A kind of “time capsule.” That is what has turned out to be Arrokoth, the farthest world visited by human technology – and formerly known by the controversial name of “Ultima Thule” – and of which three new articles in the journal “Science” reveal even more secrets. Clues that bring new ideas on how planets and planetesimals were formed, the “bricks” of our cosmic neighborhood.
The New Horizons spacecraft flew over on January 1, 2019 Arrokoth (whose technical name is MU69 2014), in the Kuiper Belt, where thousands of objects pupil at ease, including the famous Pluto planetoid. It was the first time that humanity was approaching one of the icy remains of the formation of the Solar System, beyond Neptune, orbiting the body 3,500 kilometers away from its surface and more than 6,000 million kilometers from home. A record flight.
“Arrokoth is the most distant, most primitive and most pristine object ever explored by spacecraft, so we knew it would have a unique story to tell,” says New Horizons principal investigator Alan Stern of the Southwest Research Institute in Boulder. , Colorado. “He is teaching us how planetesimals were formed, and we believe that the result marks a significant advance in the understanding of planetary and planetary formation in general.”
From “peanut” to “pancakes”
The first post-flyby images transmitted from New Horizons last year showed that Arrokoth had two lobes connected, which was initially compared to the “snowman” or “peanut” shape. However, this overflight revealed that, in reality, the body had a smooth surface and a uniform composition, in the form of two “pancakes” attached next to each other. The results were also published in «Science» last May and it was already advanced that this world would probably be decisive in our understanding of the formation of the planets of the Solar System.
During the following months, working with more high-resolution data and sophisticated computer simulations, the mission team gathered an image of how Arrokoth should have formed. His analysis indicates that the lobes of this “binary contact” object were once separate bodies that formed together and at low speed, orbited with each other and then fused gently to create the 32-kilometer-long body observed by New Horizons.
A slow cosmic dance
Thus, Arrokoth was formed 4,000 million years ago (the Solar System has about 4,600) during the gravity collapse of a cloud of solid particles in the primordial solar nebula, and not as believed based on the competitive theory of planetesimal formation hierarchical accretion call. Unlike high-speed collisions between planetesimals in hierarchical accumulation (i.e., brutal clashes between bodies), in the collapse of the particle cloud, the particles merge smoothly, growing slowly (like soap bubbles that are attached and do not lose their form). This is that there was a time, at the beginning of the formation of our cosmic neighborhood, that objects merged with each other gradually, without major shocks or violence.
“In the same way that fossils tell us how species evolved on Earth, planetesimals tell us how planets formed in space,” says William McKinnon, co-researcher at New Horizons at the University of Washington at St. Louis, and lead author of a Arrokoth training article in Science this week. “Arrokoth was not formed through violent collisions, but in a more intricate dance, in which the objects that now compose it slowly orbited each other before joining.” Something like a delicate cosmic dance that resulted in Arrokoth.
Two more studies: color and composition
Two other important tests in two other parallel studies support this conclusion. The uniform reddish color and surface composition of Arrokoth shows that it formed from nearby material, as predicted by local models of cloud collapse, rather than a mixture of matter from more separate parts of the nebula, as they might Predict hierarchical models. No water has been found, although it is not ruled out in the past. Even so, the team points out that the world is too cold to contain life as we know it.
The flat shapes of each of the lobes, as well as the remarkably close alignment of their poles and equators, also point to a more orderly fusion of a collapsed cloud. Moreover, its smooth surface with very few craters indicates that its face has remained well preserved since the end of the era of planet formation. “An object like Arrokoth would not have been formed, nor would it look the way it does, in a more chaotic accretion environment,” says Will Grundy, head of the New Horizons thematic composition team at Lowell Observatory in Flagstaff, Arizona, and lead author of a second study published in “Science.”
Scientists have used all available New Horizons Arrokoth images, taken from many angles, to determine their 3D shape, as shown in the video above. The form provides additional information about the origins of Arrokoth. The flat shapes of each of the lobes of the world, as well as the remarkably close alignment of their poles and equators, point to an orderly and smooth fusion of two objects formed from the same cloud of particles that collapses. Arrokoth has the physical characteristics of a body that joined slowly, with ‘locally sourced’ materials from a small part of the solar nebula. An object like Arrokoth would not have formed, nor would it look like it does, in a more chaotic accretion environment.
New Horizons continues to conduct new observations of additional objects from the Kuiper Belt. In addition, from summer the efforts of the ground telescopes that will hope to find other relics like Arrokoth in the vicinity will be added. Who knows if the mysterious Planet 9, that of which there is only indirect evidence, is revealed by the mission. At the moment, New Horizons is 7.1 billion kilometers from home, traveling this area at 50.400 kilometers per hour, looking for answers. .