Written by Jessica Brinegar
In 2019, the New Horizons spacecraft flew by a snowman-shaped object within the Kuiper belt, a giant ring on the outskirts of our solar system composed of rocky and icy bodies orbiting the sun. Later named Arrokoth, the two-lobed body is an example of what astronomers call planetesimal, considered to be the building blocks of planets. Arrokoth provided astronomers some insight, and a lot more questions, into how planets form.
New Mexico State University Astronomy Assistant Professor Wladimir Lyra received a three-year, $365,000 NASA-Emerging Worlds grant to further the understanding of how these planetesimals form in circumstellar disks.
“The Kuiper belt is a goldmine of information of planetesimal formation,” Lyra said. This belt of circumstellar material is home to Pluto, the dwarf planet discovered by late NMSU astronomer Clyde Tombaugh.
The research will be building off preliminary work done by graduate student and research assistant Manuel Cañas, who will continue the work with Lyra. Cañas, came to NMSU from South Carolina but is originally from Colombia. He was drawn to NMSU because of the astronomy department's research reputation.
“It's a very good school for research in astronomy," Cañas said. "Aside from research and school, I like computer programming, which is another reason why I like this project — it's very computationally intensive.”
Using computer simulations, Lyra builds models that predict planet formation mechanisms, that are then compared with the observational data collected by other researchers. These models allow him to create and verify theories of how planets form.
“For a long time, the idea was that you form planets by first forming bodies of asteroidal mass, which we call planetesimals, that then collide with each other,” said Lyra. When asked how these planetesimals begin to form, Lyra provided an amusing visualization. “You know when you don’t clean your room enough and dust bunnies form? Now imagine you don’t clean your room for 10 million years.”
The result is that these cosmic dust bunnies grow to the size of pebbles. Subsequently, interacting with the gas that they are swiftly travelling through, these pebbles naturally develop an aerodynamical process to reduce the drag (such as the V-formation that birds adopt) felt during orbit. This process, opaquely called “streaming instability,” strongly concentrates the pebbles to form planetesimals.
As these planetesimals continue to grow larger in size, an interesting trend in their densities begins to emerge. “The smaller objects, such as Arrokoth, have very low densities, like the densities of ice, and the larger mass objects like Pluto have a much higher density,” Lyra said.
"I’m really excited to apply the pebble accretion model that we’ve been working on, and see if we can reproduce the density trend observed in the Kuiper belt," said Cañas. "I find the problem to be very interesting. If planetesimals and planets are the result of the conglomeration of trillions upon trillions of similar-like pebbles, why is it that the densities of Kuiper belt objects (KBOs) differ between small KBOs and large KBOs. It’s like gathering snow in to a snow ball, and then snow balls into a snow man."
However, if the dwarf planets we see in the Kuiper belt were formed simply by the collision of smaller bodies, the smaller and larger objects have a constant they would all have the same composition, and therefore densities would only vary as the bodies compact under their own weight. Yet, attempts at matching the densities of actual Kuiper Belt objects with this theory have so far failed. This conundrum led Lyra to develop a new idea, abandoning the assumption of constant composition, which is the focus of the new grant.
“At these distances in the solar system so far away from the sun, the temperatures are so cold that water ice is as hard as rock,” he explained. Because of this, it has been assumed that ice and rock behave in the same way. However, Lyra’s team is seeking evidence that the grains of ice and rock have differential coagulation properties, or stickiness.
Lyra had me think of the ice and silicate grains in space as snowflakes and sand on earth. You can easily pick up snow and form it into a ball to throw at your friend, whereas the same game would fail on a sandy beach. In the same way, Lyra says that the ice grains in space stick together better than the silicate grains do, resulting in icy bodies forming more easily than rocky ones.
According to Lyra’s research, once the icy objects grow large enough, they begin accreting the smaller silicate grains and pebbles. The silicates travel at a slower speed than the larger icy bodies due to experiencing more gas drag during orbit. Therefore, they do not have enough energy to escape the gravitational pull of the bigger object, which results in their accretion.
Over time, these icy bodies eventually accrete enough silicate grains to form large denser objects such as Pluto. With this new theory, the positive correlation of size and density makes sense: the smaller objects are icier and more porous, whereas the larger objects have incorporated more silicates, resulting in a denser body.
“This new theory -- formation of icy planetesimals followed by differential accretion of ices and silicates --, naturally fits the observations,” Lyra said. Arrokoth, with a length of 22 miles (considered a small object for space), has a density that is a fourth of the density of water. “If you could put it in a large enough bathtub, it would float.”
As seen with the images of Arrokoth, researchers are now able to detect objects that are billions of miles away with stunning clarity. With advanced observational methods comes an increased need for scientists like Lyra, who predict the “how” behind what is observed. Asked to describe his job, he jokes, “when the observers don’t know what they are looking at, they call me.”
But then Lyra takes a more serious, and deferential, tone. “Our research has implications for the formation of Pluto. It is fitting that we are doing it here at NMSU, the academic home of Clyde Tombaugh," Lyra said. "The New Horizons spacecraft that flew past Pluto is carrying a portion of Clyde’s ashes, isn’t that mind-blowing?
"I did not get the chance to meet Clyde. If I had, I like to think that I would be telling him that I’m working toward explaining how Pluto got there for him to discover it 4.5 billion years later. And that I feel privileged and honored to carry on his legacy.”