Our Solar System is littered with rocky (and sometimes icy) objects, such as asteroids and comets.  These bodies, which are generically called “planetesimals”, are approximately 1-100 km in size, and are the building blocks for fully fledged planets. Despite their prevalence in the Solar System and importance in planet formation, however, we still do not really understand how planetesimals form.  

Currently, there are two competing ideas. The first idea is that planetesimals are formed by colliding smaller (less than km size) objects together to make larger objects.  Alternatively, these objects may form via the gravitational collapse of a large cloud of smaller particles.  The leading mechanism for triggering this collapse is known as the streaming instability, an idea originally put forth by co-author Prof. Andrew Youdin and his collaborators. In this mechanism, much smaller (cm-sized) particles group together within their parent protoplanetary disk much like birds flock to reduce their mutual aerodynamic drag. Once enough of these particles group together, they collapse due to their own gravity and form planetesimals. 

So, which of these models is the correct one? This blog article not only addresses how our group has made a significant advancement towards answering this question, but also provides an excellent example of how observational data can be used to directly test predictions made by two competing theories. 

On the observational side, Dr. Will Grundy of Lowell Observatory carried out a campaign to study cold distant planetesimals beyond the orbit of Neptune, part of the Kuiper Belt.  These Kuiper Belt Objects (KBOs), and in particular a sub-sample of them known as cold classical KBOs, are the most pristine planetesimals in the Solar System, as they have not been strongly influenced by gravitational effects from the planets or harsh radiation from the Sun.  Previous observations have revealed that a large number of these KBOs exist as binaries. Dr. Grundy’s work builds upon this and shows that nearly 80% of these binaries (see Fig. 1) orbit in a “prograde” sense (i.e., they orbit each other in the same direction as they orbit the Sun). 

With these data in hand, we realized that we had a potential test for planetesimal formation theory.  What would the streaming instability model predict for the formation of these KBO binaries compared to the collisional growth model? 

Rixin Li of the University of Arizona along with Dr. Jake Simon of the University of Colorado and the Southwest Research Institute (SwRI) carried out numerical simulations of planetesimal formation via the streaming instability collapse model (see Fig 2). These simulations show that once formed, the planetesimals possess a very large amount of angular momentum. Coupled with our previous understanding of how such angular momentum would evolve if the planetesimals were allowed to collapse to 100 km scales (this is currently prohibitively expensive in our calculations), we determined that the vast majority of these planetesimals would fragment into binary pairs. The most astonishing result, however, came when Dr. David Nesvorný of SwRI compared the fraction of these binaries that orbit in a prograde sense with the observational data obtained by Dr. Grundy – a nearly perfect match was found!    

On the other hand, previous work in studying the properties of KBO binaries in the collisional growth model has demonstrated that most of the binaries should be retrograde, clearly in conflict with the observational data. 

As Dr. Nesvorný claims in the published article, this comparison “clinches an argument in favor of” the streaming instability model. This work is a clear example of how observational data can directly test predictions made by two competing theories.  In this particular case, the streaming instability model won hands down.