A star system just 16 light-years from Earth has puzzled astronomers for more than 170 years. New research led by the University of Michigan finds that the apparent signal of a Jupiter-sized planet around the binary system Seventy Ophiuchi AB, or 70 Oph AB, was probably caused by changing activity on the stars’ surfaces. The result rules out certain giant planets while leaving open the possibility of smaller worlds, including rocky planets in the system’s habitable zone.
A planetary mystery dating back to 1855
70 Oph AB consists of two stars, 70 Oph A and 70 Oph B, orbiting each other. Researchers used highly sensitive observations to search for planets with masses equal to or greater than Jupiter. They found no such planet within the regions covered by the study.
Although the result does not reveal a giant planet, it is the most sensitive modern search for giant planets in this system. The study was not designed to detect smaller worlds, so other planets may still be present. The research received support in part from NASA and was published in The Astrophysical Journal.
The system’s planetary mystery began in 1855, when British astronomer William Jacob proposed that it contained a planet. That made 70 Oph AB the first stellar system with an officially proposed exoplanet candidate. Later investigations rejected Jacob’s claim, but the possibility of an undiscovered planet continued to interest astronomers.
“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 the 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 give scientists opportunities to study how planets form and evolve, and whether they can develop conditions suitable for life.
70 Oph AB is especially valuable because of its proximity. It is approximately 16 light-years from Earth and is the third-closest binary star system, after Alpha Centauri, at 4.2 light-years, and 61 Cygnus, at 11.4 light-years. The orbits of 70 Oph A and 70 Oph B are also among the most precisely measured stellar orbits in the sky.
Where giant planets have been ruled out
The researchers excluded planets with masses equal to or greater than Jupiter in specific regions of the system. Around 70 Oph A, they found no evidence of such a planet within 5 astronomical units. Around 70 Oph B, they ruled out these planets within half an astronomical unit. One astronomical unit is the average distance between Earth and the Sun, approximately 93 million miles.
These findings address questions that have persisted through generations of observations. Jacob’s original claim did not survive later investigation, but measurements collected over the following 171 years had not completely eliminated the possibility of a giant planet in the system’s inner regions.
One of the most puzzling clues was a series of changes in the stars’ 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, producing a detectable change, or “wobble,” in the star’s light.
Previous observations of 70 Oph AB recorded wobbles that could have been caused by a Jupiter-sized planet. The new analysis indicates that the signal instead came from changes in stellar activity.
Stellar activity mimicked a planet
The study brought together scientists from more than a dozen institutions, including NASA’s Jet Propulsion Laboratory and the California Institute of Technology’s IPAC. The team 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.
This combined data set was sensitive enough to distinguish between an apparent planetary signal and changes taking place on the stars themselves. The researchers concluded that fluctuations in stellar surface activity, rather than the gravitational pull of a giant planet, caused the mysterious radial-velocity wobble.
“Mr. Eating collected vast amounts of data from around the world and carefully considered all possibilities,” Michael Meyer said. “This is a cautionary tale about how stellar activity can mimic planetary signals by exploiting radial velocities.”
Smaller planets may still be present
Ruling out giant planets does not mean that 70 Oph AB has no planets. Worlds the size of Saturn or smaller could still orbit either star, including rocky planets that may lie in a habitable zone. This is the region around a star where conditions could allow liquid water on a planet’s surface if its atmosphere is suitable.
Such planets are important targets in the search for life beyond Earth because liquid water is essential for life as we know it. However, finding a planet in the habitable zone would not prove that life exists there.
“The new constraints will allow us to design optimal search strategies in the future,” said Meyer, the study’s senior co-author 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.”
Future missions could investigate small planets that current observatories cannot yet detect or characterize. Separating genuine planetary signals from changes in stellar activity will be essential, particularly when astronomers search for small, rocky worlds whose signals are much weaker.
“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.”
For 70 Oph AB, the findings provide a clearer picture of what is not present in the surveyed regions while preserving the possibility of smaller planets. Additional funding for the study came from the Heising-Simons Foundation.
Source: www.sciencedaily.com