Mercury
Mercury is notoriously difficult to study, thanks to its proximity to the scorching hot and blindingly bright sun. Thus, mysteries abound. For example, Mercury has a giant core — perhaps because its outer, lighter layers got brushed off by planetary collisions long ago, but scientists aren't sure. It also has a magnetic field and an atmosphere, both of unknown origin. In fact, the little planet leaks a steady stream of atmospheric particles, suggesting its atmosphere is somehow constantly regenerated. The biggest boggler of all: Mercury's highly elongated orbit is growing more oval-shaped all the time, and it could someday crash into Venus or the sun. Will its changing path (and resultant changing gravitational field) disrupt the orbits of Earth and the other inner planets, causing chaos?
Venus
Planetary scientists are still working out the details of how a once-earthlike Venus gradually morphed into the hellishly hot planet shrouded in a thick blanket of toxic gases we see today. But a bigger mystery regarding Earth's "evil twin" is why the planet's atmosphere swirls around it 60 times faster than the sphere spins itself; and speaking of Venus' spin, no one knows why it goes counter-clockwise unlike all the other inner planets, such that the sun rises in the west and sets in the east.
The most intriguing open question of all: Does Venus harbor life in its clouds? Some 30 miles up, there should be a habitable niche where pressure and temperature are earthlike. For energy, floating creatures resembling bacteria could thrive off the ample sunshine or chemicals in the atmosphere.
Earth
You might think we'd have nailed down the major bullet points about our home planet's structure and formation, but in fact, big zingers remain. We don't know, for example, how all this water got here, and we're uncertain about the nature of Earth's core, which, strangely, transmits seismic waves faster in one direction than the other. Our beloved satellite has big bogglers, too. While most scientists think the moon formed from a chunk of Earth that got knocked off during an ancient impact, the theory has a hole: the theoretical impactor, dubbed Theia, should have left a residue with distinctive characteristics, but it has not been detected.
Mars
The Red Planet, now frigid, barren and seemingly deserted, spent its first 500 million or billion years as warm, wet and geologically dynamic. Scientists don't know why it changed so drastically for the worse. They also wonder whether a more vibrant Mars once harbored life, and if it did, whether any bacteria-like Martian organisms managed to adapt to the harsher environs that took over, and are still eking out an existence there. [Seeing Things On Mars: A History of Martian Illusions]
What's more, scientists can't fully explain the planet's "hemispheric dichotomy." Smooth, younger lightly cratered lowlands dominate the planet's top half, while ancient, heavily cratered highlands characterize the southern hemisphere. As for how Mars' two funky, lumpy moons got there, their size and shape suggests they originated as asteroids and were captured by the planet's gravity; however, captured asteroids normally traverse elongated, oval orbits, while Phobos and Deimos follow circular paths around Mars.
Jupiter
Like a carefully dyed Easter egg, Jupiter is girded by lighter-hued bands called zones and darker bands called belts. But are these stripes merely surface features overlaying a uniform inner ball of gas, or are the zones and belts actually the tops of concentric cylinders that make up the planet? Whole stripes have been known to disappear without a trace; one vanished in May 2010 that was twice as wide as Earth; why? Other surface decors, such as the swirling vortex known as the Great Red Spot, are equally as mysterious: What power source drives their turbulent motion?
Furthermore, early in its history, this gas giant gobbled up great gobs of heavy elements, including more carbon, nitrogen and sulfur than are found in the sun. How did all that heavy stuff get in there, and is some of it compacted in a solid core deep below Jupiter's surface? Scientists still don't know, and are hoping to learn a thing or two when the Juno spacecraft flies past in 2016.
Saturn
For four centuries, astronomers have contemplated Saturn's eye-popping rings, but none of their attempts to explain the beautiful features have ever seemed quite right. The rings could have formed from the icy remnants of a bygone moon, or from a passing comet torn to shreds by the planet's gravity; they could be relatively young at just a few hundred million years old, or they might date back to the birth of Saturn more than four billion years ago. We just don't know. We're also yet to nail down the dynamics of giant storms and jet streams on the ringed planet's surface, as well as the dynamics of its rotation. Three different spacecraft have attempted to measure the length of Saturn's day by detecting its natural radio emissions as they soared past; all three have turned up different measurements.
Uranus
Planets are expected to radiate heat leftover inside them from their fiery formation process, but puzzlingly, Uranus radiates little or no heat into space. Perhaps the seventh planet's heat got unleashed during some cosmic smash-up in the distant past. (That collision could also have caused the planet's strange sideways spin.) Or, maybe Uranus somehow self-insulates, keeping all its heat trapped inside.
Uranus also drags around with it the craggiest astronomical object known to man — a satellite called Miranda. This strange moon has deep canyons, scrapes, terraced layers and a cliff some 12.4 miles (20 kilometers) deep – the deepest in the solar system. Miranda's geological mess may have formed from flowing ices in the moon's interior, perhaps heated by gravitational squeezing from Uranus and other moons, oozing out onto the surface. Or, perhaps the moon was shattered several times and came back together, creating its jagged and mottled features.
Neptune
Astronomers had expected Neptune to be a weatherless, featureless world in deep freeze. Instead, Voyager 2's flyby in 1989 — the only close look we've ever gotten of this 3-billion-mile-away planet — revealed a turbulent atmosphere with lighter cloud ripples and raging storms. Surprisingly, the fastest winds ever recorded in the solar system whirl on Neptune, up around 1,300 miles (about 2,100 kilometers) per hour. Driving this activity appears to be Neptune's internal heat, but as the farthest planet from the sun (farthest, that is, ever since the more-distant Pluto was kicked off the planet list in 2006), why does it hold so much heat?
Neptune's clumpy rings also confound scientists, as does its bizarre magnetic field, which emanates from a point off-set from the planet's center.
