70 Ophiuchi

70 Oph B

70 Ophiuchi is a binary star system located 16.6 light years away from the Earth. It is in the constellation Ophiuchus. At magnitude 4 it appears as a dim star visible to the unaided eye away from city lights.

The primary star is a yellow-orange main sequence dwarf BY Draconis variable of spectral type K0, and the secondary star is an orange main sequence dwarf of spectral type K4. The two stars orbit each other at an average distance of 23.2 AUs. But since the orbit is highly elliptical (at e=0.499), the separation between the two varies from 11.4 to 34.8 AUs, with one orbit taking 83.38 years to complete.

This star system was first cataloged by William Herschel in the late 18th century in his study of binary stars. Herschel proved that this system is a gravitationally bound binary system where the two stars orbit around a common center of mass. This was an important contribution to the proof that Newton's law of universal gravitation applied to objects beyond the solar system. He commented at the time that there was a possible third unseen companion affecting the orbit of the two visible stars.

In 1855, William Stephen Jacob of the Madras Observatory claimed that the orbit of the binary showed an anomaly, and it was "highly probable" that there was a "planetary body in connection with this system". This is the first attempt to use radial velocity to detect an exoplanet, and the first based on astrometric evidence.