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The Forefront of Space Science

Mysteries of Comets and the Solar System revealed by the Deep Impact mission
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Significance of observation results

Figure 3-A

Figure 3-B
Figure 3. Change in our view on the origin and evolution of the long- and short-period comets. In the previous standard theory (A), it was thought that the short-period comets formed in the outer solar system. However, recent theoretical studies and the results of the Deep Impact mission suggest that short-period comets formed in much more inner portion of the solar system than previously thought.

The newly discovered fact that the internal substances of Tempel 1, a short-period comet, are very similar to those of the long-period comets provides a very important clue to the understanding of the origin of comets. This suggests the possibility that comets from Oort cloud and those from Kuiper belt are basically made of the same substances. When this result is compared with the recent progress in the solar-system formation theory, this possibility begins to show significant implications.
Until the 1990s,the standard picture of the Oort cloud and Kuiper belt was the following. it was thought that the Oort Cloud was created around Uranus and Neptune, much closer to the Sun than its current locations, and that it was thrown to its current position several thousands to several tens of thousands AU distance away because of the influence of the gravity scattering of Uranus and Neptune. Kuiper-belt objects were thought to have formed outside Uranus and Neptune and stayed there as minor bodies quietly without being involved in the formation of such giant planets and have hardly changed their orbits since 4.5 billion years ago (Fig. 3A).

During the last several years, however, theoretical computation for planet formation of the outer solar system has progressed greatly and a new model is proposed. According to the new theory, Kuiper-belt objects are, contrary to the conventional theory, not only objects that go along stable circular orbits without much orbit changes since their formation in the outer region of the solar system (i.e., main belt objects). The theory also predicts that there are many objects (i.e., scattered-disk objects) that, although they formed in an inner part of the solar system as with Oort cloud objects, were thrown into the Kuiper-belt region because of influence of gravity scattering of such giant planets. such as Uranus and Neptune. In addition, the theory indicates that most objects approaching the Earth as short-period comets are in fact scattered-disk objects, and predicts that short-period comets are in fact made of the identical material to Oort-cloud objects (Fig. 3B).

Though I cannot discuss in detail, owing to the limitation of space, if the new theory is correct, Saturn, Uranus, and Neptune must have changed their orbital positions greatly only once during the 4.5 billion year history and moved far out in the solar system than their original location at formation. Furthermore, the rapid positional change of these giant planets must have strongly destabilized the orbits of minor bodies. As a result, this orbital change most likely caused a large-scale meteorite shower on the solar system’s inner planets, such as the Earth. This new view overturns the image of a previous static solar system based on the conventional planetary formation theory and proposes a very dynamic image.

So far, no physical evidence to support the new planet-formation theory has been found. However, the old theory cannot explain the new facts presented by the ground observations of the Deep Impact mission. Furthermore, the new facts are very consistent with the predictions by the new theory. In other words, the observation results this time strongly support the new planet-formation theory.

We observed only one comet, however. Further observations of other short-period comets are needed to reach a conclusion on the evolution of the entire planet system. In addition, we have to observe the main-belt objects of the Kuiper-belt that are believed not to have fallen into the inner solar system. Fortunately, ESA’s Rosetta spacecraft was launched for the former and NASA’s New Horizon spacecraft was launched for the latter. We expect that our understanding of the origin of our solar system will advance greatly in a several years when we can receive observation data from these spacecrafts.

The details of our research results were published in “Science” (October 14, 2005 edition).

(Seiji SUGITA)

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