The birth of a ‘snowman’ on the edge of the solar system.

A model developed at the Technion Faculty of Physics, in collaboration with German scientists in Tübingen, explains the unique properties of Arrokoth, the most distant object ever photographed in the solar system. The research team’s results shed new light on the formation of Kuiper Belt objects, asteroid-like objects at the edge of the solar system, and to understand the early stages of the formation of the solar system.

The findings, published in Nature, explain the unique characteristics of the “snowman”, formally known as Arrokoth, and the images of him were first taken last year by NASA’s New Horizons space mission.

The story begins in 2006 when the New Horizons robotic spacecraft was dispatched to take the first close-up images of Pluto and study its features and terrain. After launch, New Horizons set its course for Pluto, beginning a long journey of about nine years. In order not to waste fuel and resources, most of its systems were in sleep mode until it was close to its Pluto target.

Back on Earth, the International Astronomical Union decided to degrade Pluto from its planet-to-dwarf condition. In short, the New Horizons robotic spacecraft was sent to investigate a planet, fell asleep and woke up to discover that Pluto was no longer considered a planet. But this did not detract from the importance of the mission. New Horizons provided spectacular images of Pluto and its moon Charon, and provided invaluable scientific information that is still under investigation and will likely be studied for years. These studies will provide important information to understand the formation of the solar system, and in particular the Kuiper Belt.

But there is even more to the New Horizons adventure. While Pluto is the largest object in the furthest reaches of the solar system, it is not the only one. Beyond Neptune is a region called the Kuiper Belt, which consists of countless asteroid-like objects that range in size from a few feet to thousands of miles. Conditions in this area are different (and in particular, much colder) than your “brother” asteroid belt in the inner solar system, and Kuiper Belt objects typically consist of much icier materials.

The New Horizon spacecraft was equipped with enough resources to observe another Kuiper Belt object if an object could be found that was not too far from the spacecraft’s original trajectory. On June 26, 2014, after an extensive survey looking for such objects, the Hubble Space Telescope identified one. Following that identification, the New Horizons research team designed the spacecraft’s trajectory to pass alongside the newly found object after completing its mission to map Pluto. Five years later (and four after his encounter with Pluto in 2015), New Horizons passed by the object. On January 1, 2019, humanity took its first close-up shot of a small Kuiper Belt object when the New Horizons spacecraft passed it just 3,500 miles away.

Immediately after the arrival of his first images, the Kuiper Belt object (hitherto known as MU69 2014) was nicknamed “the Snowman” due to its unique appearance. New Horizons researchers initially called it Ultima Thule (“The Edge of the World” in Latin), due to its remote location on the edge of the solar system. But the object was eventually renamed 486958 Arrokoth, for “sky” or “cloud” in Powhatan’s now-extinct native language.

New Horizons gathered a wealth of information about the snowman – it’s a 30-kilometer contact binary consisting of two lobes of different sizes interconnected by a thin neck, which appears to be the product of two smaller Kuiper Belt objects. that collided to form Arrokoth.

Although various models have been proposed to explain the formation of Arrokoth and its peculiar properties, they faced significant challenges and were unable to explain the important characteristics of the Snowman, in particular his slow speed of rotation around himself and his large angle of inclination. . In its Nature In the article, Technion researchers introduce new analytical calculations and detailed simulations that explain the formation and characteristics of Arrokoth.

The research was led by Ph.D. Evgeni Grishin, postdoctoral fellow Dr. Uri Malamud, and his supervisor, Professor Hagai Perets, in collaboration with the German research group in Tübingen.

“A simple high-speed collision between two random objects in the Kuiper Belt would destroy them, as they are likely to be made predominantly of soft ice,” said Mr. Grishin. “On the other hand, if the two bodies orbited each other in a circular orbit (similar to the moon orbiting Earth), and then slowly spiral to come closer together and make contact, Arrokoth’s rotational speed would have been extremely high , while the measured velocity was actually quite low with respect to such expectations. The full rotation of Arrokoth takes 15.92 hours. Furthermore, its tilt angle (relative to the plane of its orbit around the sun) is very large, 98 degrees, so it is almost on the side relative to its orbit, a peculiar feature in itself. “

“According to our model, these two bodies revolve around each other, but because they revolve around the sun together, they basically make up a triple system,” he said. “The dynamics of such triple systems is complex and is known as the three-body problem. The dynamics of gravitational triple systems are known to be very chaotic. In our study, we demonstrated that the system did not move in a simple and orderly fashion. but he also didn’t behave in a totally chaotic way. “

“It evolved from having a relatively large, circular orbit to a highly eccentric elliptical orbit through slow (secular) evolution, much slower compared to the Arrokoth orbital period around the sun,” said Professor Perets. “We could demonstrate that such trajectories eventually lead to a collision, which on the one hand will be slow and will not crush objects, but on the other hand it will produce a very tilted, slowly rotating object consistent with the properties of Arrokoth.”

“Our detailed simulations confirmed this image and produced models that closely resemble the appearance, rotation, and tilt of the Arrokoth snowman,” said Dr. Malamud, in conclusion.

The researchers also studied how robust and likely such processes are, and found that they are potentially quite common with up to 20% of all wide Kuiper Belt binaries and potentially evolve similarly.

Until now, the researchers said, it was not possible to explain Arrokoth’s unique characteristics. It is a counterintuitive result, but the probability of collision in such configurations actually increases as the initial binary is more widely separated (but still linked) and the initial tilt angle is closer to 90 degrees.

“Our model explains both the high probability of collision and the unique data from today’s unified system, and indeed predict that there are many more objects in the Kuiper Belt,” said Mr. Grishin. In fact, even the Pluto and Charon system could have been formed through a similar process, and they seem to play an important role in the evolution of the binary and lunar systems in the solar system.