INTRODUCTION
The solar system is a bizarre place with its alien planets, mysterious moons and strange phenomena that are so out-of-this-world they elude explanation. Scientists have discovered ice-spewing volcanoes on Pluto, while Mars is home to a truly "grand" canyon the size of the United States. There may even be a giant, undiscovered planet lurking somewhere beyond
Uranus tilted on its side
Uranus appears to be a featureless blue ball upon first glance, but this gas giant of the outer solar system is pretty weird upon closer inspection. First, the planet rotates on its side for reasons scientists haven't quite figured out. The most likely explanation is that it underwent
some sort of one or more titanic collisions in the ancient past. In any case, the tilt makes Uranus unique among the solar system planets.
Uranus also has tenuous rings, which were confirmed when the planet passed in front of a star (from Earth's perspective) in 1977; as the star's light winked on and off repeatedly, astronomers realized there was more than just a planet blocking its starlight. More recently, astronomers spotted
storms in Uranus' atmosphere several years after its closest approach to the sun, when the atmosphere would have been heated the most.
Jupiter's moon Io has towering volcanic eruptions
For those of us used to Earth's relatively inactive moon, Io's chaotic landscape may come as a huge surprise. The Jovian moon has hundreds of volcanoes and is considered the most active moon in the solar system, sending plumes up to 250 miles into its atmosphere . Some spacecraft have caught the moon erupting; the Pluto-bound New Horizons craft
caught a glimpse of Io bursting when it passed by in 2007.
Io's eruptions come from the immense gravity the moon is exposed to, being nestled in Jupiter's gravitational well. The moon's insides tense up and relax as it orbits closer to, and farther from, the planet, generating enough energy for volcanic activity. Scientists are still trying to figure out how heat spreads through Io's interior, though, making it difficult to predict
where the volcanoes exist using scientific models alone.
Mars has the biggest volcano
While Mars seems quiet now, we know that in the past something caused gigantic volcanoes to form and erupt. This includes
Olympus Mons, the biggest volcano ever discovered in the solar system. At 374 miles (602 km) across, the volcano is comparable to the size of Arizona. It's 16 miles (25 kilometers) high, or triple the height of Mount Everest, the tallest mountain on Earth.
Volcanoes on Mars can grow to such immense size because gravity is much weaker on the Red Planet than it is on Earth. But how those volcanoes came to be in the first place is not well known. There is a debate as to whether Mars has a global plate tectonic system and whether it is active.
Venus has super-powerful winds
Venus is a hellish planet with a high-temperature, high-pressure environment on its surface. Ten of the Soviet Union's heavily shielded Venera spacecraft lasted only a few minutes on its surface when they landed there in the 1970s.
But even above its surface, the planet has a bizarre environment. Scientists have found that its upper winds flow 50 times faster than the planet's rotation. The European Venus Express spacecraft (which orbited the planet between 2006 and 2014) tracked the winds over long periods and detected periodic variations. It also found that the hurricane-force winds
appeared to be getting stronger over time.
Spacecraft has visited every planet
We've been exploring space for more than 60 years, and have been lucky enough to get close-up pictures of dozens of celestial objects. Most notably, we've sent spacecraft to all of the planets in our solar system — Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune — as well as two dwarf planets, Pluto and Ceres.
The bulk of the flybys came from
NASA's twin Voyager spacecraft, which left Earth in 1977 and are still transmitting data from beyond the solar system in interstellar space. Between them, the Voyagers clocked visits to Jupiter, Saturn, Uranus and Neptune, thanks to an opportune alignment of the outer planets.
There could be life in solar system, somewhere
So far, scientists have found no evidence that life exists elsewhere in the solar system. But as we learn more about how
"extreme" microbes live in underwater volcanic vents or in frozen environments, more possibilities open up for where they could live on other planets. These aren't the aliens people once feared lived on Mars, but microbial life in the solar system is a possibility.
Microbial life is now considered so likely on Mars that scientists take special precautions to sterilize spacecraft before sending them over there. That's not the only place, though. With several icy moons scattered around the solar system, it's possible there are microbes somewhere in the oceans of Jupiter's Europa, or perhaps underneath the ice at Saturn's Enceladus, among other locations.
Mercury is still shrinking
For many years, scientists believed that Earth was the only tectonically active planet in the solar system. That changed after the Mercury Surface, Space Environment, Geochemistry and Ranging (MESSENGER) spacecraft did the first orbital mission at Mercury, in high definition and getting a look at the features on its surface.
In 2016, data from MESSENGER (which had crashed into Mercury as planned in April 2015) revealed cliff-like landforms known as fault scarps. Because the fault scarps are relatively small, scientists are sure that they weren't created that long ago and that
the planet is still contracting 4.5 billion years after the solar system was formed.
There may be a huge planet at the edge of the solar system
In January 2015, California Institute of Technology astronomers Konstantin Batygin and Mike Brown announced – based on mathematical calculations and on simulations – that there could be a giant planet lurking far beyond Neptune. Several teams are now on the search for this theoretical "
Planet Nine," which could take decades to find (if it's actually out there.)
This large object, if it exists, could help explain the movements of some objects in the Kuiper Belt, an icy collection of objects beyond Neptune's orbit. Brown has already discovered several large objects in that area that in some cases rivaled or exceeded the size of Pluto. (His discoveries were one of the catalysts for changing Pluto's status from planet to dwarf planet in 2006.)
