Meet Ceres, the Dwarf Planet

Dwarf planet Ceres in full color, as seen by NASA's Dawn spacecraft on its first orbit in 2015. NASA/JPL-Caltech/UCLA/MPS/DLR/IDA
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Dawn's Trip to Ceres

Dwarf planet Ceres in full color, as seen by NASA's Dawn spacecraft on its first orbit in 2015. NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

The ongoing exploration of the solar system keeps rewarding scientists with amazing discoveries at distant worlds. For example, a spacecraft called Dawn revealed the first-ever close-up looks at a world called Ceres. It orbits the Sun in the main Asteroid Belt, and Dawn spacecraft made its way there after encountering and studying an asteroid called Vesta. Together, these little worlds are revamping what planetary astronomers understand about their part of the solar system

Dawn Reveals an Old World

Ceres is an ancient world that formed early in solar system history. Its exploration by Dawn is essentially a step back to the epochs of time when the planets were still accreting together from chunks of rock and ice swirling in a disk surrounding the newborn Sun. Ceres has a rocky core but an icy surface, which gives some indication of where it may have formed. It also has an ocean beneath the surface, and a thin atmosphere hovering just above the icy crust.

Some of Dawn's images feature a set of bright spots on the surface. They are salt and mineral depositsleft behind as geysers of water escape to space. The existence of those geysers proves the existence of that hidden ocean. 

Facts about Ceres

Like Pluto, Ceres is a dwarf planet. It was once considered a planet, but recent debates have pushed back into the category of a dwarf. It clearly orbits the Sun, and seems to be rounded by its own gravity, but some consider that it hasn't cleared its orbit of material yet (hard to do, since it's in the Asteroid Belt). 

As worlds go, Ceres is fairly smal l— around a thousand kilometers across. It's the largest object in the belt, and makes up about a third of the total mass of the Asteroid Belt. Compared to other solar system bodies (moons and other dwarf planet candidates), Ceres is bigger than the tiny world Orcus (in the Kuiper Belt) and smaller than Saturn's moon Tethys.

How did Ceres Form? 

The big questions that planetary scientists want to answer about Ceres involve its formation history. We know that it dates back to when the main planets were still forming, but what process brought the pieces of "proto-Ceres" together to make a dwarf planet? It's very likely that Ceres was made from smaller particles in the protoplanetary nebula. As they orbited the Sun, these materials smashed together to make larger ones. This is exactly how the larger worlds formed, too. Eventually, enough of those pieces stuck together to form a protoplanet, which is  essentially a "baby" planet that can grow larger if conditions are right.

If things had gone a little differently, infant Ceres might have joined with one or more of its neighbors to form a larger world. Instead, it remained about its current size. Since it had enough mass to have a decent gravitational pull, its shape gradually became rounded over time. Ceres's surface was battered by impacts from other objects early in its history. Its interior was heated by a combination of those impacts and probably also by the decay of radioactive elements deep in its core. The Ceres we see today is the result of 4.5 billion years of change, a rounded world that somehow survived bombardment without breaking apart. 

Dawn's orbit has gone as low as 700 kilometers above the surface, and its cameras have returned some very close-up looks. Astronomers hope to send more missions to Ceres in the future. There's one on the drawing boards from China, and other spacecraft will head out to the worlds of the outer solar system. 

Why Study the Outer Solar System?

Worlds such as Ceres and Pluto, as well as others that exist out in the "deep freeze" of the solar system, provide important clues to the origin and evolution of the solar system. The planets we know were not "born" in the places we see them in today. They've gone through complicated histories of formation and migration to their current positions. For example, the outer gas giants likely formed much closer to the Sun and then moved outward to the colder parts of the solar system. Along the way, their gravitational influence affected other worlds and scattered smaller moons and asteroids. 

This tells astronomers that the early solar system was a dynamic, ever-changing place. The interactions between planets as they migrated sent smaller worlds hurtling out to new orbits, even as the gas giants lumbered out to their current orbits. Comets were sent to the distant Oort Cloud and Kuiper Belt, and they contain some of the earliest and oldest materials of the solar system. Worlds like Dawn and dwarf planet Pluto (which was explored in 2015 by the New Horizons mission) continue to be active, and that piques our interest. Why do they have ice volcanoes? How do their surfaces change? These and many other questions are begging to be answered, and future missions to those and other worlds will provide answers.