The New Horizons probe, launched in 2006, made a historic flyby of Pluto and its moon Charon in 2015. After that, it continued to fly farther out into the Kuiper belt, a ring of asteroids orbiting far from the sun. Objects in the Kuiper belt are thought to be largely unchanged for millions of years because they receive so little radiation from the sun and they are so sparse as to have very few collisions with other asteroids. That means that Kuiper belt objects can give us a glimpse into the primordial state of our solar system and what the Earth may have looked like when it was beginning to form. Specifically, New Horizons later made a close flyby of Arrokoth, formerly called Ultima Thule. Arrokoth has a two-lobed “space-potato” shape that suggests that it was formed by two asteroids joining together. This makes it a valuable model for studying how Earth and other planets formed out of smaller particles, a process known as “accretion”. Although New Horizon’s flyby happened more than a year ago, the images took so long to transfer to Earth that the results were only published recently, in three papers in the same volume of Science.

One theory of how our solar system formed proposes that a massive gas cloud that  collapsed into solid “planetoids” large enough to generate their own gravitational fields. These planetoids would then collide into each other – quickly and violently –  until they formed larger and larger objects to the point of planets and moons. This model is called “hierarchical accretion”. In contrast, the paper from McKinnon and colleagues presented evidence the two lobes of Arrokoth merged gently over an extended period of time. The analysis of the surface of Arrokoth suggests a model of “pebble accretion” where small objects slowly pull each other closer together until drag from the gas cloud slows them down and they collide. The researchers also analyzed the rotations of the two lobes to see if they were tidally locked before joining. This means that the denser side of each lobe was aligned towards the other lobe because it experiences more gravitational pull. For example, the moon is tidally locked to the Earth so we always see the same side of it. They found that the rotations of the two lobes supports a model where the two were tidally locked, again arguing that they were in close proximity for a long time before colliding. This would explain why the two lobes are Arrokoth are still close to spherical after having collided.

The ‘slow and steady collision’ theory  is also supported by a close inspection of the surface in a paper Spencer et al. The age of the asteroid was estimated, based on the density and size of craters, to be around 4 billion years old. With the age of our solar system estimated to be 4.6 billion years, this supports Arrokoth being a primordial object similar to what was found at the beginning of our solar system. Although collisions are more dramatic in the hierarchical accretion model, they are so infrequent that the large-scale buildup of mass necessary to make a solar system might actually be slower than what is possible through pebble accretion. It is impossible to go back in time to see how exactly the Earth and other planets formed, but the Kuiper belt gives us a valuable snapshot into what their past might have looked like. Even though there undoubtedly were violent collisions during the formation of the planets, it seems that most of the mass was built up slowly and more peacefully. 

The New Horizons probe will continue to document the Kuiper belt for as long as it has power, which is expected to last another 10 years. It may be limited, however, by how much propellant it has left to maneuver into close flybys. Between a dramatic flyby of Jupiter, snapping the first high-resolution images of Pluto and documenting the surface of Arrokoth, the mission has already been an outstanding success. 

Managing correspondent: Julian Segert

Original articles: The solar nebula origin of (486958) Arrokoth, a primordial contact binary in the Kuiper Belt

The geology and geophysics of Kuiper Belt object (486958) Arrokoth

Color, composition, and thermal environment of Kuiper Belt object (486958) Arrokoth

Press article: The Most Distant World We’ve Ever Explored Just Shed Light on How Planets Are Born Science alert

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