Pluto has a bizarre atmosphere
Pluto's observed atmosphere broke all the predictions. Scientists saw the haze extending as high as 1,000 miles (1,600 km), rising higher above the surface than the atmosphere on Earth. As data from New Horizons flowed in, scientists analyzed the haze and discovered some surprises there, too.
Scientists found about 20 layers in Pluto's atmosphere that are both cooler and more compact than expected. This affects calculations for how quickly Pluto loses its nitrogen-rich atmosphere to space. NASA's New Horizons team found that tons of nitrogen gas escape the dwarf planet by the hour, but somehow Pluto is able to constantly resupply that lost nitrogen. The dwarf planet is likely creating more of it through geological activity.
Rings are everywhere in the solar system
While we've known about Saturn's rings since telescopes were invented in the 1600s, it took spacecraft and more powerful telescopes built in the last 50 years to reveal more. We now know that every planet in the outer solar system – Jupiter, Saturn, Uranus and Neptune – each have ring systems. That said, rings are very different from planet to planet. Saturn's spectacular rings, which may have come from a broken-up moon, are not repeated anywhere else.
Rings aren't limited to planets, either. In 2014, for example, astronomers discovered rings were discovered around the asteroid Chariklo. Why such a small body would have rings is a mystery, but one hypothesis is perhaps a broken-up moonlet created the fragments.
Jupiter's Great Red Spot is shrinking
Along with being the solar system's largest planet, Jupiter also hosts the solar system's largest storm. Known as the Great Red Spot (since it's big and ruddy-colored), it's been observed in telescopes since the 1600s. Nobody knows exactly why the storm has been raging for centuries, but in recent decades another mystery emerged: the spot is getting smaller.
In 2014, the storm was only 10,250 miles (16,500 km) across, about half of what was measured historically. The shrinkage is being monitored in professional telescopes and also by amateurs, as telescope and computer technology allow high-powered photographs at an affordable cost. Amateurs are often able to make more consistent measurements of Jupiter, because viewing time on larger, professional telescopes is limited and often split between different objects.
Most comets are spotted with a sun-gazing telescope
Comets used to be the province of amateur astronomers, who spent night after night scouring the skies with telescopes. While some professional observatories also made discoveries while viewing comets, that really began to change with the launch of the Solar and Heliospheric Observatory (SOHO) in 1995.
Since then, the spacecraft has found more than 2,400 comets, which is incredible considering its primary mission is to observe the sun. These comets are nicknamed "sungrazers" because they come so close to the sun. Many amateurs still participate in the search for comets by picking them out from raw SOHO images. One of SOHO's most famous observations came when it watched the breakup of the bright Comet ISON in 2013.
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.)