51 Ophiuchi

51 Ophiuchi (51 Oph) is a star located approximately 410 light years away in the constellation Ophiuchus, northwest of the center of the Milky Way. It is notable for being "a rare, nearby example of a young planetary system just entering the last phase of planet formation". There is uncertainty about the stellar classification of this star. It has the nominal classification of B9.5IIIe, a B-type giant star with emission lines. However, it has also been classified as an A0 II-IIIe star and as a Herbig Ae/Be star.

51 Ophiuchi has a disk of dust and gas that appears to be a young debris disk and is probably a planetary system in the late stages of formation. This system resembles Beta Pictoris, a well known star with a large debris disk, in several ways: spectral type, the presence of an edge-on disk with both gas and dust, and the presence of variable blue-shifted absorption lines suggesting in-falling comets.

The distance to 51 Ophiuchi is much greater than the distance to Beta Pictoris, and its debris disk is relatively compact. As a consequence, the disk around 51 Ophiuchi requires an interferometer to resolve, in contrast to that of Beta Pictoris, which has been observed using visual spectrum imaging. Recent observations of 51 Ophiuchi made with the Keck Interferometer Nuller at the W. M. Keck Observatory show that the disk has two components: a central cloud of large particles (exozodiacal dust) surrounded by a much larger cloud of small silicate particles extending to about 1,000 astronomical units. The inner disk has a radius approximately four times the distance between the sun and the Earth, with a density of around 100,000 times that of the dust in the Solar System.