Taking it for a spin —
Light from stars at the end of their lives is intense enough to rip asteroids to bits.
We tend to view the bodies of the Solar System as creations of gravity, which pulled their parts together and holds them in place as they orbit. But as we saw with ideas about the formation of Arrokoth, there are lots of situations where gravity is essentially a constant for long periods of time. And given enough of that time, relatively small forces like friction from sparse gas clouds or pressure from the light of the Sun can add up and create dramatic changes. In fact, a remarkable number of these potential influences have been identified and simulated.
One of these has been named the YORP effect, for its developers, Yarkovsky, O’Keefe, Radzievskii, and Paddack. It describes how light can alter the rotational properties of orbiting bodies. In a recent edition of the Monthly Notices of the Royal Astronomical Society, Dimitri Veras and Daniel Scheeres decided to calculate what happens as the Sun ages, the intensity of its light increases dramatically, and the entire asteroid belt gets YORPed.
A (perhaps too) bright future
It’s pretty widely understood that, as the Sun ages, it will expand until its outer edges come close to the Earth’s orbit. What’s less widely recognized is that it will get quite a lot brighter than it is at present. Other stars with masses similar to the Sun can get thousands of times brighter than the Sun in the last stages of their fusion-driven lives, allowing effects that might otherwise be a bit weak to become dominant.
The list of said effects would almost certainly include the YORP effect. It exists because photons carry momentum, which they can transfer to objects when they’re absorbed. If an object is a flat, uniform disk, illuminating it will only produce an even force pushing it away from the light source. But most small bodies like asteroids are anything but even. Differences in reflectivity and/or an irregular surface shape can lead to uneven forces on the asteroid, starting it spinning. And over time, the spin will gradually increase as long as the light is present and no other force intervenes.
The spinning becomes significant because most of the bodies in the asteroid belt are rubble piles—collections of rocks and dust barely held together by gravity. As a rubble pile spins up, there’s little to hold it together, and it runs a risk of shedding material into space. A number of binary asteroids or asteroids with small “moons” have been discovered; the moon may represent rocks that were formerly part of the asteroid but were spun off at some point in its past. Binary asteroids can occur when increasing spin rates fragment the asteroid more evenly.
To find out how asteroids would respond to steadily increasing light pouring out of a bloated Sun, Veras and Scheeres decided to create a model of asteroids and light exposure, start it off with a Sun-like star of double the mass, and allow the star to evolve into a bloated giant.
Spin is in, asteroids out
The short answer is that bad things happen to asteroids. Pretty much all of them. “Survival occurs only for the largest asteroids at the furthest distances from the star,” the researchers conclude. Assuming a realistic internal strength provided largely by gravity and friction, survival is only an option for very small bodies with a radius below a half-kilometer, or at extreme distances, where the body is well beyond Neptune.
Their calculations also showed that there’s likely to be a hierarchical disassembly of the asteroids—once the original body fragments, most of its pieces will spin up and fragment, too. While it’s possible for some bodies to only partially fragment, the conditions that allow this are extremely narrow; most asteroids end up completely disassembled into their component parts. Veras and Scheeres estimate that something like the asteroid Itokawa would end up releasing as many as 17,500 individual rocks as it spun to pieces.
“The result can be a relentless fission cascade,” the authors write. “This cascade would cease—or at least our fragmentation model would no longer be applicable—at a value of [radius length] corresponding to the constituents of the asteroid.”
The authors argue that this probably explains several features of white dwarfs, the objects left behind after stars enter their giant phases. Forty of the white dwarfs we’ve studied appear to have debris disks orbiting them. That’s unexpected because gravitational effects should allow disks like these to coalesce into larger objects early in the evolution of the system surrounding the star. The YORP-driven process described here can explain how these larger objects are disassembled back into a debris disk shortly before white dwarf formation.
Another unexpected feature of white dwarfs is the presence of heavier elements on their surface, when gravity should pull them toward the star’s interior. This implies that the material we’re seeing got there recently, which implies that parts of the former exosolar system are regularly falling into the white dwarf. That can be explained by another effect driven by light, the Poynting–Robertson effect, which slows down dust particles in orbit as they radiate light that is higher energy in the direction of their travel due to the Doppler effect. Thus, all the dust that’s left over from the asteroid disassembly may be finding its way into the remains of the star that disassembled it.