Artist's impression of Chariklo, a small, asteroid-like object with a ring system. The surprising discovery was announced this week. Image copyright Don Davis.
Artist's impression of Chariklo, a small, asteroid-like object with a ring system. The surprising discovery was announced this week. Image copyright Don Davis.

These Are Some of the Solar System’s Biggest Surprises

The universe is full of surprises, but two discoveries in the outer solar system are dominating astronomy news this week.

First, astronomers reported yesterday that they have found a distant, tiny world – a small, icy body that lives in the darkness far beyond the orbit of Neptune. Called 2012 VP113, the world’s existence challenges theories describing the infant solar system, and flames speculation that a large planet hides on the fringes of detection. 2012 VP113 and its sibling Sedna are now the two farthest-flung, roundish objects we’ve spied whose gravitational allegiance lies with the sun.

Next, we learned of an asteroid-like body with rings. Called Chariklo, the ringed world is a Centaur – an icy, rocky object that lives between the orbits of Jupiter and Neptune. Chariklo is the first not-planet in the solar system known to have a ring system. And it isn’t just any old ring system – there are two bright, glimmering icy rings encircling the enigmatic, 248-kilometer-wide world. For decades, scientists had thought a small body’s gravity would be too weak to cling to rings like Saturn’s. “When it appeared, it was a complete surprise,” said Felipe Braga-Ribas, a planetary scientist at the National Observatory in Brazil, who discovered Chariklo’s rings hiding in a few seconds of observational data. “We started trying to understand it.”

While discussing the discovery with various planetary scientists, it became apparent that such surprises are the norm in planetary science. It seems the solar system has no shortage of unexpected offerings. “It’s one of these discoveries that you just don’t expect to have happen. But that’s the story of planetary science,” said Joseph Burns, a planetary scientist at Cornell University, while talking about Chariklo. “We go out and see what nature says.”

In honor of this week’s outer solar system double feature, I thought it would be fun to ask some scientists which discoveries they consider the most surprising. Whether recent or decades-old, nearby or faraway, groundbreaking or just “wow,” the discoveries described below invoke a delightful tour through the solar system’s many eclectic treasures.

And please, whether scientist or not, feel free to comment on this post and share your most surprising solar system moment – what has our planetary neighborhood surprised you with?


Q: Which discovery (or discoveries) in the solar system has most surprised you, and why? (responses have been lightly edited for length and clarity)

Ryan Anderson, astrogeologist, U.S. Geological Survey

For me, the biggest recent surprise was the discovery of plumes at Europa. A lot of times the big “surprises” make perfect sense in retrospect. We think Europa has liquid water under the ice and that the ice is shot through with fractures, so it makes sense that it might have plumes much like Enceladus at Saturn. But still, making sense of the discovery in retrospect doesn’t change the fact that it’s surprising and exciting when you first hear about it.

Erik Asphaug, planetary scientist, Arizona State University

My biggest surprise was seeing a comet split up into a dozen pieces in 1993, with the discovery of Shoemaker-Levy 9 when I was graduating from Arizona. Wow! I remember thinking, “How many comets get made in this way?” Twenty years later we’ve all been blown away by the sequential space missions to Wild 2, Tempel 1, and Hartley 2. They are all so different. After all this time we still know basically nothing about comets as geologic bodies, which makes them the most fun and rewarding objects of investigation.

Fran Bagenal, planetary scientist, University of Colorado

Volcanoes on Io, which revealed the most geologically active object in the solar system, and moons around asteroids (how did they get there?).

Michele Bannister, postdoctoral fellow, University of Victoria

How active so many of the icy worlds (moons, dwarf planets) of the Solar System are. Not only active in the great, slow past of geological time, but active now: We can see their surfaces change in our lifetime. Geysers on Enceladus, possible plumes on Ceres and Europa, suggestive geyser-features on Triton, storms and rivers on Titan…we’re living in a Solar System that is changing and dynamic.

Bill Bottke, planetary scientist, Southwest Research Institute

The Nice model, where the giant planets possibly started in a very different configuration than they have today; the ~200 km diameter naked iron core represented by the asteroid Psyche; the discovery of the Kuiper belt and more recently, Sedna and its brethren; not quite a discovery, but the paucity or absence (depending on who you believe) of ancient rocks on Earth that are older than 4 billion years; ice within the permanently shadowed craters on the Moon and Mercury; the prediction that many icy moons have deep oceans; the equatorial ridge around Iapetus, and the absolute weirdness of Miranda; how much our view of the solar system has fundamentally changed since the advent of fast computers and efficient numerical integration codes.  (Exoplanets!)

Mike Brown, astronomer, Caltech

Since the story on the second Sedna body is coming out, I am reminded just how incredibly surprised we were when we discovered Sedna. So surprised that we didn’t believe it for about a month until we got multiple confirmations and ruled out every other possibility. It was the only thing ever found so far away and it lived in an area of space where nothing should have been. I always said, at the time, that this is the best part of doing science, because when you find something that is not supposed to be there, you have learned something new about how the solar system works.

Joseph Burns, astronomer, Cornell University

If you go back long enough, a big [surprise] is just the nature of natural satellites. When I was growing up as an academic, satellites were supposed to be just bombarded, barren, cratered objects. Uninteresting. Why would you want to look at them? And then we got in orbit around Mars and saw Phobos and Deimos, and they were pretty bizarre-looking. And then Voyager went out – and suddenly you see Io. And it’s got volcanoes and sulfur, it looks like a pizza and has some sort of bizarre surface. And it turns out, when you go to every system, every one of the satellites is unique – and now, three of the possible abodes for life in the solar system are on moons.

Athena Coustenis, planetary scientist, Observatoire de Paris-Meudon, CNRS

For me, the most surprising discovery were the jets at Enceladus, because it demonstrated that we could find liquid water under the surface of the icy moons at 10 astronomical units, and challenges all the conventional habitable zone models…

Luke Dones, planetary scientist, Southwest Research Institute

We went in a decade from, “only Saturn has rings” to ring systems around all four giant planets. The ring arcs of Neptune were particularly surprising. Also, the very complicated orbital structure of the Kuiper Belt/trans-Neptunian region/inner Oort Cloud/whatever you want to call it. Most of the populations beyond Neptune could have formed closer to the Sun and moved out to their current locations, but there’s a “cold classical” population that seems like it’s always been where it is now.

And, the first extrasolar planets were found around a pulsar, of all places.

Lindy Elkins-Tanton, director of the Department of Terrestrial Magnetism, Carnegie Institution for Science

I was really surprised when Mercury’s magnetic field was found to be offset to the north; we’re used to a magnetic field whose pole can wander relative to the spin pole of the planet, but what about one whose magnetic equator is north of the planet’s equator? This surprise gives me the exciting idea that we may not understand magnetic dynamos very well after all!

Jay Farihi, astronomer, University College London

Comets in the main asteroid belt – they’re like comet-spies in the inner system where they don’t belong. Some main belt asteroids sometimes exhibit characteristics of comets – tails, outflows, etc. They are still poorly understood; it’s not clear if ice and volatiles are evaporating or being released from collisions or rotational break up.

David Grinspoon, astrobiologist, U.S. Library of Congress

I have been repeatedly surprised by the level of activity in small worlds that we “knew” should be old and dead, before we started exploring the outer solar system. Volcanoes on Io, geysers on Enceladus and now evidence of surface water on Asteroid Vesta?  The apparent prevalence of liquid water environments far beyond the sunny inner solar system is a delightful, ongoing set of surprises.

Avi Loeb, astrophysicist, Harvard University

The discovery of ice on Jupiter’s moon Europa – with potentially liquid water under it – was most surprising to me because this environment might host life (in other words, there might be fish in that water). Another surprising discovery was of Sedna, which is a Pluto-size object on an eccentric orbit extending thirty times farther than Neptune’s distance from the Sun. 

Ralph Lorenz, planetary scientist, Johns Hopkins University Applied Physics Laboratory

I wrote a paper in 1995 anticipating that we would not find sand dunes on Titan, which may be one of the most outstandingly bad predictions in planetary science (for interesting reasons) since some 15 percent of Titan turned out to be covered in giant dunes. Bigger picture, Titan turned out to be much more diverse than anyone remotely expected.  Pre-Cassini, everyone thought in one-dimensional terms – Titan is the same everywhere. We thought it would be wet (no dunes), and yet it has vast deserts.  It turns out climate (latitude) controls a lot – it’s wet too, just around the north polar regions. And nobody, not even us, expected us to be able to see the bottom of Ligeia Mare, all the way down to 170 meters. Titan’s seas must be amazingly clear.

Franck Marchis, planetary astronomer, Carl Sagan Center, SETI Institute

Asteroids are mini geological worlds with complex surface activity, differentiated interiors, complex evolution histories, and moons. Also, the satellites of Saturn – their shape, structure and evolution (captured or formed from the ring?) – and the complex interactions between Saturn, its ring system and its satellites. Io (with its outburst eruptions) and Europa and Jupiter’s magnetic field. There is still a lot to learn about the interaction of satellites and the giant planet’s magnetosphere — maybe a future promising way to detect moons around exoplanets?

Sarah Milkovich, planetary geologist, NASA’s Jet Propulsion Laboratory

I’d say three things: The plume at Enceladus, because this tiny moon is spitting out enough water to form the E-ring around Saturn! The plumes at Europa, because we didn’t see them from the Jovian system with the Galileo spacecraft, but from Earth with Hubble. The recurring slope linea (RSL) at Mars, because the idea of any kind of liquid water-related activity on Mars today is rather mind-boggling.

Catherine Neish, planetary scientist, Florida Institute of Technology

I think my top three are: 1. Ice on Mercury’s poles. Although I was only 11 when this was discovered using ground-based radar, I still think this stands out as one of the neatest discoveries in planetary science. The fact that there is water ice on the closest planet to the Sun just seems so incongruous (even though in retrospect it makes perfect sense). 2. Plumes on Enceladus. Water jetting directly into space! How neat is that? 3. Titan: The desert planet. Prior to the Cassini mission, a lot of people thought that Titan would be covered by a sea of liquid hydrocarbons. The Cassini-Huygens mission revealed it to be much more of a desert planet, with vast sand dunes near the equator, and only smaller lakes and seas near the poles.

Alex Parker, planetary astronomer, University of California, Berkeley

In recent memory, the most exciting and surprising series of confirmed discoveries were the detection of Pluto’s packed system of four small moons outside its very large moon Charon. They were unexpected, are in a surprisingly delicate configuration, and their origin and survival remains challenging for theorists to explain. They’re also particularly exciting in light of the fact that we now get to explore them up close when New Horizons visits the Pluto system next year!

Carolyn Porco, planetary scientist and Cassini imaging lead, Space Science Institute

I was part of the Voyager mission to the outer solar system.  Every stop was packed with surprises. But the best surprise of all was the spectacular geysering activity we on Cassini have found at the south pole of Enceladus.  We suspected that moon might have geysers of some sort.  But never did we imagine they’d be the phenomenally dramatic and huge things they turned out to be. It all comes down to a failure of imagination and our inability to divine the variety and spectacle of the phenomena that Mother Nature can create.

Christopher Russell, Geophysicist, Dawn mission principal investigator, UCLA

I will give you three answers, all from the moons of the outer solar system: The magnetic field of Ganymede (which seems to be generated by a magnetic dynamo similar to that in the Earth – no other moon is like that today, although our moon once was), the plume of Enceladus and the lakes on Titan. These moons were like small planets.

Mark Showalter, planetary astronomer, SETI Institute

I’ll vote for Saturn’s F ring as first imaged by Voyager 1. It showed the so-called “braids” (which aren’t really braids, of course) and it was the first time we all realized that a ring didn’t have to be circular and uniform.

Let me pass along a quick anecdote. At the moment that first image came down, one of the Imaging team scientists was doing an on-camera interview, which I was watching. Somewhat befuddled, he blurted out, “This is high on the list of things we didn’t expect to see!” I always wondered what else was on that list.

Linda Spilker, Cassini project scientist, Jet Propulsion Laboratory

For me, the discovery of the icy jets spewing out of the south polar region of Saturn’s tiny moon Enceladus was a big surprise. Enceladus is only 500 km in diameter and should have frozen solid long ago yet today it lofts icy particles and gas into space, creating Saturn’s diffuse E ring. The discovery of liquid methane lakes and seas at the north pole of Saturn’s giant moon Titan was another big surprise. Titan has a thick nitrogen atmosphere and methane plays the role on Titan, with clouds and rain, that water plays on Earth, creating river channels and filling lakes and seas which contain more than 100 times as much hydrocarbons than all of the oil and gas on Earth.

Alan Stern, New Horizons principal investigator, Southwest Research Institute

River valleys on Mars, volcanoes on Io, and the discovery that dwarf planets dominate the population of planets in our solar system. More generally, we should not be surprised at being surprised at the richness of nature — that’s a hallmark of planetary science.

Anne Verbiscer, planetary scientist, University of Virginia

I know this sounds awfully self-serving, but the discovery that surprised me the most was that of Saturn’s Phoebe Ring!  Yes, we planned our Spitzer observations with the intent of finding a ring, but we were really surprised (and delighted) to find that it was there!

View Images
New orbit diagram for the outer solar system. The Sun and Terrestrial planets are at the center. Jupiter, Saturn, Uranus and Neptune are in purple solid circles. The Kuiper Belt (including Pluto) is the dotted light blue region just beyond the giant planets. Sedna’s orbit is shown in orange, and 2012 VP113’s orbit is shown in red. (Scott Sheppard, Carnegie Institution for Science)

This post has been updated to include more responses