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Planetary News: Asteroids and Comets (2005)Ceres: An Embryonic World?By Amir AlexanderSeptember 13, 2005
Only weeks after Michael Brown and his associates announced the discovery of 2003UB313, the so-called “10th planet,” astronomers’ attention has been drawn to a much older addition to our Solar System. Ceres, the largest object in the Asteroid Belt, known for over 200 years, appears to be a far more complex and planet-like object than had been previously imagined. In an article published on September 8 in the journal Nature, authors Peter C. Thomas of Cornell, Joel Parker, Alan Stern, and E. F. Young of the Southwest Research Institute, Lucy McFadden of the University of Maryland, C. T. Russell of UCLA, and M. V. Sykes of the Planetary Science Institute suggest that Ceres might not be the plain space-rock previously suspected, but rather a tiny world all its own. Ceres was the first object found in the Solar System (apart from the Sun) which was not itself a planet. Discovered by Italian astronomer Giuseppe Piazzi in 1801 using a telescope located atop the Royal Palace in Palermo, Sicily, it was soon recognized as too small to be considered a “proper” planet. Its diameter (since measured) is around 480 kilometers, giving it a surface area about the size of the state of Texas. When in subsequent years other objects like it, though smaller, were discovered in the same region of space, astronomers realized that Ceres was not alone. It was, rather, the "big brother" in a family that includes thousands of space rocks, all orbiting the Sun between Mars and Jupiter. It became known as the “Asteroid Belt.” The presence of the Asteroid Belt in the wide gap between Mars and Jupiter had puzzled previous generations of astronomers. Many hypothesized that the rocky debris is all that remains of an ancient planet that came to a catastrophic end. Today planetary scientists know that the asteroids are, in fact, a planet in the making that never completed its formation process. When the giant Jupiter was formed 4.6 billion years ago, its enormous gravity cleared the region around it from most of the rocky debris that would normally accrete to form a planet. The asteroids are what remains of the far denser ancient debris field, and they orbit just outside the gravitational field of their giant neighbor. In themselves, however, they lack the necessary mass to form into a single viable planet. Because of their unique history, planetary scientists view the asteroids as fossils, left largely unchanged from the earliest days of the Solar System. “The asteroid belt,” write Peter Thomas and his collaborators, “preserves a record of both this earliest epoch of Solar System formation and variation of conditions within the solar nebula.” It is therefore a unique source of information about the history and development of the Solar System. Viewed in these terms, planetary scientists did not generally consider Ceres to be a promising object of study. The asteroid Vesta, only slightly less massive than Ceres, and discovered just 6 years after it, has a clearly layered structure, an external basaltic coat of volcanic rock and sharp topographical features. Ceres, by contrast, appears to be of low density and devoid of any obvious external features. Scientists have therefore long hypothesized that Ceres is a homogeneous undifferentiated mass, similar to the numerous small carbonaceous meteorites that populate the Asteroid Belt. The Nature article, however, sharply challenges this view, and suggests that the largest asteroid may in fact be a far more complex object than previously suspected. In December 2003 and January 2004 Thomas and his collaborators observed and recorded the entire rotation of Ceres around its axis using the Hubble Space Telescope. Because Ceres' period is slightly longer than 9 hours, the Hubble required six orbits around the Earth to capture all the phases of the asteroid's’ rotation. When they examined the images, the researchers found that Ceres was an almost perfect spheroid, only slightly bulging along its equatorial plane. The outline of a section of the asteroid, furthermore, turned out to be an almost perfect ellipse. This is significant. Most asteroids are highly irregular in shape, their miniscule gravity being too small to influence their overall outlines. Planets, by contrast, are near spheroids, somewhat flattened at the poles and slightly bulging at the equator. This shape represents the equilibrium between the planet’s gravity pulling inwards, and the internal pressure pushing outwards, with the slight equatorial bulge produced by the planet’s rotation. When it arrives at this equilibrium, a planetary mass is arranged in the optimal way regarding the forces acting upon it, and is therefore considered “relaxed.” Since Ceres’ shape corresponds so closely to this shape, the authors concluded that it is in a “relaxed” state – in fact, the only known “relaxed” asteroid. Since Ceres is relaxed, the authors argue, the precise shape of its section reveals a great deal about its inner structure. Given the best estimate of its mass, if Ceres were an undifferentiated homogeneous object, it would bulge around 40 kilometers at the equator. This means that its equatorial radius would be 40 kilometers greater than the polar radius. The Hubble images, however, show Ceres’ bulge to be significantly smaller – around 30 kilometers. This discrepancy, Thomas and his collaborators argue, can be easily explained if one assumes that Ceres is not homogeneous but layered, with a dense core embedded in a lighter coating. Specifically, a dense rocky core surrounded by a mantle of water ice, covered in turn by a lighter outer crust would give Ceres the very shape the Hubble observed. All this leads to a surprising conclusion: Ceres, far from being a simple homogeneous space rock, resembles nothing so much as the rocky planets of the inner Solar System – Mercury, Venus, Earth, and Mars. Like them it has a dense core within a mantle, which is, in turn, surrounded by an outer shell. “Ceres is an embryonic planet” concludes Lucy McFadden, one of the paper’s authors. Had it not been for the gravitational perturbations from Jupiter billions of years ago, she adds, Ceres would undoubtedly have become a full-fledged planet. Tiny Ceres, in all likelihood, will never be recognized as a planet in its own right. Nevertheless, the planet-like qualities of this seemingly familiar object join with other recent discoveries to confuse a question that once seemed trivial: what is a planet? With giant “planets” orbiting distant stars, Pluto-sized objects orbiting at the edges of our own Solar System, and now familiar asteroids appearing suspiciously planet-like, the simple category of "planet" seems to be under assault from all directions. But however we eventually choose to classify it, there is no doubt that the Ceres, ancient relic of the primordial Solar System, has much to teach us about the origins and development of the planets we know today. |
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