Why Some Huge Planets Hug Stars So Tight

Stars form in cloudy nebulas and, shortly after genesis, consume most of the gas of their birthplace and use the surrounding dust and leftover gas to form planets, according to standard theory.

The gas and dust collapse into a rotating ?circumstellar? disk and are drawn toward the star. Planets are thought to sometimes migrate inward after birth, too. But scientists don?t yet know what drives the inward spiraling motion.

A new model suggests magnetic instabilities in the disk cause gas to fall onto the star and also helps drag young planets into their final orbits.

?Astronomers observe gas crashing down upon the surfaces of young stars by virtue of the ultraviolet radiation they emit, but a way to transport this gas from the disk to the star has not been convincingly specified,? said study team member Eugene Chiang at the University of California, Berkeley.

The new model, detailed online in the June 8 issue of the journal Nature Physics, could also help explain why some planets outside our solar system orbit so close to their parent stars.

The magnetic instability arises from the fact that gas in the circumstellar disk orbits at different speeds depending on its distance from the star. Radiating throughout the disk like spokes on a bicycle wheel are magnetic field lines.

Chiang likens the magnetic field lines to rubber bands binding the inner and outer gas rings together. Because the inner ring rotates faster than the outer one, the magnetic field ?rubber bands? stretch in the direction of the rotation.

?What does that do? It pulls back on the inner ring and speeds up the outer one,? Chiang told SPACE.com. This acts to slow down the inner ring, causing it to lose momentum and spiral inward to crash onto the star.

Chiang and coauthor Ruth Murray-Clay, also of UC Berkeley, say that recently observed ?transitional disks??gaps in the circumstellar disk that are free of dust?around young stars support their model.

The stellar wind of young stars blows dust out of the transitional disk regions, but has no effect on gas. The magnetic instability the researchers are hypothesizing only works if the spinning gas has sufficient electrical charge. Dust tends to absorb charges and reduce electrical conductivity.

Because the inflowing gas drags embedded objects, including young planets, along with it toward the star, the new model also has implications for planet formation. Hot Jupiters are giant gas planets that orbit closer to their parent stars than Mercury does to our sun and as a result have extremely high surface temperatures.

The new model suggests that planets riding the wave of inflowing gas toward the inner region of their solar systems can be halted by magnetic instabilities in the immediate vicinity of the star.

?Once disrupted, disk gas can no longer drag the planets inward,? Chiang said.

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