When it comes to underdog planets, Ceres might be at the top of the list. Sure, you've probably heard about Pluto's demotion to dwarf planet. But before Pluto, there was Ceres.
Ceres also once enjoyed full membership in the solar system's planetary fraternity. When astronomers discovered it in 1801, it was the only object known between Mars and Jupiter. Its story echoes Pluto's. After astronomers found more bodies in similar orbits - objects that became part of what's now known as the asteroid belt - they reclassified Ceres as an asteroid.
To explore it is to excavate the solar system's history
It's not just any asteroid, though. It's still the biggest one there is, accounting for about a third of all the mass in the asteroid belt. Ceres is big enough for gravity to have made it round, which qualifies it as a dwarf planet as well. Despite this humble status, Ceres is proving to be way more interesting than just another space rock.
For instance, Ceres is one of the most watery worlds in the solar system, with water comprising about 15 percent of its mass and a third of its volume, according to the latest estimates. Most of the water is locked up in ice, but scientists think that deep in its interior, some of it may be liquid. As water is necessary for life, this has implications for habitability.
That's not to say Ceres is home to aliens but such a world contains vital clues about the origins of life - on Earth and beyond.
As the only icy body of its kind in the inner solar system, Ceres represents one of the key building blocks that formed the planets. "It's an example of the last step before planet-hood," says Andy Rivkin, a planetary scientist at Johns Hopkins University in the US. Ceres is a remnant of the past, and to explore it is to excavate the solar system's history.
The Dawn spacecraft, now in orbit around Ceres, is doing just that. Its mission is far from over, but it's already revealed a wondrous, cratered surface, discovering a world that's not a dull chunk of ice and rock, but one that's alive and kicking.
A unique world
By the end of the 18th century, astronomers knew of seven planets, having just added Uranus to the mix in 1781. But between Mars and Jupiter was a curiously wide gap. According to an empirical law known as the Titius-Bode law, the distance between the sun and the planets followed a distinct pattern. If the pattern held, there should be a planet right in that gap.
So when an Italian monk named Giuseppe Piazzi discovered Ceres on New Year's Day in 1801, astronomers rejoiced. Not only had they finally found the missing planet, but it was also only the second time anyone had identified a new planet in modern times.
Even though astronomers found three more bodies in similar orbits over the next few years, Ceres kept its planetary status. It remained a planet for almost 50 years, after astronomers discovered Neptune (which, incidentally, didn't follow the Titius-Bode pattern).
When it was discovered, it was thought of as a planet, it's been downhill since then
By then, thanks to improved telescopes, astronomers were finding more and more objects near Ceres. They realised that it was just one of an entirely new class of bodies. Instead of a planet, Ceres was dubbed a "minor planet" or an asteroid.
"Poor Ceres got demoted," says Chris Russell, a planetary scientist at the University of California, Los Angeles. "In 1801, when it was discovered, it was thought of as a planet, and it's been downhill since then."
Ceres has languished as just another member of the asteroid belt, albeit the largest. Still, most people thought it was simply a big rock. "Even among scientists, I think we only really realised how weird or how different it was in last 10 to 15 years," Rivkin says.
The solar system is a wet place
Scientists could only make more accurate measurements of its mass about 15 year ago, he says, revealing Ceres to be less dense than previously thought. This was a key revelation, showing that Ceres must contain huge quantities of ice mixed with rock.
That ice in itself isn't unique. As multiple spacecraft have discovered, the solar system is a wet place. Comets, the moons of Saturn, Jupiter and even Pluto and its moon Charon, have lots of ice. Some places, like Europa and Ganymede, may contain vast subsurface oceans. Saturn and Jupiter have other small, icy moons comparable to Ceres as well. But when you consider Ceres's size, silicate rocky composition, and its location in the asteroid belt, it's one of a kind.
It wasn't always special. About 4.5 billion years ago, as the planets were just beginning to form, the solar system was home to perhaps tens of thousands of icy bodies like Ceres. They collided and merged to form the worlds we know today - including Earth.
Icy bodies like Ceres were key ingredients not just for planets, but also for life on Earth
If they didn't coalesce into planets, they got smashed into smaller pieces, becoming the floating detritus that fills the asteroid belt. Luckily for us, Ceres survived.
Icy bodies like Ceres were key ingredients not just for planets, but also for life on Earth. To fill its oceans, where life first evolved, icy objects such as comets and Ceres-like bodies must have crashed into the planet, delivering water in the process.
Then there's the prospect of life existing on Ceres itself. About 950 km across and 80 times less massive than the moon, Ceres is too small to have an atmosphere, and neither liquid water nor ice can survive for long on its surface. But, below the surface is a different story.
When Ceres first formed, it was hot enough to melt ice, which made it easier for water and rock to separate. That allowed heavier, rocky material to sink toward the centre while the water remained, forming the mantle.
Ceres cooled from the outside in, so while its outer layers are now a frozen mixture of ice and clay, some of the water in its interior might still be liquid. Just maybe, somewhere inside Ceres is an environment warm and wet enough for life.
While no one expects to find aliens on Ceres, it does have some advantages over other possible life-harbouring worlds like Europa and Ganymede. Ceres is closer to the Sun, so it enjoys some extra warmth that might be needed to nurture life.
It's also free of the radiation-filled environment that Jupiter's magnetic field creates around Europa and Ganymede. "I would say Ceres is more user friendly to life than we would get in the interior of Europa or Ganymede," Russell says.
In 2014, Ceres got even more interesting. The Herschel space telescope detected water vapour spraying from the dwarf planet - the first definitive detection of water from an object in the asteroid belt. The vapour seemed to come and go at different times, prompting speculation for cryogenic volcanoes or other geologic activity.
To figure out what exactly was triggering those apparent plumes, scientists would need to take a closer look. Fortunately, a spacecraft was already on its way.
Dawn launched in 2007, and after more than a year exploring Vesta, the second most massive asteroid, it headed towards Ceres. As the round, cratered world came into focus, scientists noticed a big white spot, shining bright in stark contrast to the drab, grey surface.
Such watery eruptions could be a direct link to wet, potentially habitable environments
Upon getting closer, Dawn revealed it was not one spot but two, so bright that UFO enthusiasts speculated they were signs of Ceresian beings.
But scientists had a few more likely hypotheses. The spots could be sunlight reflecting off patches of recently formed ice. Or Dawn was actually peering straight into cryogenic volcanoes or geysers.
Not only would that mean Ceres was geologically active, but also that water was reaching the surface. Such watery eruptions could be a direct link to wet, potentially habitable environments below.
By the time Dawn entered an orbit around Ceres in March 2015, scientists found many other smaller spots across its surface. As for the big ones, they turned out to be a cluster of smaller dots, all inside a 90 km-wide crater named Occator.
Now that Dawn has spiraled into an orbit just 1,470 km above Ceres, scientists have pretty much ruled out geysers or volcanoes, says Russell, who is the principal investigator of the Dawn mission.
After more careful measurements, researchers found that those spots were not reflecting as much light as they thought. At best, they were only bouncing back 50 percent of any incoming light, which is not shiny enough to be an icy geyser.
The spots are not likely to be patches of ice either. Surface ice does not last very long before sublimating away, and Dawn's camera and infrared spectrometer has not detected any chemical signs of ice. So far, Russell says, the most likely possibility is that the spots are salt - not table salt, but chemical salts like magnesium sulfate.
That might not sound as exciting as erupting geysers, but salt still points to flowing water. "That's just as good an indicator of water because the salt isn't just going to float out of the body itself," Russell says. "It's going to be carried out by liquid to get it to the surface."
What scientists do not yet know is whether water is depositing salt now, or if the salt formed last year, or even millions of years ago. But by comparing old Hubble images of Ceres with Dawn's, researchers can determine if the spots have changed - and if water is still delivering salt to the surface today.
Meanwhile, Dawn continues to reveal more surprises. It has detected a thin layer of haze filling most of Occator crater, but remaining just below the rim.
Ceres is going to be a great place for a follow-up mission
Dawn is not yet close enough to measure any details about the haze, other than the fact that it's scattering light. But if it turns out to be water vapour - perhaps related to the watery process that's depositing salt on the surface - it could be the same vapour that Herschel detected last year.
Maybe the strangest thing, other than the spots, is a lone, pyramid-shaped mountain that towers 6 km above the surface. At first glance, Russell says, it's similar to the ice mountains the New Horizons spacecraft recently discovered on Pluto. The same processes that formed the mountains might be at work on both dwarf planets.
The mountain could also be a permafrost mound called a pingo, which can be seen in places like Alaska and perhaps Mars, Russell says.
On Earth, pingos form when a mound of ice grows underneath frozen soil, getting as high as 50 metres. Dawn scientists are now scouring the images of Ceres for similar mounds. Due to the weaker gravity on Ceres, Russell explains, pingos might be able to grow as high as the pyramid mountain.
Eventually, maybe humans will pay a visit
Dawn will continue to collect data from its current orbit until autumn, before dropping to a mere 375 km from the surface in December. Dawn will then continue circling Ceres indefinitely, at least until its primary mission is over next summer. But the end of Dawn isn't necessarily the end for exploring Ceres.
"Ceres is going to be a great place for a follow-up mission," Rivkin says. "Something like a Ceres rover." Ceres would also be a great testing ground for a potential lander on Europa. Both the European Space Agency and NASA have started planning missions, but only to orbit the icy moon. With the possibility of a subsurface ocean, drilling into its surface would be more exciting.
Drilling on a moon like Europa is still a long way off, however. Getting to Ceres would be a simper mission as it's much closer and in a relatively radiation-free environment. Eventually, maybe humans will even pay a visit, Rivkin says. "Ceres is a good spot for a post-Mars target."
On the tiny chance that there is life on Ceres, it will take a lander to find it. "I have no evidence at all and I don't expect we would get any evidence for life on Ceres," Russell says. "But if there were a mission that wanted to go out and explore whether there was evidence of life on Ceres, I would certainly vote for it."
Even if Ceres was not home to alien critters, the world itself seems to be alive, with whiffs of water vapour, frozen mountains, maybe flowing water and whatever else Dawn will uncover in the coming months. In Russell's eyes, that kind of activity should make Ceres a bona fide planet.
In fact, when the International Astronomical Union voted to make Pluto and Ceres dwarf planets in 2006, they could have gone the other way and declared both planets. The first proposed definition of a planet said it was anything orbiting a star that's big enough to be round, which would have included Ceres and other objects in the outer solar system, expanding the planetary club to 12.
But alas, astronomers adopted a more stringent definition, requiring a planet to not only be round, but to have cleared its orbit. This rules out Ceres and Pluto because of their places in the asteroid and Kuiper belts, respectively.
For now, Ceres remains officially a dwarf planet. Whatever you call it, though, you can't say it's boring.