When you hear the phrase “spiral arms” you probably think of galaxies. Lots of galaxies have intense arcs of stars that spiral away from their center, including our Milky Way. Not all galaxies have spiral arms, and galaxies arent the only celestial items with spiral arms. About a 3rd of protoplanetary disks around young stars have spiral arms, and we now believe we know why.
In galaxies spiral arms are caused by density waves within the stellar disk. The density waves create a kind of traffic jam result, where specific stars wander into and out of spiral arms, but the overall spiral structure is kept. The spiral structure is more kept by the higher density of gas and dust in the arms, which triggers star production within the arms.
Protoplanetary disks have a comparable structure to young galaxies. They are both a flat disk of gas and dust orbiting a huge central bulge. The distinction in scale and age indicates we cant merely state that spiral arms in galaxies and planetary disks have the exact same cause.
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One design for planetary disk spirals is that they form similar to spiral nebula. Basically, gravitational instabilities within the disk trigger density variations that rapidly develop into a spiral structure. The issue with this idea is that, unlike galaxies where stars only engage gravitationally, the gas within a disk puts in pressure which would work to disrupt the spiral structure.
Another concept is that the spiral structure is activated by the existence of a large protoplanet. A Jupiter-sized object within a planetary disk would produce turbulence and a gravitational tug that might trigger spiral arms to form, like ripples in a pond. The only problem with this idea is that large protoplanets have never ever been seen within a spiral protoplanetary disk. That is, till now.
Simulation of how a protoplanet could develop disk spiral arms. Credit: L. Krapp and K. Kratter/University of Arizona
Astronomers have actually found a Jovian protoplanet orbiting a young star understood as MWC 758, which is about 500 light-years from Earth. The planet, called MWC 758c, is about twice the mass of Jupiter and orbits its young star at a range of about 100 AU, or more than three times the distance of Neptune from the Sun. The spiral disk system has actually been understood for a while, but previous observations revealed no evidence of a world.
The group intends on following up their ground-based observations with observations from the James Webb Space Telescope (JWST). This ought to offer even more comprehensive images, which will help them understand how the world formed and the interactions between the world and the spiral structure of the system.
The MWC 758 system is just a few million years of ages, but it is otherwise fairly similar to our own solar system. Its quite possible that the Suns protoplanetary disk had a comparable spiral structure, which would have played a vital function in the development of Earth.
Recommendation: Dong, Ruobing, Joan R. Najita, and Sean Brittain. “Spiral arms in disks: planets or gravitational instability?” The Astrophysical Journal 862.2 (2018 ): 103.
Recommendation: Wagner, K., et al. “Direct images and spectroscopy of a huge protoplanet driving spiral arms in MWC 758.” Nature Astronomy (2023 ).
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Not all galaxies have spiral arms, and galaxies arent the only celestial objects with spiral arms. In galaxies spiral arms are caused by density waves within the galactic disk. The density waves create a kind of traffic jam effect, where specific stars wander into and out of spiral arms, however the overall spiral structure is preserved. The spiral structure is additional maintained by the higher density of gas and dust in the arms, which triggers star production within the arms.
One model for planetary disk spirals is that they form similar to spiral galaxies.