Paris, 26 February 1998

European astronomers discover planet formation around dying star

Astronomers at the universities of Amsterdam, Louvain, Groningen and Utrecht have found proof that planets can form around old, dying stars. In the vicinity of the Red Rectangle, an old binary star in the Monoceros constellation, they have detected a ring of matter constituting the first stage of planet formation. Their results will be published in Nature on 26 February. It had previously been assumed that planets can form only round new-born stars.

Young stars are frequently surrounded by a flattened ring of gas and particles left over from the star formation process. The particles can join together to form larger and larger pieces that eventually grow to the size of planets. Observations made using the Dutch-German short wavelength spectrometer (SWS) on board the Infrared Space Observatory (ISO), a mission of the European Space Agency (ESA), have shown that such rings are rich in silicates. The particular form and composition of the silicate, crystalline olivine, occurs on the Earth and in comets, which are the remnants of abortive planet formation. It thus appears to be an important constituent in the formation of planets.

From SWS observations it now appears that the ring of matter around the Red Rectangle binary contains large particles with a great deal of crystalline olivine. The Red Rectangle consists of two old neighbouring stars, one of which was a red giant and is now in the process of becoming a white dwarf. This star is thus very old. The olivine is of the same type as that found in the rings round young new-born stars, on the Earth and in comets. It is the first time that this formative constituent of planets has been found in the ring round an old, dying star. Towards the end of their lives, stars of the same type as our sun swell up to form red giants and emit large quantities of gas and matter. Most of this matter escapes the star's gravity and is lost in space. But if the dying star is part of a binary system, its companion may prevent the gas and material from escaping. This will give rise to a stable, flat ring around the double star, which may remain in existence for a considerable time. In this ring, planet formation may occur.

Illustration : The illustration shows two infrared images of the Red Rectangle. The left-hand picture shows the cloud of gas and matter emitted by the red giant (Dr H. van Wickel, Louvain). The right-hand picture shows a detail of the cloud nucleus (Prof. G. Weigelt and Dr R. Osterbart, Bonn), in which the ring can be seen as a dark band. Above each picture is the spectrum made with the SWS. It exhibits a number of conspicuous peaks appearing as stripes, which are caused by the presence of crystalline olivine.

The illustration can be viewed on

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