When you hear the phrase spiral arms, you probably think of galaxies. Many galaxies, including our own Milky Way, have arcs of bright stars receding from their centers. But not all galaxies have spiral arms, and galaxies are not the only celestial bodies with spiral arms. About a third of protoplanetary disks around young stars have spiral arms, and we think we now know why.

In galaxies, the spiral arms originate from density waves within the galactic disk. Density waves create a kind of traffic jam effect where individual stars enter and exit spiral arms, but the overall spiral structure is preserved. The spiral structure is supported by a higher density of gas and dust that causes star formation in the arms.

Protoplanetary disks have a structure similar to young galaxies. Both are flat disks of gas and dust swirling around a large central ridge. But the difference in scale and age means that we cannot simply say that the spiral arms in galaxies and planetary disks have the same cause.

One pattern of planetary disk spirals is that they form in a similar fashion to spiral galaxies. In fact, gravitational instabilities inside the disk cause density fluctuations that quickly turn into a spiral structure. The problem with this idea is that, unlike galaxies, where stars interact only gravitationally, the gas inside the disk exerts pressure that can destroy the spiral structure.

Another idea is that the helical structure is due to the presence of a large protoplanet. A Jupiter-sized object within the planetary disk will generate turbulence and gravitational force that can cause spiral arms to form, similar to ripples in a pool. The only problem with this idea is that large protoplanetaries have never been observed within a spiral protoplanetary disk. So, until now.

Modeling how a protoplanet might form spiral disk arms. Credits: L. Krapp and K. Kratter/University of Arizona.

Astronomers have discovered a proto-Jupiter planet about 500 light-years from Earth orbiting a young star known as MWC 758. The planet, named MWC 758c, has about twice the mass of Jupiter and orbits its young star at a distance of about 100 AU, more than three times Neptune’s distance from the Sun. The spiral disk system has been known for a long time, but previous observations had not shown any signs of planets. This is because the MWC 758c is particularly red, meaning it’s either too cold or surrounded by too much dust. Low red wavelengths are difficult to observe with ground-based telescopes due to the thermal noise of the Earth’s atmosphere. To finally observe the gas giant, the Large Binocular Telescope Interferometer (LBTI), which specializes in infrared and near-infrared observations, is needed.

The team plans to continue their ground-based observations from the James Webb Space Telescope (JWST). This should provide more detailed images to help them understand how the planet was formed and the interaction between the planet and the spiral structure of the system. The MWC 758 system is only a few million years old, but other than that it looks very similar to our own Solar System. It is quite possible that the protoplanetary disk of the Sun, which played a decisive role in the formation of the Earth, has a similar spiral structure. Source