2015: The Year of the Dwarf Planets

Dawn image of Ceres taken February 19, 2015 from a distance of 46,000 km. NASA image

Today a NASA spacecraft called Dawn settled into orbit around the gigantic asteroid Ceres, having already sent back intriguing photos of the cratered body.

Ceres, which is about 950 kilometres in diameter depending on where the measurement is taken, was discovered in 1801 and for a time was thought to be the widely rumoured planet that was supposed to orbit between Mars and Jupiter. Instead Ceres became known as the greatest of the asteroids, most of which lie in a belt between the two planets.

Ceres is no longer an asteroid - today it's classified as a dwarf planet. It gained that distinction as part of the more controversial decision made in 2006 by the International Astronomical Union to designate Pluto a dwarf planet rather than a full planet. But back to Pluto in a minute.

As Dawn has drawn closer to Ceres, it has revealed the once mysterious face of this body, including its many craters and unusual bright spots, including two right next to each other in the same crater. When looking at these bright spots, it must be remembered that Ceres is actually a fairly dark body and the brightness of the spots is just relative to the darkness of the rest of Ceres' surface. 

The Dawn spacecraft, which is equipped with an ion propulsion system, was launched by NASA  in 2007 and on its way to Ceres paused at the giant asteroid Vesta for 14 months. In a few weeks, Dawn will lower its orbit around Ceres and deliver even higher resolution photos of this dwarf planet.

By then other NASA scientists should be looking at images that will bring Pluto into focus for the first time. The New Horizons spacecraft, launched nine years ago, is closing in for its July 14 flyby of Pluto. Already New Horizons is sending back photos that match the resolution of the best photos of Pluto available so far from the Hubble Space Telescope. The latest New Horizons photos show Pluto and three of its five known moons. 

Just as Dawn is revealing the face of Ceres, New Horizons will unveil Pluto and its moons, starting with its largest moon Charon. Dawn's findings will help reveal how the solar system was formed, and New Horizons will, in the words of co-investigator William McKinnon, give us our first glimpse at the solar system's "third zone," the dwarf planets inhabiting the Kuiper Belt on the fringes of oursolar system. 

Astronomers have discovered more than 200 objects in the Kuiper Belt, and with Hubble's help, New Horizons will be directed toward another Kuiper Belt object after it passes by Pluto this summer.

So keep an eye out for the images and other data coming to us from Dawn and New Horizons. 2015 is going into the books as the Year of the Dwarf Planets.

1996 Hubble Space Telescope Images of Pluto. Space Telescope Science Institute image