A mysterious star system just 16 light-years away may be hiding habitable planets

Led by the University of Michigan, the new study examined 70 Ophiuchi AB, also known as 70 Oph AB, a nearby binary star system in which two stars orbit each other. Using an exceptionally sensitive analysis, scientists searched for planets as massive as Jupiter or larger. Instead of finding one, they determined that such planets do not exist within the regions they examined.

That result might initially seem disappointing, but it represents an important advance. The researchers achieved the most sensitive modern search for giant planets in this particular system, using an approach that was not intended to detect smaller worlds. The work, supported in part by NASA, therefore leaves open the possibility that other planets remain undiscovered.

Among those possibilities are planets the size of Saturn or smaller, including rocky worlds that might exist within the system's habitable zone. This is the region around a star where temperatures could allow liquid water to remain on a planet's surface, provided the planet has suitable atmospheric conditions. Because liquid water is essential to life as we know it, such worlds are especially interesting to astronomers searching for life beyond Earth.

A Planetary Mystery Dating Back to 1855

The mystery surrounding 70 Oph AB began long before modern planet hunting. In 1855, British astronomer William Jacob proposed that the system contained a planet, making it the first star system to host an officially proposed exoplanet candidate. His claim was eventually rejected, but the possibility of undiscovered planets continued to intrigue astronomers for generations.

"It's a mix of disappointment and excitement," said Yiting Li, a postdoctoral researcher in the University of Michigan's Department of Astronomy. "We're finding out that there isn't a Jupiter-sized planet, but we're also opening the latest chapter on this historic system."

Unlike our solar system, which has only one star, roughly half of all stellar systems in the Milky Way contain at least two. These multiple star systems offer valuable opportunities to investigate how planets form, change over time, and potentially develop conditions suitable for life.

70 Oph AB is particularly attractive because of its proximity. Located approximately 16 light-years from Earth, it is the third closest binary star system to our planet, following Alpha Centauri (4.2 light-years) and 61 Cygni (11.4 light-years).

The new findings, published in The Astrophysical Journal, also establish the orbits of the system's two stars, 70 Oph A and 70 Oph B, as some of the most precisely understood stellar orbits in the sky.

Future NASA Missions Could Search for Smaller Worlds

Knowing which planets are absent can be just as valuable as discovering new ones. By eliminating certain possibilities, the researchers have provided information that could make future searches more efficient and help astronomers focus on planets that remain within reach of upcoming observatories.

"The new limit enables us to design optimum search strategies in the future," said Michael Meyer, a senior co-author of the study and chair of the University of Michigan's astronomy department.

"In fact, the University of Michigan is part of a team studying a space mission called SHERA that will focus on binaries, where this system is one of a handful of targets. 70 Oph AB will certainly be a target of flagship missions, such as NASA's Habitable World Observatory."

Such missions could help scientists study smaller planets that current observations cannot yet detect or characterize. Rocky planets are especially important targets because they offer the possibility of environments similar to Earth, although finding one in a habitable zone would not by itself establish that life exists there.

Scientists Discover the True Source of a Mysterious Stellar Wobble

The research brought together scientists from more than a dozen institutions, including NASA's Jet Propulsion Laboratory and IPAC at the California Institute of Technology.

Their analysis ruled out planets with masses equal to or greater than Jupiter's within specific parts of the system. Around 70 Oph A, no such planets were found within 5 astronomical units. Around 70 Oph B, they were excluded within half an astronomical unit. An astronomical unit is the average distance between Earth and the Sun, approximately 93 million miles.

These limits resolve a question that had persisted despite increasingly advanced observations. Although William Jacob's original claim from 1855 did not withstand later examination, measurements collected over the following 171 years had never completely eliminated the possibility of giant planets in the system's inner regions.

One particularly puzzling clue involved changes in the stars' radial velocity, which describes how quickly an object moves toward or away from Earth. Astronomers can use these changes to search for planets because an orbiting planet's gravity causes its host star to move slightly back and forth. This subtle motion produces a detectable "wobble" in measurements of the star's light.

For years, observations of 70 Oph AB revealed a wobble that could have been explained by a Jupiter-sized planet. The new research, however, points to a different cause.

To investigate, the team combined roughly a century of historical measurements with new observations from two major 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 resulting analysis was sensitive enough to distinguish the apparent planetary signal from changes occurring on the stars themselves. The researchers concluded that variations in stellar surface activity over time, rather than the gravitational influence of a giant planet, were responsible for the mysterious wobble.

"Yiting assembled an absolutely huge amount of data from all over the world and carefully explored all possibilities," Meyer said. "It's a cautionary tale about how stellar activity can mimic planet signals using the radial velocity."

The Search for Hidden Planets Is Far From Over

Despite ruling out giant planets in the surveyed regions, the findings do not mean 70 Oph AB is planet-free. Smaller planets could still orbit either star, and the system remains a promising destination for future observations.

The results illustrate an important challenge in the search for planets beyond our solar system. Stars are not perfectly stable objects, and changes in their activity can produce signals that resemble those created by orbiting planets. Separating these effects is essential for identifying genuine exoplanets, particularly smaller rocky worlds.

For 70 Oph AB, the latest discovery represents progress rather than a final answer. Scientists now have a clearer picture of what the system does not contain, along with better information for determining where to look next.

"The study sets an upper limit, not a ban, on planets," Li said. "Given how prevalent planets are in our universe and in the Milky Way, I would wager that we'll find there are other planets there."