70 Ophiuchi

Lore & Background

In Ptolemy's 2nd-century Almagest, this star system is listed as a 4th magnitude star, the 28th (or 4th outside the constellation figure) in Ophiuchus, and is star No. 261. It was first catalogued as a binary by William Herschel in the late 18th century, who proved it is a gravitationally bound system where the two stars orbit a common center of mass—an important contribution to showing that Newton's law of universal gravitation applies beyond the Solar System. The star was once part of the obsolete constellation Taurus Poniatovii, but after the International Astronomical Union officially recognized constellations, it was placed in Ophiuchus. In Chinese astronomy, the asterism Zōng Rén (宗人, Official of Religious Ceremonies) consists of 66, 67, 68, and 70 Ophiuchi; 70 Ophiuchi itself is called Zōng Rén sì (宗人四), the fourth star of Zōng Rén.

70 Ophiuchi is a variable star with a combined magnitude range of 4.00 to 4.03. The type of variability is uncertain, and it is not clear which component causes the variations. It has been suspected of being either a BY Draconis variable or an RS Canum Venaticorum variable, although the latter would require a close-in companion that has not been detected. It is suspected that 70 Ophiuchi B is the variable star, with a measured period of 1.92396 days.

The primary star is a yellow-orange main sequence dwarf of spectral type K0, while the secondary is an orange dwarf of spectral type K4. They orbit each other with a semi-major axis of 23.2 AU, but because the orbit is highly elliptical (e=0.499), the separation varies from 11.4 to 34.8 AU, with one orbit taking 88.38 years.

Reader's Guide

70 Ophiuchi holds a significant place in the history of exoplanet detection. In 1855, William Stephen Jacob of the Madras Observatory claimed that the binary's orbit showed an anomaly, making it 'highly probable' that a 'planetary body' existed in the system—the first known attempt to use astrometric methods to detect an exoplanet, though Friedrich Bessel had applied similar methods 10 years earlier to deduce Sirius B. T. J. J. See made a stronger claim for a dark companion in 1899, but Forest Ray Moulton soon proved that a three-body system with the specified orbital parameters would be highly unstable. Both claims were later shown to be erroneous. In 1932, Louis Berman announced a 'third dark companion' around 70 Oph A with an 18-year period and mass 0.1–0.2 solar masses; in 1943, Dirk Reuyl and Erik Holberg claimed a planetary companion with mass 0.008–0.012 solar masses and a 17-year period, causing a sensation, but later observations discredited it. Negative results do not completely rule out planets: a 2006 McDonald Observatory team set limits on planets with masses 0.46–12.8 Jupiter masses and separations 0.05–5.2 AU. Radial velocity observations have detected no exoplanets around either star; for 70 Oph A, Jupiter-mass planets interior to 5 AU are excluded, while for 70 Oph B, planets more massive than 0.25–0.3 Jupiter masses within 0.5 AU are excluded. Around 70 Ophiuchi A, planets with a mass similar to Saturn or less within the habitable zone are still possible.

Did You Know?

• 70 Ophiuchi was first catalogued as a binary star by William Herschel in the late 18th century.
• In 1855, William Stephen Jacob made the first known attempt to detect an exoplanet using astrometric methods, claiming a planetary body in this system.
• The two stars vary in separation from 11.4 to 34.8 AU due to an orbital eccentricity of 0.499.
• In Chinese astronomy, 70 Ophiuchi is the fourth star of the asterism Zōng Rén (宗人).