Planet precursors. Green silicate crystals, called olivine, swim in the dusty disk surrounding a brown dwarf, in this artist's conception.

And the Meek Shall Inherit Some Earths

Though it's not quite a star, the humble brown dwarf may still have hidden talents. New evidence suggests that dust clouds orbiting the celestial objects could be breeding grounds for planets, just like those around true stars.

Planet-building begins when the dust grains swirling around young stars stick together and grow crystals. The dust also tends to flatten into a thin disk over time. Crystallization requires high temperatures, which are thought to be supplied by the star's radiation. As a result, astronomers had assumed that brown dwarfs, which weigh roughly between 1 and 9 percent of the Sun and have cores that do not get hot enough to burn hydrogen as normal stars do, are too cool to transform their local dust into planets.

To test this, astronomer Dániel Apai of the University of Arizona and his colleagues used the Spitzer Space Telescope to study a subset of brown dwarfs in a nearby star-forming region, which is 1 million to 3 million years old. In 6 of their 8 targets, the team found infrared emissions indicative of silicate dust grains. They then observed that this dust was growing, crystallizing and settling towards a flatter disk. The results could mean that brown dwarfs can form planets, the researchers report 21 October in Science. "We think if they start, they will finish," Apai says.

Still, there are many uncertainties in how planets form and not all dust grains will become planets, says astronomer Leonardo Testi of the Osservatorio Astrofisico di Arcetri in Florence, Italy. Theory predicts that these pebble-sized objects will tend to destroy each other when they collide, rather than clump together, he says. Moreover, scientists have a hard time tracking such grains because, as they grow larger, they emit longer wavelengths that are increasingly difficult to detect. The possibility that brown dwarfs may harbor planets is intriguing, however, says Testi, especially since these objects constitute some of our nearest neighbors.

Related sites
Apai's home page
California & Carnegie Planet Search

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