A pair of small alien worlds, Ceres and Pluto, move into the spotlight this year as spacecraft arrive at their cosmic shores for the first time.
NASA's Dawn spacecraft released its first views of Ceres on Monday, already hinting at previously unknown craters. Still ahead for NASA's New Horizons probe is former planet Pluto, billions of miles from Ceres and the king of a distant, icy realm.
Both are dwarf planets, mini-worlds that just don't make the cut as official planets. It's a vast population of worldlets that scientists don't know much about.
But if all goes according to plan, that will change starting now. And it's about time the little guys got some attention.
"There are more dwarf planets than all the gas giants and terrestrial planets combined," said Alan Stern, principal investigator of NASA's New Horizons mission, which will visit Pluto in July. "They dominate the planetary population of our solar system."
There are more dwarf planets than all the gas giants and terrestrial planets combined.
Dwarf planets, the last class of unexplored worlds in Earth's neighborhood, may contain well-preserved tales of the solar system's formation. Unlike the more familiar rocky and gassy planets, which have edited and erased those histories, Pluto, Ceres, and friends have filed away those records in their geology and distant locations.
"They're like the debris left over from a shipwreck," says Caltech astronomer Mike Brown. "You can't really figure out what happened from looking at the carcass of the ship, but you can see from the debris where everything flowed, what the currents were like."
Close Encounters of the Watery Kind
In early March, NASA's Dawn spacecraft will slip into orbit around Ceres. If it succeeds, Dawn will be the first spacecraft to chase down and orbit two worlds. The first one it visited was massive asteroid Vesta, a dry and dusty body that Dawn spent 14 months mapping and studying starting in 2011.
Within a few weeks, Dawn will be close enough for its images of Ceres to be better than the Hubble Space Telescope's. Ceres is so tiny—just 590 miles (950 kilometers) wide, about one-third the size of Earth's moon—that detailed images can be captured only by a spacecraft on its doorstep. A powerful telescope like Hubble can make amazing images of a huge object like a galaxy, but can't resolve fine features on a tiny one.
Discovered in 1801 by an Italian priest, Giuseppe Piazzi, Ceres has been a puzzling spot of light for more than two centuries. Its size and seemingly large amount of water, including tufts of water vapor, make it unlike anything else in the asteroid belt between Mars and Jupiter. (See the solar system and its cosmic neighborhood in an interactive graphic "19 Trillion Miles to Explore.")
It alone accounts for more than 30 percent of the rocky realm's mass. Scientists want to know why Ceres is so different from everything else in its neighborhood—and the Dawn spacecraft, which has already studied Vesta, is uniquely positioned to tell them that.
"Ceres has been able to hide itself very well," says UCLA's Christopher Russell, Dawn's principal investigator. Fragments of Vesta have fallen to Earth, he notes, but there are no pieces of Ceres lying around providing a preview of what might be there. "What is its role in the early solar system? Why is it so different?"
Close Encounter of the Extremely Cold Kind
On July 14, NASA's New Horizons spacecraft will fly by Pluto and its abnormally large moon, Charon.
Launched in 2006, New Horizons will be the first to bring distant, reddish Pluto into focus. Since its discovery nearly 85 years ago by Kansas farm boy Clyde Tombaugh, Pluto has been been confounding astronomers with one puzzling observation after another. (Watch a video about Tombaugh's discovery of Pluto.)
The best images that scientists have of Pluto are just blurry blobs. It's much too far away—averaging 40 times farther from the sun than Earth—to get a good look at. Even New Horizons, the fastest spacecraft ever launched, took nearly a decade to get there.
But scientists have a few guesses about what they might find on Pluto. Observations already hint at a dynamic, shifting surface that varies dramatically in both brightness and color. Some scientists suspect they might find evidence for icy volcanic eruptions.
One thing is certain: The 1,430-mile-wide (2,300 kilometers) world hovering on the fringe of the observable solar system will provide plenty of surprises.
Now, with New Horizons on approach and already collecting data (the mission's science campaign began January 15), the team is inching closer to solving Pluto's mysteries and unraveling the secrets of the icy Kuiper Belt, a region stuffed with countless comets, worlds like Pluto, and other leftovers from the dawn of the solar system.
"This is our first really good chance to do what we've done in the inner solar system," says Brown, who discovered several distant dwarf worlds similar to Pluto. "We finally get to see one up close and start to learn about how this new class of objects, that we didn't really realize was out there, works. Which is just going to be super cool."
A Surprising Family Tree?
Though billions of miles now separate Pluto and Ceres, some scientists suspect the two worlds might have both been born in the icy Kuiper Belt, where Pluto now reigns.
"Ceres, if you look at it, is kind of like a junior version of a Kuiper Belt object. It has the same density," says William McKinnon, a planetary scientist at the Washington University in St. Louis. "The question is, where did it actually form? Was there enough ice in the primordial asteroid belt to build such a world?"
In some ways, Ceres is more similar to Pluto than it is to the other asteroids in its neighborhood: It's round and icy, with its insides separated into layers.
The question is, where did it actually form?
"It sure smells like something that came from the outer solar system," Brown says.
If Ceres did start out in the Kuiper Belt, it may have been hurled inward during a period of violent upheaval early in the solar system's history, eventually settling into its current orbit in the asteroid belt.
Though the New Horizons encounter with Pluto will be fleeting, the craft will send data back to Earth for 16 months as it sails into the Kuiper Belt. There, perhaps, another target world awaits. NASA's Hubble Space Telescope has already started scouting for its next world to explore.
There will be no third world for Dawn, though. When its mission at Ceres ends, scientists will leave the craft in orbit around the watery sphere, a whirling perpetual monument to Earth's first exploration of the dwarf realm.
Correction: An earlier version of this story incorrectly stated that Pluto is 14,300 miles wide. The correct distance is 1,400 miles.