Stellar activity, not a giant planet, explains nearby wobble

For years, observations of 70 Oph AB revealed a wobble that could have indicated a Jupiter-sized planet. The new analysis instead traced it to variations in stellar surface activity over time.

The team combined roughly a century of historical measurements with observations from 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.

More than a dozen institutions contributed, including NASA's Jet Propulsion Laboratory and IPAC at the California Institute of Technology. The analysis separated the apparent planetary signal from changes occurring on the stars.

Michael Meyer, a senior co-author and chair of Michigan's astronomy department, said: “It's a cautionary tale about how stellar activity can mimic planet signals using the radial velocity.” He also praised Li for assembling an exceptionally large worldwide data set and testing possible explanations.

Takeaway: The wobble came from the stars, so it was not evidence of a giant planet.

Which planets did the survey exclude?

• 70 Oph A: The survey excluded planets with at least Jupiter's mass within 5 astronomical units.
• 70 Oph B: It excluded them within 0.5 astronomical unit.

An astronomical unit, or AU, is the average distance between Earth and the Sun, about 93 million miles.

The researchers achieved the most sensitive modern search for giant planets in this particular system, using an approach not designed to detect smaller worlds.

Eliminating possibilities can make later searches more efficient because observers can focus on planets that remain within reach. Meyer said the new limit enables teams to design optimal search strategies.

Takeaway: The survey rules out giant planets in two specific zones, not across the entire system.

Could habitable planets still be there?

Planets as small as Saturn or smaller could orbit either star. Some could be rocky worlds inside the habitable zone, the region around a star where temperatures could permit liquid water on a planet's surface.

Such a world would also need suitable atmospheric conditions. Because liquid water is essential to life as we know it, these targets attract special attention from astronomers searching for life beyond Earth.

A habitable-zone planet would not prove that life exists there. Current observations also cannot yet detect or characterize some smaller candidates, so excluding giant planets does not settle whether 70 Oph AB hosts small worlds.

Takeaway: Small rocky planets can still fit the observations.

Where will observatories look next?

The University of Michigan is part of a team studying the SHERA space mission, which focuses on binary stars. 70 Oph AB is one of a handful of systems targeted for study.

Researchers also expect 70 Oph AB to attract attention from flagship missions, including NASA's Habitable World Observatory. Such observatories could detect and characterize smaller planets that current measurements cannot.

NASA provided partial support for the study.

Takeaway: Better limits should let future telescopes spend less time on excluded possibilities.

What has the study settled?

Li said the study sets an upper limit, not a ban. Given how common planets are in the universe and Milky Way, she expects researchers to find others in the system.

More broadly, stars are not perfectly stable, so their activity can imitate the signal produced by an orbiting planet. Astronomers must separate those effects before announcing a discovery, particularly for smaller rocky worlds.

The 32-author paper, “A Century of Radial-velocity and Astrometric Monitoring of 70 Oph AB: New PFS Data and Constraints on Possible Planetary Companions,” appeared in The Astrophysical Journal, volume 1009, issue 2, article 199 (2026), DOI 10.3847/1538-4357/ae9f52. The Heising-Simons Foundation supplied additional funding.

Takeaway: This study narrows the possibilities rather than closing the search.