70 Ophiuchi: a 1855 puzzle 16 light-years away, and still no gas giant
· BehindLines newsroom
A giant planet suspected since 1855 probably does not exist. A team led by the University of Michigan reviewed a century of measurements of 70 Ophiuchi AB, a pair of stars about 16 light-years away, and found no body with the mass of Jupiter or more in the regions probed. According to ScienceDaily, which relays the university's release, smaller worlds remain possible.
A quiet neighbour
By both sources' account, 70 Ophiuchi AB is the third-closest binary system to Earth, after Alpha Centauri (4.2 light-years) and 61 Cygni (11.4 light-years). Roughly half of the star systems in the Milky Way contain at least two stars. In 1855, the British astronomer William Jacob proposed a planet there, the first official exoplanet candidate in such a system; the idea did not hold up.
What the team did
The researchers combined historical radial-velocity measurements with new observations from the 6.5-metre Magellan Clay Telescope (Las Campanas Observatory, Chile) and the 2.7-metre Harlan J. Smith Telescope (McDonald Observatory, Texas). The work, led by postdoctoral researcher Yiting Li, appears in The Astrophysical Journal; Michael Meyer, chair of the astronomy department, is senior co-author. The team spans more than a dozen institutions, including NASA's Jet Propulsion Laboratory and IPAC at Caltech.
Results and their limits
No planet of Jupiter's mass or more shows up within 5 astronomical units of star A, nor within 0.5 astronomical units of star B. The radial-velocity "wobble" that might have betrayed a giant is better explained by activity on the stars' surfaces. Yiting Li stresses that the study sets "an upper limit, not a ban": Saturn-sized or rocky planets, including in the habitable zone, are not excluded, and that alone would not prove life. Michael Meyer sees the limit as a way to design better searches; the university takes part in studying the SHERA space mission, which targets binary systems, 70 Ophiuchi AB among them.