Old Parents for New Planets?

Astronomers have discovered the ingredients of planets in an unusual locale: the dusty disk surrounding an ancient binary star system. The finding, reported in tomorrow's Nature, is a surprise, because most planets are thought to form near young stars.

In the standard picture of planet birth, a huge cloud of dust and gas collapses. The result is a newborn star surrounded by a planet-forming disk that is rich in dust particles containing iron, silicon, oxygen, and other elements--the same elements that make up the planets in our solar system. But a Dutch and Belgian team observing with the European Space Agency's Infrared Space Observatory (ISO), a satellite carrying an infrared telescope that can measure the size and composition of dust grains, has found such a dust disk in an unusual place.

When the researchers focused ISO's instruments on a gas and dust cloud called the Red Rectangle, located 1000 light-years away, they found dust grains about 10 micrometers in diameter that may be the makings of future planets. The chemical signatures suggested tiny crystals of olivine, a mineral widespread on Earth that contains the elements magnesium, iron, silicon, and oxygen. This find was surprising, because the Red Rectangle is not near a young star. Instead, it contains gas and dust blown off by a couple of elderly stars.

One of the stars at its center is a red giant, which has a relatively cold surface where molecules formed in the star's outer layers can collect into dust particles, says Rens Waters of the University of Amsterdam in the Netherlands. Once the dust escapes into the nebula, says Waters, "we believe that these particles are now increasingly sticking together," a process that could ultimately lead to the formation of planets. "Here we have a clue to planet formation that is very different," says Michel Guélin of the Institute for Millimeter Radioastronomy near Grenoble, France.

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