About 5 billion years ago, the Earth was forming. Gas and dust accumulated alongside the young Sun’s protoplanetary disk, perhaps pushed a little by the resonant gravity of Jupiter and other large worlds. We can imagine that during its formation, the Earth swept up orbital debris, leaving a void in the disk that was visible from many light years away. While we know this story to be quite accurate, the idea that planets like Earth always fill the gaps in a protoplanetary disk is probably not the case. For decades, the idea of planets forming from the debris of young stars has been supported by low-resolution images of disks around stars like Fomault. The gas and dust surrounding young stars is often cold and faint, making them difficult to study. But advanced radio telescopes like ALMA have now captured detailed images of these disks. Many of them have annular spaces that are largely free of debris, and some of these contain visible protoplanets. So the general consensus is that holes in the disk indicate the presence of planets, even if we cannot observe them directly. But a new study reveals that things are much more complicated.
A protoplanetary disk surrounds the young star HL Tauri, as illustrated by ALMA. ALMA reveals some of the disk’s substructure, such as the space where planets can form, but it is difficult to accurately measure how much dust is in the disk. Image source: ESO/ALMA The team looked at N-body simulations of early disks, in which three to seven protoplanets interacted with gas, dust and gravel inside the disk. Their model is complex enough to examine not only how these planets accumulate material and evolve, but also how the orbiting planets might move through gravitational interactions and how they interact with their disks. can change the shape or orientation of the orbit relative to the disk. . They simulated these systems over a period of 100 million years, long enough to study how planets can settle into stable orbits.
One of the things they discovered is that in a young disk, five to seven protoplanets quickly develop unstable orbits. In their simulation, stability dissipated in 40,000 years, representing the blink of an eye for the universe. It will take longer for the planets to fill the space on the disk. This means that when we see 5 or more gaps in a protoplanetary disk, not all of them can be filled by planets. It’s possible that the rings are created by the orbital resonance of a particularly large planet, in the same way that Jupiter prevents planets from forming in its asteroid belt. Another discovery is that the orbits of planets can move and shift significantly, which again would not allow them to clear their orbital paths. Smaller worlds in particular will likely spend their first few days moving across the disk in a chaotic manner. Our young Earth may have migrated significantly as it formed, spending most of its time hidden among gas and dust rather than visible in a void. This means we usually don’t see Earth-like worlds forming in a protoplanetary disk, because we can’t distinguish them from the disk’s overall glow. All this means that we cannot make a simple connection between the number and size of voids observed in early planetary disks and the number and distribution of exoplanets that we observe in evolved star systems. Planet formation is a complex dance, and although we know some of the steps, there is still much to learn.