Pluto
And then there is Pluto. Discovered in 1930, it had been classified as the ninth planet for ~75 years. On Aug 24, 2006 the International Astronomical Union (IAU) adopted a new definition of what it takes to be planet and Pluto did not meet that definition. If you want more information on this, please click here. Now Pluto is classified as a dwarf planet and #134340 in the minor planet catalog. However, after the New Horizon mission visited Pluto in 2015 and produced so much new information, we include it here. This image compares our knowledge of Pluto before and after the New Horizon mission (Hubble space telescope vs camera on orbiter).
In terms of size and density Pluto is very similar to Neptune’s moon Triton. New Horizons, which has returned spectacular results had a long way to go. In 2006 Pluto appeared like this in New Horizons Long Range Reconnaissance Imager:
After New Horizon's encounter with Pluto, we now know that it looks like this:
Pluto is not the only dwarf planet. Located inside the Asteroid Belt between the orbits of Mars and Jupiter is the dwarf planet Ceres, which is currently the focus of the Dawn Mission. If you have 2:37 min to spare, you may want to check this recent video, giving some of the highlights of the Dawn mission this far:
Here is an update the Dawn mission December 2018: Evidence for carbon-rich surface on Ceres
