Strange dwarf planet Ceres may have formed at the icy edges of the solar system

Paul M. Sutter is an astrophysicist at SUNY Stony Brook and the Flatiron Institute, host of "Ask a Spaceman" and "Space Radio," and author of "How to Die in Space." Sutter contributed this article to Space.com's Expert Voices: Op-Ed & Insights.

Ceres, the most dominant member of the asteroid belt, doesn't look like the other asteroids. Most notably, it has a lot of ammonia on its surface, which other asteroids tend to lack. The only place to get a lot of ammonia is in the outer regions of the solar system in the early days of its formation. 

In a new paper, researchers propose a possible explanation for why Ceres is so different from its neighbors: Perhaps the dwarf planet began in an orbit beyond Saturn and, during a massive reshuffling of the outer planets, got planted into the asteroid belt, where it has remained to this day.

A Ceres matter

With a radius of 296 miles (476 kilometers), Ceres is by far the largest object in the main asteroid belt, the loose collection of rocks between the orbits of Mars and Jupiter. The vast majority of asteroids are far smaller than Ceres; in fact, by itself, Ceres accounts for over one-third of the entire mass of the belt.

And Ceres is weird.

Related: Biggest mysteries of the dwarf planet Ceres

NASA's Dawn spacecraft spent several years in orbit around Ceres, mapping and studying its surface. That mission found that the surface is a mixture of water ice minerals bound with water, like clay and carbonates. Beneath that is a mantle rich in water ice surrounding a core of mainly solid rock.

Ceres has a relatively low density (2.2 grams per cubic centimeter) and a rather low albedo, meaning it's not very reflective. This makes Ceres very similar to the most common kind of asteroid, the C-type — or carbonaceous chondrite — asteroids. Indeed, Ceres' location within the main asteroid belt puts it close to other C-type asteroids.

But most C-type asteroids don't have that much water and don't have so many clays. And Ceres also has a lot of ammonia on its surface, which almost all other asteroids lack. Ammonia isn't common in the inner solar system, where the intense heat of the sun evaporated it in the early days of the system's formation. Instead, ammonia survived only beyond the "ice line," the distance from the sun where volatile elements like ammonia could survive.

Indeed, Ceres doesn't really look, smell or act like an asteroid. Instead, the dwarf planet has more in common with the most distant objects in the solar system — the members of the Kuiper Belt, like Pluto, Charon and Eris.

Scientists have proposed explanations for each strange trait. Maybe Ceres formed inside the asteroid belt, in a really weird way that allowed it to maintain a lot of water. Maybe it formed without disrupting the rest of the main belt. Maybe over the course of billions of years, a lot of ammonia floated in from the outer solar system and found itself on the surface of the dwarf planet (and not the other asteroids).

Or maybe, the authors of the new paper propose, Ceres was born beyond the orbit of the giant planets and got sent into the asteroid belt long ago.

A song of ice and also ice

The early solar system was a chaotic place. Thousands of small celestial bodies called planetesimals jostled and fought for dominance, aggregating into planets only to be smashed apart again. Among these multitudes was a vast population sitting outside the giant planets, according to the new paper, posted to the preprint database arXiv.

The most common model of the early solar system, called the Nice model (named for the city in France where a conference on solar system formation took place), states that the giant planets of the solar system formed closer to the sun, and closer to one another, than their present-day locations. Some versions of the Nice model also

When those planets began to migrate to their modern positions, it shook things up a lot. The large population of Kuiper Belt objects were disrupted, their orbits perturbed by the motions of the giant worlds. Some of them were ejected completely from the solar system. Some crashed into each other and were obliterated. Some were captured, as in the case of Neptune's moon Triton.

And perhaps others made their way into the inner solar system.

Homeward bound

But even if Ceres-type worlds made it into the asteroid belt, it wasn't smooth sailing from there. Astronomers already knew that the disturbances of the outer system affected even the asteroids, causing the belt to lose up to 80% of its objects. So it was a serious challenge for something like Ceres to end up in the asteroid belt and persist to the present day.

It required a lot more objects like it. The researchers estimated that if at least 3,500 Ceres-size objects existed beyond the orbit of Saturn in the early solar system, at least one of them would make it into the asteroid belt and stay there. Those estimates are based on computer simulations that tracked the evolution of planetary orbits during the disruptive phase of the outer-planet migration. Thousands of Ceres-size objects sounds like a lot, but it's actually well within the range that other models suggest for populations of the outer system. 

The conclusion: The most likely explanation for Ceres is that it was born in the icy edges of the solar system and was the luckiest member of a group of objects that were displaced when the giant planets shifted their positions.

The asteroid belt and the Kuiper Belt remain some of the best laboratories for understanding the formation of the solar system. Any model of planetary formation and evolution must include their effects on the smallest objects orbiting the sun. And so Ceres just may be the major clue we need to understand what happened in the solar system over 4 billion years ago.

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