Do Giant Planets Form Quickly Or Slowly?

May 14, 1998

After decades of intensive searching, astronomers have found the first giant planets orbiting around other stars like the Sun. We now know that giant planets similar to Jupiter are not rare in the universe, and exist around a number of stars in the Sun's neighborhood of the galaxy. However, all of these planets are thought to be about as old as the stars around which they orbit, typically billions of years old, like the Sun. These planets are middle-aged, then, as planets go. In order to find out what these planets were like when they were infants, and in particular to learn what processes led to their formation, astronomers need to search for them around much younger stars.

In a paper published in the May 14 issue of Nature, Alan Boss of the Carnegie Institution of Washington explains that by looking for wobbles in young stars, a clear-cut test can be applied that will determine the mechanism through which giant planets form out of the disks of gas and dust that swirl around the youngest stars. Once a giant planet is formed in orbit around a young star, the star will wobble back and forth as the star and planet orbit around their common center. A wobbling star, therefore, implies the presence of an otherwise unseen planetary companion.

There are two ways in which giant planets are thought to have formed. One mechanism is very slow, and requires about a million years for a massive solid core to form from collisions between smaller ice and rock bodies, followed by about 10 million years during which the solid core gains a gaseous atmosphere from the disk and grows to its final, Jovian size. Boss has found that the second mechanism is much faster, and requires only about a thousand years for the disk itself to clump up directly into a large ball of gas and dust that will form the giant planet.

If giant planets form by the solid-core mechanism, then young stars will not begin to wobble until they are no longer adolescents, i.e., until after they are 10 million years old or older. However, if giant planets form by the disk-clumping mechanism, then even the very youngest stars will wobble. In fact, Boss shows, the youngest stars will begin to wobble appreciably even during the process where the disk begins to form clumps. NASA's planned Space Interferometry Mission (for launch in around 2005) will have the power to detect easily these tiny, infantile motions. By looking closely at a large sample of young stars in nearby star-forming regions (e.g., in the Taurus constellation), astronomers will soon be able to find the clues that will solve the puzzle of how giant planets form.

Alan Boss is a staff member at the Carnegie Institution of Washington's Department of Terrestrial Magnetism (DTM) in northwest Washington, D.C. DTM, led by Sean C. Solomon, is one of the institution's five research centers. In addition to astronomy, Carnegie conducts research in the earth sciences, plant biology, and developmental biology. The institution's administration building, housing the office of president Maxine F. Singer, is in Washington, D.C..
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Carnegie Institution for Science

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