Young stars have large dust disks, but as they mature these are
usually lost. In 1983, the Infrared Astronomical Satellite (IRAS)
discovered that a normal star (Vega) had a ring of cool dust around
it. Other normal stars have been found to show the same type of cool
dust. These dust discs may be the sites where planets failed to
form, or the remnant, left after planet formation. After the survey
of the sky by IRAS, astronomers have waited for the ISO satellite
(Infrared Space Observatory) to investigate the dust discus in more
detail, since most of the dust discs can be seen only in the
infrared. ISO has been used to investigate the size of the dust
discs are dominated by silicate dust (small grains of sand), whilst
others show the signature of complex organic molecules. The region
around Vega has been mapped. This reveals a feature larger than the
star alone, showing that ISO can resolve the discs around nearby
stars. The dust disc around Vega could be as large as 900 AU across
(the Earth is 1 Astronomical Unit from the Sun).