70 Ophiuchi – a Binary Star System Located 16.6 Light-years Away from Earth

70 Ophiuchi system is a conjectured planetary system orbiting both stars of the binary system. It is a binary star system located 16.6 light-years away from the Earth. This binary system is located about 16.6 light-years away in the northeastern part of Constellation Ophiuchus, the Serpent Holder – east of Muliphen (Gamma Ophiuchi). It is in the constellation Ophiuchus. At magnitude 4 it appears like a dim star visible to the unaided eye away from city lights.


This star system was first cataloged as a binary star 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. Both components are K-type stars that are smaller, cooler, and dimmer than our Sun. The components orbit around the center of mass in 83.4 days at a separation of 23.2 AU and vary from 11.4 to 34.8 AU.

Binary star systems

  • 70 Ophiuchi Ab – It is the innermost planet of the primary star with an orbital period of 59.4 days. Its gravitational field is very similar in strength to Earth’s at 102.9%.
  • 70 Ophiuchi Ac – It is a planet similar in size to Ab but its mass is a little bit lower. It takes 141.6 days to orbit once around the star at distances varying from 0.30 to 0.73 AU. The surface temp is 132°F.
  • 70 Ophiuchi Ad – It is a sub-Saturn mass planet that takes a year and a half to revolve once around the star. The orbital distance varies from 0.93 AU to 1.65 AU.
  • 70 Ophiuchi Ae – It is a sub-Saturn mass planet taking 3.4 years to revolve once around the star and takes 161⁄2 to rotate once in the same direction as its orbit.
  • 70 Ophiuchi Bb – It is a Mercury-like planet in terms of size and orbit. The surface temperature is 65°F, nearly matching with Earth’s 59°F.
  • 70 Ophiuchi Bc – It is a planet bigger and more massive than Earth. 70 Oph Bc orbits at 0.63 AU with a circular orbit, revolving around the star once every seven months.
  • 70 Ophiuchi Bd – It is a super-Earth 7.7 times more massive than Earth. Its density is lower than Earth’s but its surface gravity is stronger than Earth’s.
  • 70 Ophiuchi Be – It is the outermost planet in this system. It is a gaseous planet massing 20 Earth masses and sizing over 41⁄2 times that. It takes 31.2 days to complete one rotation.


70 Ophiuchi is a multiple hypergiant star in the constellation of Ophiuchus. It is a variable star with a magnitude range for the two stars combined of 4.00 to 4.03. It is located 16.6 light-years away in the constellation Ophiuchus in the caelregio Tarandus. The type of variability is uncertain and it is not clear which of the two components causes the variations. It has been suspected of being either a BY Draconis variable or an RS Canum Venaticorum variable, and a period of 1.92396 days has been measured. 70 Ophiuchi is situated close to the celestial equator, as such, it is at least partly visible from both hemispheres at certain times of the year.

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