Dwarf Planet Ceres

PIA19064-Ceres-DwarfPlanet-StillImage-20150414

Previously, I told you the fascinating story of Ceres’s discovery and complicated identity crisis; now I’m ready to tell you about the dwarf planet specifically.

 

Let’s take a quick trip out beyond our Moon, past Mars (if you find yourself at Jupiter, you’ve gone too far), and into the realm that is commonly known as the asteroid belt. Now, contrary to popular depictions, the asteroid belt isn’t crammed full of asteroids. As a kid, I remember seeing illustrations of the asteroid belt that made it look as densely packed as Saturn’s rings. That depiction is a gross exaggeration. In fact, while there are billions of bodies orbiting out in the asteroid belt–it is believed that there are somewhere between 1 and 2 million asteroids with a diameter of 1 kilometer or more–the area is still mostly just empty space. If you were to board a rocket that would fly through the asteroid belt, the chances of actually smacking into anything are extremely slim. Of the asteroids in the belt that have a diameter of 10 km or more, a collision is only likely to occur about once every 10 million years. So anyway, this is the home of one such body, the dwarf planet Ceres.

Compared to the other bodies in the asteroid belt, Ceres is huge. Ceres has a diameter of 950 kilometers (590 miles). This is a little smaller than the width of Texas or Montana. Ceres comprises between a quarter and a third of the mass of the entire asteroid belt. King among the asteroid belt, Ceres falls short when facing up against the other planets in our solar system. Compared to Earth and the Moon, Ceres has the mass of .00015 that of the Earth and .0128 that of the Moon. (For some perspective, it would take almost 80 Cereses to equal the mass of just the Moon.)

Size comparison of Earth, the Moon, and Ceres.

Size comparison of Earth, the Moon, and Ceres.

 

Ceres orbits the Sun at an average distance of 415 million kilometers (257 million miles), in a nearly circular orbit. At this distance from the Sun, and at the speed that Ceres is traveling, one year on Ceres is equivalent to 4.6 years on Earth.

Ceres is believed to consist of a thin, dusty crust situated above a fairly thick layer of water-ice. At the center of the dwarf planet is a thick rocky core.

Cutaway image showing Ceres's layers.

Cutaway image showing Ceres’s layers.
“Ceres Cutaway” by NASA, ESA, and A. Feild (STScI)

 

Ceres, of course, has less mass than the Earth, and thus you would weigh less standing on a scale on Ceres than you would on Earth.  If you weigh 150 pounds on Earth, then you weigh a mere 4.2 pounds on Ceres!

Ceres is one of the latest planets to be explored by high-tech modern spacecraft. NASA’s Dawn spacecraft is currently orbiting the dwarf planet at a just recently arrived distance of only 2,700 miles above its surface. For about a month, Dawn will orbit and study Ceres from this location. The spacecraft will complete an orbit every three days, constantly kicking images and other important data back to Earth. For some perspective, the resolution Dawn can obtain while imaging Ceres is somewhat comparable to what it would be like for you to observe a soccer ball from 10 feet away. Subsequent to the 2,700 mapping orbit, Dawn will venture even closer to the dwarf planet providing better and better views of Ceres. By the end of 2015, Dawn will be concluding its mission at an altitude of only 230 miles. Dawn’s cameras at this distance will be able to produce images with a resolution 850 times greater than that of what Hubble would be able to produce. Now, that soccer ball is a mere 3.3 inches away! At this distance, Dawn will be in a fairly stable orbit around the dwarf planet and is expected to become its satellite into perpetuity.

I’ll have more to share about Dawn soon.

For now, let’s all celebrate the fact that we’re still exploring–exploring not just planets and asteroids and moons, but exploring actual worlds. Let’s celebrate the fact that we’re learning new things about this particular world on a daily basis and that this will continue for many months to come. And, let’s celebrate the fact that with all that we know today, it’s a tiny amount compared to what we still get to learn in the future. Having a lot to learn, I think, is much more exciting than already knowing it all.

Ceres–Either the Most or Second-Most Popular Dwarf Planet

It has been nearly a decade since the International Astronomical Union (IAU) formally defined the word ‘planet’, resulting in the reclassification of Pluto as a ‘dwarf planet’. Some people still remain upset about the decision, considering the new classification as a demotion. If you roll with the kinds of people that I do, battle-lines have been drawn around the issue and many a friendship have been lost in the process. I don’t want to rekindle those debates (this is likely inevitable, however, as Pluto will be in the news quite a bit in the coming months as New Horizons is finally about to have its encounter with the dwa… whatever-you-want-to-call-it), so let’s take a look at a dwarf planet that appears to have finally found comfort in its classification: Ceres.

Color view of Ceres as imaged by Hubble in 2004 - Credit: NASA, ESA, J. Parker (Southwest Research Institute), P. Thomas (Cornell University), L. McFadden (University of Maryland, College Park), and M. Mutchler and Z. Levay (STScI)

Color view of Ceres as imaged by Hubble in 2004 – Credit: NASA, ESA, J. Parker (Southwest Research Institute), P. Thomas (Cornell University), L. McFadden (University of Maryland, College Park), and M. Mutchler and Z. Levay (STScI)

If you thought Pluto’s designation was complicated and controversial, just wait until you Ceres’s story.

Ceres has had a bit of an identity crisis of its own. Italian astronomer Giuseppe Piazzi discovered Ceres on New Years Day, 1801. He at first thought it was a star, but observed its movements against the stellar backdrop over the course of a few days and determined it to be a planet. He took a conservative approach in his announcement however, by referring to it as a comet.

I have announced this star as a comet, but since it shows no nebulosity, and moreover, since it had a slow and rather uniform motion, I surmise that it could be something better than a comet. However, I would not by any means advance publicly this conjecture. – Giuseppe Piazzi in a letter to fellow Italian astronomer Barnaba Oriani

With the help of other astronomers and using a method for calculating orbits developed by Carl Friedrich Gauss, it was confirmed that the object was not a comet, but in fact some sort of small planet. German astronomer Johann Bode had been promoting his hypothesis that planets orbited their host stars at distances that could predicted by mathematics. This hypothesis predicted a planet should exist between Mars and Jupiter. When Bode heard news of Piazzi’s discovery of an object at precisely that location, he rushed to announce that the missing planet had been located and even went as far as to name it himself. The name he gave: Juno. Piazzi, however, had taken the liberty as the new planet’s discoverer to give it the name ‘Ceres Ferdinandea’, honoring the patron goddess of Sicily and King Ferdinand of Bourbon. Piazzi rightfully objected to Bode’s stake on naming rights:

“If the Germans think they have the right to name somebody else’s discoveries they can call my new star the way they like: as for me I will always keep it the name of Cerere and I will be very obliged if you and your colleagues will do the same.” Piazzi in a letter to prominent astronomer and editor of scientific journals, Franz Xaver von Zach.

Piazzi’s name ultimately won out, though it was shortened to its currently-accepted name: Ceres.

"Giuseppe Piazzi" by F. Bordiga - Image from Smithsonian Institute Library

“Giuseppe Piazzi” by F. Bordiga – Image from Smithsonian Institute Library

After more objects were discovered orbiting in the same area, Sir William Herschel, in 1802, labeled these new objects, including Ceres, as asteroids (though the term asteroid, which means “star-like”, wasn’t commonly accepted until the early 1900s).

So thus, Ceres became the first, and largest, of the asteroids that orbit between Mars and Jupiter in a loose collection that we collectively refer to as the asteroid belt. But Ceres’s identity crisis wasn’t over just yet. Ceres was king of the asteroids until 2006, when that controversial IAU reclassified it as a dwarf planet. 1

From star, to comet, to planet, to asteroid, and finally to dwarf planet, Ceres looks to Pluto and remarks, “Psh… and you think you had it bad.”

Now that this introduction is out of the way, stay tuned for more information about Ceres. I’ll tell you about this fascinating world and get you up to speed on NASA’s Dawn spacecraft that will be arriving at Ceres in March of this year.

Animation of Ceres as viewed by the Dawn spacecraft on January 13, 2015. - Source: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA/PSI

Animation of Ceres as viewed by the Dawn spacecraft on January 13, 2015. – Source: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA/PSI

(Much of the information in this post came from Giuseppe Piazzi and the Discovery of Ceres, G. Foderà Serio, A. Manara, and P. Sicoli, published in Asteroids III by the University of Arizona Press)


  1. Since Pluto’s reclassification from planet to dwarf planet was viewed by many as a demotion, I wonder if it’s safe to refer to Ceres’s reclassification from asteroid to dwarf planet as a promotion.