"It's likely that some of Neptune's reddest asteroids formed even farther from the sun in the solar system's early days, before migrating inward and getting caught in Neptune's orbit, the researchers added. Studying them could open a window into how asteroids in the early solar system formed and how their composition has changed over the past 4.6 billion years."
My note. It is very difficult to model the postulated, protoplanetary disc using the solar nebula model. There are a variety of changes to the MMSN found in different models and simulations. An example is a 1977 MMSN model and the amount of mass in earth masses proposed in the ecliptic.
The Distribution of Mass in the Planetary System and Solar Nebula,
https://ui.adsabs.harvard.edu/abs/1977Ap&SS..51..153W/abstract
The arXiv paper,
https://articles.adsabs.harvard.edu/pdf/1977Ap&SS..51..153W, 19-Feb-1977. The table in the paper shows MMSN values 0.01 to 0.07 solar masses and disk mass distribution for the planets so they can evolve from the disk. Total disk mass ranges 0.01 to 0.07 solar masses, 3.329428E+03 (3329.428) earth masses up to 2.330599E+04 (23305.99) earth masses in the protoplanetary disk said to evolve into the solar system we see today. The 1977, six-page paper at the end “4. Conclusions” indicates the MMSN creates an anomalously low mass region for Mercury, Mars, and the asteroids. Some values for the MMSN range 0.01 to 0.1 solar masses in the disk. 0.1 solar mass disk = 3.329428E+04 (33,294.28) earth masses. Much work is done now to refine the MMSN, dust, gas, and mass distribution in the postulated protoplanetary disk that creates the solar system we see today using a timescale of some 4.5 billion years or so. Comparing the 1977 MMSN to present day ALMA disk observations should be interesting too. Looks like much juggling takes place to get the protoplanetary disk to fall into place and create the solar system we see today. When I read the more current computer simulation models and reports like phys.org publishes or sometimes, space.com, I look for tables like the 1977 paper disclosing disk mass for different regions used in the simulations to create the solar system. Hard to find such clarity it seems.