Why Some Huge Planets Hug Stars So Tight

Stars formin cloudy nebulas and, shortly after genesis, consume most of the gas of theirbirthplace and use the surrounding dust and leftover gas to form planets, accordingto standard theory.

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

A new modelsuggests magnetic instabilities in the disk cause gas to fall onto the star andalso helps drag young planets into their final orbits.

?Astronomersobserve gas crashing down upon the surfaces of young stars by virtue of theultraviolet radiation they emit, but a way to transport this gas from the diskto the star has not been convincingly specified,? said study team member EugeneChiang at the University of California, Berkeley.

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

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

Chianglikens the magnetic field lines to rubber bands binding the inner and outer gasrings together. Because the inner ring rotates faster than the outer one, themagnetic field ?rubber bands? stretch in the direction of the rotation.

?What doesthat do? It pulls back on the inner ring and speeds up the outer one,? Chiang toldSPACE.com. This acts to slow down the inner ring, causing it to losemomentum and spiral inward to crash onto the star.

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

The stellarwind of youngstars blows dust out of the transitional disk regions, but has no effect ongas. The magnetic instability the researchers are hypothesizing only works ifthe spinning gas has sufficient electrical charge. Dust tends to absorb chargesand reduce electrical conductivity.

Because theinflowing gas drags embedded objects, including young planets, along with ittoward the star, the new model also has implications for planetformation. HotJupiters are giant gas planets that orbit closer to their parent stars thanMercury does to our sun and as a result have extremely high surfacetemperatures.

The newmodel suggests that planets riding the wave of inflowing gas toward the innerregion of their solar systems can be halted by magnetic instabilities in theimmediate vicinity of the star.

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

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