In the Search for Exoplanets, Some Failures Are A Success
The next decade promises to be transformative for exoplanet science. A new generation of extraordinarily powerful observatories — including the Extremely Large Telescope (ELT), the Giant Magellan Telescope, and NASA's Habitable Worlds Observatory (HWO) — is poised to carry humanity's search for worlds beyond our solar system into breathtaking new territory. These instruments will move us decisively past the era of mere exoplanet detection and into an age of deep characterization: direct imaging of planetary bodies, atmospheric spectroscopy, and radial velocity measurements of unprecedented precision. Most tantalizingly, they hold the potential to detect biosignatures — chemical indicators of life — on rocky planets orbiting nearby stars, particularly the cool, abundant M dwarf stars that populate our galaxy.
But this extraordinary scientific promise comes with a formidable practical constraint: observing time on these future flagships will be among the most competed-for resources in the history of astronomy. Researchers who wish to use these telescopes must make a compelling, evidence-based case for their targets. That requires knowing, with considerable precision, where to look and what to expect before the telescopes ever open their mirrors. This preparatory science — careful, methodical, and sometimes unglamorous — is every bit as vital as the headline-grabbing discoveries it enables.
Targeting a Historic Binary Star System
A new study led by astronomers at the University of Michigan represents exactly this kind of essential groundwork. The team turned their attention to 70 Ophiuchi AB, a nearby binary star system that has fascinated astronomers for well over a century, using the powerful technique of radial velocity (RV) measurement to hunt for terrestrial planets that might one day be directly imaged by next-generation observatories. Their findings are published in The Astrophysical Journal in a paper titled "A Century of Radial-velocity and Astrometric Monitoring of 70 Oph AB: New PFS Data and Constraints on Possible Planetary Companions." The lead author is Yiting Li, a postdoctoral researcher in the Department of Astronomy at the University of Michigan.
The choice of 70 Ophiuchi AB as a target is scientifically strategic. Roughly half of all Sun-like stars in the Milky Way exist in binary or multiple-star systems, yet planet detection around such systems poses unique observational challenges. The widely used transit method — which detects the slight dimming of a star as a planet passes in front of it — struggles in binary systems because the light from two stars complicates the photometric signal. The radial velocity method, which measures the subtle Doppler shift in a star's light caused by the gravitational tug of an orbiting planet, is far better suited to these environments. Understanding how planets form and survive in binary systems is therefore a crucial, and still incompletely answered, question in planetary science.
What Is 70 Ophiuchi AB?
Located just 5.1 parsecs (approximately 16.6 light-years) from Earth, 70 Ophiuchi AB is one of the closest binary star systems to our solar system, making it an exceptionally attractive target for both current and future observations. Both stars are main sequence stars estimated to be approximately 6.2 billion years old — older than our own Sun, which is about 4.6 billion years old — suggesting a system that has had ample time for planetary systems to mature and stabilize.
- 70 Oph A: Approximately 0.89 solar masses, making it the more Sun-like of the two components, though cooler and less luminous than our star.
- 70 Oph B: Approximately 0.73 solar masses, a dimmer, orange K-type star.
- The two stars orbit each other with a period of roughly 88 years, at a mean separation that places them in a dynamically complex but scientifically rich configuration.
- The system has been observed telescopically for over 200 years, making it one of the most historically monitored stellar binaries in the sky.
In fact, 70 Ophiuchi holds a peculiar place in the history of astronomy. In the 1940s, astronomer Kaj Strand controversially claimed to have detected a planet orbiting one of its stars through astrometric measurements — an announcement that was later refuted. This history of false positives makes the current study's rigorous methodology all the more meaningful, and speaks to the enduring fascination this system holds for planet hunters.
"At a distance of 5.1 pc, the 70 Oph AB binary star system is one of the most favorable targets for future direct imaging and astrometry missions surveying mature, terrestrial planets." — Li et al., The Astrophysical Journal
The Observations: A Century of Data, Anchored by New Measurements
To conduct their search, the research team used the 6.5-meter Magellan II Clay Telescope at the Las Campanas Observatory in Chile, equipped with its highly sensitive Planet Finder Spectrograph (PFS). During 2025, they accumulated an impressive dataset: 499 radial velocity measurements of 70 Oph A and 334 measurements of 70 Oph B. These new observations were then combined with archival RV data and spectrometry spanning several decades, incorporating over 27 years of RV baseline data for 70 Oph A alone — one of the longest continuous RV baselines for any stellar target.
This combination of new, high-precision measurements with an extensive historical archive is precisely what makes the study so powerful. Long time baselines are essential for detecting the slow, subtle gravitational signatures of planets in wide orbits — the kind of planets that a future mission like the Habitable Worlds Observatory might one day image directly. The longer the baseline, the more confident astronomers can be that they are not missing a slowly varying planetary signal hiding within the noise.
A Null Result With Real Scientific Value
Despite the richness of the dataset and the sensitivity of the observations, the team did not find evidence of planets. The radial velocity variations they observed in both stars were ultimately traced not to the gravitational pull of orbiting planets, but to stellar activity — intrinsic variability in the stars themselves, modulated by their own rotation periods. This is a well-known and persistent challenge in exoplanet science: active stellar surfaces, including starspots, faculae, and convective motions, can produce RV signals that mimic the signatures of orbiting planets with remarkable fidelity.
"We find that the long-term RV variability of both components is consistent with stellar activity modulated by rotation periods, and we detect no coherent planetary signals in either component." — Li et al., The Astrophysical Journal
A null result — the scientific term for a study that finds no evidence of what it was looking for — is often mischaracterized as a failure. In reality, it is precisely the opposite. In science, knowing what is not there is just as important as knowing what is. The absence of evidence, when the search has been conducted with sufficient sensitivity, is itself meaningful evidence. Each null result refines our statistical understanding of planetary populations, helps calibrate our detection methods, and — critically — tells future observers what they can safely rule out before committing precious telescope time.
"It's a mix of disappointment and excitement. We're finding out that there isn't a Jupiter-sized planet, but we're also opening the latest chapter on this historic system." — Yiting Li, University of Michigan
What the Data Rules Out — and What It Leaves Open
The study's results place specific, quantitative upper limits on the masses and orbital configurations of any planets that might inhabit the 70 Ophiuchi AB system:
- For 70 Oph A: Jupiter-mass planets are excluded in orbits inside approximately 5 astronomical units (au) from the star.
- For 70 Oph B: Planets with masses of approximately 0.25–0.3 Jupiter masses (roughly Saturn-mass) are excluded inside 0.5 au.
- Importantly, the data still permits Saturn-mass planets or smaller to exist in stable orbits around 70 Oph A out to approximately 2.5 au — a region that could potentially encompass habitable zone territory.
- The existence of Earth-sized planets in the system remains entirely consistent with the data and cannot be ruled out with current technology.
These constraints are not mere footnotes — they are the blueprint for future observational campaigns. By knowing precisely where large planets do not exist, mission planners for observatories like the Habitable Worlds Observatory and the proposed SHERA mission can design optimized search strategies, focusing their limited observing time on the orbital regimes most likely to yield detectable Earth-like planets.
"The new limit enables us to design optimum search strategies in the future. 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 Worlds Observatory." — Michael Meyer, University of Michigan
The Stellar Activity Problem: A Cautionary Tale
One of the study's most important contributions to the broader field is its detailed disentangling of stellar activity signals from genuine planetary signatures. This challenge — sometimes called the stellar noise floor — is one of the most pressing technical obstacles in modern exoplanet science. As radial velocity instruments become ever more precise, they inevitably pick up more of the intrinsic variability of the stars themselves, not just the gravitational tugs of planets.
The Sun, for example, exhibits RV variations of several meters per second due to its own activity, including sunspots, granulation, and oscillations. Detecting an Earth-mass planet in the habitable zone of a Sun-like star requires sensitivity to RV signals of roughly 10 centimeters per second — far smaller than typical stellar noise. Developing robust techniques to distinguish stellar noise from planetary signals is therefore one of the frontier challenges of the field, and studies like this one, which carefully characterize the activity of specific target stars, provide invaluable data for that effort.
"Yiting assembled an absolutely huge amount of data from all over the world and carefully explored all possibilities. It's a cautionary tale about how stellar activity can mimic planet signals using the radial velocity." — Michael Meyer, University of Michigan
Implications for Habitability in Binary Systems
Beyond the immediate results, the study carries significant implications for our understanding of habitability in binary star systems. Using more than a century of combined RV and astrometry data, the team has achieved an exceptionally precise characterization of the mutual orbits and dynamical masses of 70 Oph A and B. This makes 70 Ophiuchi AB one of the best-characterized nearby solar-type binary systems in the sky — a benchmark object for theoretical models of planetary formation and orbital stability.
In binary systems, the gravitational interplay between two stars creates complex dynamical environments that can either stabilize or destabilize planetary orbits. The key parameters — orbital eccentricity, inclination, and the closest approach distance (periastron) between the two stars — determine the boundaries of what are called S-type stable zones (regions around each individual star where planets can maintain stable long-term orbits) and P-type stable zones (regions around both stars at once, in a circumbinary orbit). Even small changes in eccentricity or periastron distance can dramatically reshape these zones and the extent of the habitable zone (HZ) within them.
"This is especially valuable for habitability studies in binary stars, where even modest changes in eccentricity or periastron distance can substantially modify the extent of long-term stable zones. 70 Oph AB presents a unique opportunity for the search and characterization of habitable planets in binary systems." — Li et al., The Astrophysical Journal
Crucially, the team notes that 70 Ophiuchi is one of the nearest binary systems where the habitable zone can be spatially resolved — meaning that a future direct-imaging telescope could, in principle, take an actual picture of a planet in the habitable zone of either star. This makes the system a uniquely valuable laboratory for the study of life's potential in binary environments, which, given the prevalence of binary stars in the galaxy, could have profound implications for estimates of the overall abundance of habitable worlds in the universe. For more on habitability research, the NASA Astrobiology Institute provides extensive resources on the latest findings in this field.
Looking Forward: The Planets That Are Almost Certainly There
Perhaps the most scientifically provocative aspect of this study is its implicit conclusion: the absence of detected planets is almost certainly not an absence of planets. Decades of exoplanet science, culminating in the extraordinary statistical harvest of missions like NASA's Kepler Space Telescope and the ongoing TESS mission, have established beyond reasonable doubt that planets are the rule rather than the exception around virtually all types of stars. A planetary system around 70 Ophiuchi AB is not merely possible — it is statistically expected. The current study simply cannot yet detect the smaller planets that are most likely there.
This perspective reframes the null result not as an endpoint, but as an invitation. The upper limits established by Li and colleagues define the gap in our current detection capabilities — and that gap is precisely where Earth-like planets could be hiding, waiting for the next generation of instruments to reveal them.
"The study sets an upper limit, not a ban, on planets. 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." — Yiting Li, University of Michigan
The researchers are optimistic that 70 Ophiuchi AB will remain one of the highest-priority targets for future flagship missions. Its proximity, its well-characterized stellar parameters, its long observational history, and the newly established constraints on its planetary architecture all make it an ideal target for direct imaging campaigns with instruments like the Habitable Worlds Observatory and the SHERA mission. If Earth-like planets in habitable zones of binary star systems
