In our solar system, we have unique names for many of the worlds we study. Planets...asteroids and comets...dwarf planets...a star. But what about those things we don't have a name for yet? What about objects we've never studied up close?
At 12:33 a.m. on New Year's Day, the New Horizons spacecraft will fly by one of these mysterious worlds, which orbits the Sun in the Kuiper Belt, a vast region of objects beyond the orbit of Neptune.
The object is nicknamed Ultima Thule. We don't know whether Ultima is more rocky or icy, or exactly what material it it's made of. So what can we call it, other than "Kuiper Belt Object?"
We know Ultima is not a moon, because it doesn't orbit another body. We know it;s not a planet or a dwarf planet like Pluto because it's not nearly big enough or round enough.
So what about those smaller types of worlds? Could we call it an asteroid or comet?
Asteroids are mostly rocky and mostly orbit in the inner solar system—the asteroid belt between Mars and Jupiter, for example, and those in two distinct groups that trail and lead Jupiter in the gas giant's orbit around the Sun (these are called Trojan asteroids).
Some orbits bring asteroids close to Earth—those are the Near Earth Objects and we keep a close eye on them.
What about a comet? In general, comets we know of are like the smaller asteroids in size, but the main different is that they contain more ice than asteroids do. They also generally come from farther out in the solar system, where water would always be frozen. (Some comets come from the Oort Cloud, others from the Kuiper Belt.)
When a comet comes close the Sun, the intense heat vaporizes some of the comet's ice, which carries dust along with it, giving the comet its telltale coma and bright tail (and sometimes even multiple tails).
There are some exceptions to these rules, of course. Some small bodies are icy, but just never come close enough to the Sun to produce a coma or tails. And some asteroids may have ice. The Centaurs, for example, are a group of rocky and icy bodies that orbit between Jupiter and Neptune. These are thought to be escaped Kuiper Belt Objects.
In fact, more and more scientists are starting to think of comets and asteroids not as two completely different types of objects, but more as two ends of a spectrum. What really differentiates them is their location in the solar system—which can teach us things about how the solar system formed and evolved. An object near Mars holds different information than an object near Neptune.
How did those bodies get where they are?
Can their compositions and locations tell us that they were pushed around by Jupiter, or are perhaps pieces of a planet that never fully formed?
Kuiper Belt Objects are also distinct, like asteroids or comets, but we're not totally sure what makes them different yet besides their position about 2.3 billion miles (1.4 billion kilometers) beyond the orbit of Neptune (that's 30-70 Astronomical Units from the Sun, or 30-70 times the distance between the Sun and Earth.
Scientists have some ideas what we might find there, but the solar system is always one for surprises. One thing is for sure—Ultima Thule will help us understand more about the evolution of the solar system. That's the beauty of exploring the cosmos—as we discover new types of objects beyond our atmosphere, we understand our place in the solar system a little more.