On January 1st, 2019, during its flyby of the Kuiper Belt over six billion kilometers away from Earth, NASA’s New Horizons space probe encountered and imaged Arrokoth. The finding is currently the most distant object ever explored by a spacecraft. After data on the unusual rocky mass was used by the New Horizons team to generate a nuanced understanding of the formation of planets, their findings were published on February 13th, 2020. 

The images captured show that Arrokoth with two connected lobes and a uniform color and composition, somewhat in the likeness of a snowman. Its uniformity suggests that it has been well-preserved since its formation billions of years ago. This indicates that it can serve as a kind of “fossil” to inform scientists of how planets form. The images found also serve to characterize it as a planetesimal (planetary building block) that was most likely shaped through a gentle merger of two separate bodies originated in the same gravitationally collapsing cloud. 

Two contrasting theories have long competed to explain the formation of a planetesimal: hierarchical accretion and particle-cloud collapse (or pebble accretion). Both theories are rooted in similar principles. They agree that gravitational collapse of a cloud of gas and dust assumes the form of a protoplanetary disk (a 2D circular structure), and that a young star emerges at the center of that spinning disk, causing the dust grains to amass and create larger bodies capable of pulling in other objects with their own gravitational force. This is where the disagreement between the two theories arises. The theory of hierarchical accretion – the prevailing paradigm in the 20th century –  postulates that planetesimals initially grow out of the aggregation of small particles coming from disparate parts of the disk to eventually form larger masses that collide with each other more frequently and violently. Conversely, the more recent cloud collapse theory suggests that these larger bodies are created through more gentle merging. Pebble accretion outlines how planetesimals are formed – often in pairs – as small particles, which build up in their local gas cloud, collect into separate streams that are carried along with the gas. When these streams of particles become dense enough, they begin collapsing into the center under their own gravity, forming planetesimals relatively quickly.

The exploration of Arrokoth has revealed that the formation of the 36-kilometer-wide object can only be explained by the particle-cloud collapse theory. Observations about its color and composition indicate that both lobes were formed from a single cloud of matter in the solar nebula, rather than from an amalgamation of material from separate areas of the nebula. Additionally, digital models of the merger show that, after forming in the same local cloud of matter, the two separate lobes created a co-orbiting binary, eventually fusing into one celestial body. Simulations indicate that the joining of the lobes occurred at a speed of 15 kilometers per hour, at most. Moreover, the similarity between Arrokoth and other masses in the Kuiper Belt indicates that its formation is most likely representative of the way in which other bodies were created, which is consistent with the theory of pebble accretion.

Pebble accretion could also aid in answering why planets can form with such ease. Astronomers estimate that the number of stars in our galaxy ranges from 100 billion to 400 billion, meaning that there could be between 800 billion and 3.2 trillion planets in the Milky Way alone. It would seem that planet formation requires a multitude of perfect conditions; yet, the prevalence of planets suggests otherwise. Unlike hierarchical accretion, which relies on somewhat random collisions between particles or planetesimals that would likely involve a much longer process, the logic of pebble accretion allows for a relatively quick build-up, providing a partial explanation for the ubiquity of planets. 

According to New Horizons Principal Investigator Alan Stern, parallels can be drawn between the debate on planet formation and the debate on the origins of the universe of the mid-1900s, when three competing theories aimed at explaining the inception of the cosmos. It was not until the cosmic microwave background was discovered in 1964 that the controversy ended, solidifying the Big Bang as the most widely accepted theory. Stern implies the same could happen now, as the data gathered from Arrokoth validates the theory of pebble accretion, potentially settling the dispute between the two theories of planetary formation. 

When a scientific dispute such as this one is resolved and one theory comes out on top, it can provide an opportunity to narrow the focus of further inquiry on a particular subject so that resources, such as research funding, can be more efficiently employed in the search for new knowledge. In all likelihood, these findings will pave the way for new discoveries in the field of planetary science.

Sources: AAAS, NASA, Space.com, Knowable Magazine