Origins of Existence by Fred Adams

Author:Fred Adams
Language: eng
Format: epub
Publisher: Simon & Schuster

FORMATION OF PLANETS BY GRAVITY

The formation of planets can also take place through the action of gravity alone. In principle. By itself, gravitational condensation cannot account for the giant planets in our solar system. Gravity is an equal-opportunity force and does not distinguish between gas and rocky particles. Any objects formed solely through the action of gravity would have the same chemical composition as the starting raw material. In our solar system planets formed by gravity would have the same composition as the Sun. But they don’t—our giant planets are enriched in heavy metals. In other solar systems, where giant planets are being found in unexpected places, the formation of planets through the action of gravity remains viable. It also remains possible that our giant planets formed through a gravitational condensation in the early solar nebula, as long as some other mechanism provided the observed enrichment in heavy elements.

Circumstellar disks—the birthplaces of planets—can be unstable if their temperatures are low and their masses are large. Many young stars are accompanied by massive disks, which are susceptible to breaking up. If the star and disk have nearly equal masses, for example, the system lives at a point of dangerous instability. These extreme cases can readily precipitate gravitationally bound bodies containing about one percent of the total. This natural mass scale is about ten times that of Jupiter, more than enough to form a giant planet. The time required for this gravitational condensation to develop is relatively short; a full-fledged secondary body can be produced after several orbits, where each orbit takes ten to one thousand years. This condensation mechanism is much faster than the accumulation process, which can take millions of years. If a disk is sufficiently heavy, then gravity can make large planets at a rapid rate.

The difficulty in this scenario of planet formation, when applied to our solar system, is to understand how the giant planets become enriched with heavy elements and how these metals ended up as rocky cores. Dust grains, the carriers of heavy elements, naturally tend to settle toward the center of a young planet because the grains feel less force from pressure than the surrounding gas. Unfortunately, the time required for the dust to settle greatly exceeds the time it took the planets to form.

Although gravitational instability may not account for the production of planets in our solar system, nothing prevents this mechanism from operating in other solar systems. Indeed, the collection of solar systems associated with other stars is far more diverse than we had ever imagined. Blessed with such wide variations, solar systems throughout the galaxy probably construct their planets through a variety of different methods, all competing with one another. And while our solar system is rather smooth and well ordered, this conservative configuration may prove to be the exception.

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