But smaller worlds may yet hide in the two-star system's habitable zones
Study: A Century of Radial Velocity And Astrometric Monitoring of 70 Oph AB: New PFS Data and Constraints on Possible Planetary Companions (DOI: 10.3847/1538-4357/ae9f52)
New research led by the University of Michigan is helping set the stage for future observatories to characterize rocky planets outside our solar system, planets that could prove life isn't confined to Earth.
The new research represents the most sensitive modern search for giant planets in a nearby binary system-a system with two stars orbiting each other-known as 70 Ophiuchi AB, or 70 Oph AB. That search, however, didn't discover any large planets. In fact, it did the opposite: With some important caveats, it ruled out planets with the mass of Jupiter and larger.
Although that may sound like a letdown, the null result is noteworthy for a few reasons. For one, there is the record-setting sensitivity the team achieved for the system. Further, despite that sensitivity, the researchers' work, supported in part by NASA, wasn't designed for smaller planets.
That leaves the door open for planets the size of Saturn or smaller within the system's habitable zone. This is the region where the temperature wouldn't be too hot or too cold for liquid water, a key ingredient for life as we know it, to exist on a rocky planet's surface.
This work also represents the latest update to a system with a rich history. In 1855, the British astronomer William Jacob claimed the system had a planet, making 70 Oph AB the first host of an official exoplanet candidate. Although Jacob's claim itself hasn't held up, the possibility of planets endures.
"It's a mix of disappointment and excitement," said Yiting Li, a postdoctoral researcher in the U-M 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."
While our solar system has but a single star, roughly half of the stellar systems in the Milky Way have at least two stars. These systems are enticing targets for future astronomical studies to search for clues on how planetary systems form, evolve and perhaps even support life. 70 Oph AB is of particular interest given its proximity to Earth. At about 16 light-years away, it's the third-closest binary system to Earth, just behind Alpha Centauri (4.2 light-years) and 61 Cygni (11.4 light-years).
Additionally, with this new study, published in The Astrophysical Journal, the orbits of the two stars, 70 Oph A and 70 Oph B, are among the best characterized in the sky.
"The new limit enables us to design optimum search strategies in the future," said Michael Meyer, a senior co-author and chair of the U-M 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."
The new study was performed by a team that included experts at more than a dozen institutions, including NASA's Jet Propulsion Laboratory and IPAC at the California Institute of Technology. The researchers were able to rule out planets with the mass of Jupiter or greater within 70 Oph AB's inner regions. That is, there aren't any within 5 astronomical units-one astronomical unit is the distance between the Earth and the sun-of 70 Oph A and none within half an astronomical unit of 70 Oph B.
Although Jacob's initial 1855 claim didn't survive scrutiny, more sophisticated measurements made over the next 171 years also hadn't nixed the possibility of Jupiter-sized planets in 70 Oph AB's inner regions. Even modern measurements of the stars' radial velocity-their speed toward or away from Earth-had a "wobble" that could, until now, be explained by a Jupiter-sized planet, Li said.
The team's new analysis included a century's worth of data, as well as key new observations. These came from the 6.5 meter Magellan Clay Telescope at the Las Campanas Observatory in Chile and the 2.7 meter Harlan J. Smith Telescope at the McDonald Observatory in Texas. The sensitivity of their approach allowed the researchers to determine this wobble was due to variations on the surface of the stars over time, not a planet.
"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."
That said, the researchers stressed that this study turns the page, rather than closes the book on exoplanets in this historic binary system.
"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."
Additional support for the research was provided by the Heising-Simons Foundation.
