Astronomers studying the nearby binary star system Seventy Ophiuchi AB have found that a long-standing signal once attributed to a Jupiter-sized planet was instead caused by activity on the star’s surface. Led by the University of Michigan, the study rules out certain giant planets in the regions examined while leaving open the possibility of smaller planets, including rocky worlds in the system’s habitable zone.
A planetary mystery dating back to 1855
Seventy Ophiuchi AB, also known as 70 Oph AB, is a binary system in which two stars orbit each other. Located approximately 16 light-years from Earth, it is the third-closest binary star system to Earth, after Alpha Centauri at 4.2 light-years and 61 Cygnus at 11.4 light-years.
The system’s planetary mystery began in 1855, when British astronomer William Jacob proposed that it contained a planet. That claim was eventually rejected, but the possibility of an undiscovered planet continued to interest astronomers for generations.
“I feel a mixture of disappointment and excitement,” said Yi-Ting Li, a postdoctoral fellow in the University of Michigan’s astronomy department. “While we are discovering that no Jupiter-sized planet exists, we are also beginning the latest chapter in this historic system.”
Unlike our solar system, which has one star, about half of the star systems in the Milky Way contain at least two stars. These multiple-star systems offer opportunities to study how planets form and evolve, and whether they can develop conditions suitable for life.
The orbits of 70 Oph A and 70 Oph B are also among the most precisely measured stellar orbits in the sky, making the system an especially useful target for planetary searches.
Ruling out giant planets
The University of Michigan-led team conducted an extremely sensitive search for planets with masses equal to or greater than Jupiter. Rather than finding such a planet, the researchers determined that no planet of that size exists within the areas they surveyed.
Near 70 Oph A, the analysis ruled out planets with at least Jupiter’s mass within 5 astronomical units. Around 70 Oph B, it ruled out such planets within half an astronomical unit. An astronomical unit is the average distance between Earth and the Sun, approximately 93 million miles.
The result does not show that 70 Oph AB has no planets. The researchers’ approach was designed to detect giant planets, so smaller worlds could still remain undiscovered. Those could include Saturn-sized planets or smaller bodies, including rocky planets in the system’s habitable zone—the region where temperatures could allow liquid water to exist on a planet’s surface if suitable atmospheric conditions are present.
Liquid water is essential for life as we know it, making potentially habitable rocky planets important targets in the search for life beyond Earth. However, finding a planet in the habitable zone would not prove that life exists there.
Stellar activity mimicked a planet
One of the system’s most puzzling clues was a series of changes in its radial velocity, the speed at which an object moves toward or away from Earth. An orbiting planet’s gravity can make its host star move back and forth slightly, creating a detectable shift—or “wobble”—in measurements of the star’s light.
Observations of 70 Oph AB over the years showed wobbles that could have been explained by a Jupiter-sized planet. To investigate, the researchers combined nearly a century of historical measurements with new 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.
The combined analysis was sensitive enough to distinguish between an apparent planetary signal and changes taking place within the star itself. The researchers concluded that fluctuations in stellar surface activity, rather than the gravitational influence of a giant planet, caused the mysterious wobble.
“Mr. Eating collected vast amounts of data from around the world and carefully considered all possibilities,” Meyer said. “This is a cautionary tale about how stellar activity can mimic planetary signals by exploiting radial velocities.”
Future missions could search for smaller worlds
Knowing which planets do not exist can help astronomers plan more effective searches. By ruling out certain giant planets, the researchers have narrowed the possibilities and provided information that could help future observatories focus on planets that remain detectable.
“The new constraints will allow us to design optimal search strategies in the future,” said Michael Meyer, 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 binary star-focused space mission called SHERA, and this system is one of the few targets. 70 Oph AB will definitely be a target for flagship missions like NASA’s Habitable World Observatory.”
Such missions could study small planets that current instruments cannot yet detect or characterize. Rocky planets are especially valuable targets because they could offer conditions similar to those on Earth, although their presence in a habitable zone alone would not establish that they support life.
The study involved scientists from more than a dozen institutions, including NASA’s Jet Propulsion Laboratory and the California Institute of Technology’s IPAC. Additional funding came from the Heising-Simons Foundation.
Although the search ruled out a giant planet in the surveyed regions, smaller planets may still orbit either star. The system therefore remains a promising target for future observations—and a reminder that stellar activity can produce signals that resemble those of orbiting planets.
“This study puts a cap on the planet, not a ban,” Lee said. “Given how widespread planets are in our universe and in our Milky Way galaxy, I’d wager that we’ll find other planets out there, too.”
Source: www.sciencedaily.com
