AU, more than three times the distance between Neptune and the Sun. The spiral disk system has been known for a long time. some time, but previous observations have not shown evidence of a planet. This is because MWC 758c is particularly red, which means it is either very cold or covered in a lot of dust. The low red wavelengths are difficult for ground-based telescopes to observe due to thermal noise from the Earth’s atmosphere. It took the Large Binocular Telescope Interferometer (LBTI), which specializes in infrared and near-infrared observations, to finally observe the gas giant. The team plans to continue their ground-based observations with observations from the James Webb Space Telescope (JWST). This should provide even more detailed images, which will help them understand how the planet formed and the interactions between the planet and the spiral structure of the system.

source: https://www.nature.com/articles/s41550-023-02028-3