Organic ring around nearby star

Astronomers have known since 1991 that a ring of dust, a likely vestige of planet formation, surrounds the nearby star HR 4796A. Researchers now report the first evidence that the ring contains complex organic molecules.

Alycia Weinberger of the Carnegie Institution of Washington (D.C.) and her colleagues observed the ring with a near-infrared camera on the Hubble Space Telescope. Comparing those images with Hubble images made in visible light, the team showed that the dust scattered much more infrared light than visible light. And that’s the same pattern displayed by a group of organic compounds called tholins.

Astronomers have found tholins in comets and the atmosphere of Saturn’s moon Titan, but the new study provides the first strong evidence for the material outside the solar system, Weinberger’s team says in the Feb. 1 Astrophysical Journal Letters.

A youthful 8 million years old, HR 4796A lies 220 light-years from Earth. Astronomers believe the star’s narrow ring, which has a radius nearly twice that of Pluto’s average distance from the sun, may be left over from a planet-making disk of gas and dust particles. Tholins may coat comets or other ice particles that lie within the chilly confines of the ring, the team suggests.

No one knows whether the star has planets. But if it does, comets from the ring could pepper inner orbs with organic compounds in much the same way that comets are thought to have delivered these building blocks of life to the primitive Earth, says Weinberger.

Bruce Macintosh of the Lawrence Livermore (Calif.) National Laboratory says tholins “are the most likely” explanation for the disk’s color. Observations at a longer infrared wavelength, taken by Macintosh and his colleagues with the Keck II telescope on Hawaii’s Mauna Kea, may shed further light on whether tholins are present.

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