Astro Bob: Dwarf planet Ceres plies September skies

"Astro" Bob King is a freelance writer for the News Tribune. Read more of his work at

Ceres just before orbit.jpg
Ceres just before orbit. (NASA)

NASA’s Dawn spacecraft made this color closeup of cratered Ceres, the largest asteroid and also the only dwarf planet within the orbit of Neptune. You can see it as a faint “star” in binoculars in the Southern Fish this month. NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

How would you like to see a dwarf planet? Pluto’s the most famous, but it requires a good-sized telescope and a dark sky. There are currently five known dwarf planets. Of them only Ceres, which is also the largest asteroid, is the only one accessible to backyard skywatchers. All you need to see it is a pair of binoculars or a small telescope.

Dwarf planets and regular planets share two key characteristics:

  • They orbit the sun independently — they’re not satellites or moons of other planets.
  • They’re massive enough to mold themselves into spheres

But the two differ in that planets are big enough to clear their orbits of other small bodies like stray asteroids or comets. Planets wield their gravity to remove smaller bodies near their orbits by collision, capture, or otherwise pushing them out of the neighborhood. Dwarf planets simply aren’t massive enough to do this.


Ceres orbits within the asteroid belt between Mars and Jupiter at an average distance of 257 million miles (413 million km) from the sun. NASA / McREL

Ceres orbits in the asteroid belt between Mars and Jupiter 2.8 times farther from the sun than Earth. With a diameter of 588 miles (946 km) it’s about a quarter the size of our own moon. While the moon is primarily composed of rock Ceres is loaded with ice and water-rich minerals like clay and carbonates. Briny liquids have bubbled up from below its surface and left bright salt deposits on the dwarf planet’s dark surface. Sadly, the salt isn’t the kind you love on your french fries but other varieties such as sodium carbonate and magnesium sulfate.

Dawn’s close-up view of Occator Crater (57 miles / 92 km across) reveals the bright dome Cerealia Facula and neighboring patches called Vinalia Facula, both rich in sodium carbonate and ammonium chloride deposited by a slushy brine within or below the dwarf planet’s crust. NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

This icy, salty, clayey little world beckons on September nights from the constellation of Piscis Austrinus the southern fish. Like all asteroids it looks exactly like a star. Only the largest telescopes or the Hubble can distinguish a shape. Ceres reached opposition on August 29 at magnitude 7.7, the same brightness as the planet Neptune. It’s faded a bit since then to 7.9, but that’s still within range of 50mm binoculars from the outer suburbs and countryside. For more light-polluted areas you’ll need a small telescope instead.

Piscis Austrinus is located well to the left of the Saturn-Jupiter duo low in the southern sky. While the constellation is faint, Fomalhaut is bright. Stellarium

Back in August you had to stay up closer to midnight to see Ceres, but in mid-September it comes into good view starting around 10:30 p.m. for observers in mid-northern latitudes. The moon is also out of the evening sky, making the next two weeks ideal for bagging your first dwarf planet.


Point your binoculars at Fomalhaut and slide one field of view up and to the right (past Epsilon) to see the “Party Hat,” outlined in green. Ceres moves to the right across the asterism this month. Positions are shown every three nights. Stellarium

Locate the bright star Fomalhaut and you’re practically there. It’s the only first magnitude star low in the southeastern sky at that time. Point your binoculars at Fomalhaut and use the detailed map to “star hop” to the figure I’ve outline in green — I’ve dubbed it the Party Hat asterism but feel to make your own star patterns. Whatever works. Ceres will appear as a faint point of light that moves to the right (west) across the asterism over the next couple weeks.

This photo was taken with a mobile phone through a pair of 50mm binoculars last night, September 12. It gives you a good idea of how Ceres looks in a star field. Piqui Diaz

So many astronomical objects either look like fuzz or points of light. The more you know about Ceres going in, the better you’ll be able to appreciate what you’re seeing. I like hands-on and always remember concepts like dwarf planet vs planet better if I actually look at both. To learn more about Ceres, check out NASA’s in-depth (but easy to understand) Ceres page .

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