Stellar Activity, Not a Giant Planet, Explains 70 Ophiuchi's 170-Year-Old Wobble

In 1855, a British astronomer named William Jacob proposed that a planet circled one of the stars in 70 Ophiuchi, a pair of stars about 16 light-years from Earth. That made it the first star system with an officially proposed exoplanet candidate. His claim was later rejected. Now, 171 years after he made it, a University of Michigan-led team has explained the signal that kept the idea alive: it came from the stars themselves, not from a giant planet.

The system, known as 70 Oph AB, is a binary in which two stars orbit each other. The researchers searched it for planets as massive as Jupiter or larger and found none in the regions they examined. Around 70 Oph A, they excluded such planets within 5 astronomical units. Around 70 Oph B, they excluded them within half an astronomical unit. An astronomical unit is the average distance between Earth and the Sun, about 93 million miles.

The long-standing puzzle involved radial velocity, a measure of how fast a star moves toward or away from us. A planet's gravity tugs its star slightly back and forth, producing a wobble that shows up in the star's light. For years, the wobble in 70 Oph AB looked like what a Jupiter-sized planet might cause. Measurements over the past 171 years had never fully eliminated that possibility in the system's inner regions.

To test it, the team, led by postdoctoral researcher Yiting Li, combined roughly a century of historical measurements with new observations from two telescopes: the 6.5-meter Magellan Clay Telescope at Las Campanas Observatory in Chile and the 2.7-meter Harlan J. Smith Telescope at McDonald Observatory in Texas. The analysis was sensitive enough to separate the apparent planetary signal from changes on the stars. The team concluded that variations in stellar surface activity over time produced the wobble. Scientists from more than a dozen institutions took part, including NASA's Jet Propulsion Laboratory and IPAC at Caltech. NASA supported the work in part, and the Heising-Simons Foundation provided additional funding. The results appear in The Astrophysical Journal.

Michael Meyer, a senior co-author and chair of Michigan's astronomy department, said Li assembled an enormous amount of data from around the world and carefully explored every possibility. He called the result a cautionary tale about how stellar activity can mimic planet signals in radial velocity data. The study also pins down the orbits of 70 Oph A and 70 Oph B as some of the most precisely understood stellar orbits in the sky.

The result does not make the system empty. The team's approach was built to find giant planets, not small ones, so Saturn-sized and smaller worlds remain possible, including rocky planets in the habitable zone, where temperatures could allow liquid water on a surface if the atmosphere is suitable. Finding such a planet would not by itself show that life exists there. Li described the finding as an upper limit, not a ban, and said that given how common planets are in the Milky Way, she would wager other planets are there. She called the result a mix of disappointment and excitement.

The system matters partly because of where it sits. Roughly half of all stellar systems in the Milky Way hold at least two stars, and 70 Oph AB is the third closest binary to Earth, after Alpha Centauri at 4.2 light-years and 61 Cygni at 11.4.

Meyer said the new limit helps in designing future search strategies. Michigan is part of a team studying a space mission called SHERA, which would focus on binaries, with 70 Oph AB among a handful of targets. He expects the system to be a target for flagship missions such as NASA's Habitable World Observatory, which could study smaller planets that current instruments cannot yet detect.