A Binary Within a Binary: Unraveling the Secrets of the Logos-Zoe System in the Outer Solar System
In the distant Kuiper Belt—a region of the Solar System beyond Neptune filled with icy bodies—lies the Logos-Zoe system, a peculiar and intriguing binary. Logos (formally known as (58534) 1997 CQ29) and its companion Zoe are part of a class of objects called Cold Classicals, which have very stable orbits and distinct red-colored surfaces. Discovered in 1997 and 2001 respectively, Logos and Zoe initially appeared to be a fairly standard wide binary pair. However, recent observations suggest that Logos may itself be a contact binary—a system of two bodies touching or nearly touching—potentially making the entire system a rare example of a triple configuration.
Historical Observations Reveal a Mystery
For over twenty years, astronomers have observed the Logos-Zoe system using both space- and ground-based telescopes. High-resolution imaging from the Hubble Space Telescope (HST) allowed researchers to measure the brightness of Logos and Zoe separately. These measurements showed that Logos had significant changes in brightness over short timescales—indicating rapid rotation or an irregular shape. Zoe, on the other hand, showed less variability and over much longer timescales, hinting at a much slower rotation or a different orientation. Ground-based telescopes, which could not resolve the two bodies separately, confirmed that one of the objects is highly variable.
New Observations and Lightcurve Analysis
Between 2019 and 2023, new data were collected using the Lowell Discovery Telescope and the Magellan-Baade Telescope. These observations provided a continuous lightcurve, a graph that shows how the brightness of the system changes over time. By analyzing this lightcurve, the researchers found a periodic signal with a rotation period of 17.43 ± 0.06 hours and a large brightness variation of 0.70 ± 0.07 magnitudes. Such a high amplitude suggests that Logos is not a simple sphere or ellipsoid. Instead, the shape of the lightcurve—marked by sharp dips and peaks—points strongly toward Logos being a contact binary.
Modeling the System with Candela
To test the contact binary hypothesis, the team used Candela, a sophisticated modeling tool based on earlier software called licht. Candela builds 3D models of binary systems and simulates how they would appear from Earth. Using this software, the researchers modeled Logos first as a triaxial ellipsoid and then as a contact binary. The contact binary model matched the observed lightcurve much better than the ellipsoid model, especially in reproducing the U- and V-shaped dips in brightness. This strongly supports the idea that Logos is composed of two connected components.
The Elusive Companion: What We Know About Zoe
While Logos reveals itself through its lightcurve, Zoe remains enigmatic. The available data suggest it could be a slow rotator, potentially taking days or even months to complete a spin. Its shape is also unclear—Zoe could be spherical, elongated, or even a contact binary like Logos. Sparse data from HST suggest a brightness change of about 0.6 magnitudes across several years, which might indicate an elongated shape. However, without a consistent lightcurve, Zoe’s physical characteristics remain uncertain. Future observations will be crucial in solving this part of the puzzle.
Predicting a Rare Celestial Event
One of the highlights of the paper is the prediction of the upcoming "mutual event season," which will occur between 2026 and 2029. During this time, Logos and Zoe will periodically pass in front of each other from our perspective, causing eclipses and occultations. These events are rare, occurring only once every 151 years for this system. Using Candela, the researchers modeled several possible shapes and rotational states for Zoe and explored how these would affect the timing and brightness of the mutual events. Even with uncertainties, this season represents a unique chance to refine models of both Logos and Zoe.
Why Mutual Events Matter
The team emphasizes that mutual events offer an exceptional opportunity to learn more about the physical and orbital properties of distant systems like Logos-Zoe. By carefully observing how the system’s brightness dips during these events, scientists can estimate the sizes, shapes, and densities of the objects involved. This is especially valuable in the outer Solar System, where such details are often elusive. These insights help astronomers better understand how the Solar System formed and evolved, particularly the icy objects that may be the building blocks of planets.
Final Thoughts
This study of Logos-Zoe reveals a dynamic and complex system in the outer reaches of the Solar System. Logos is likely a close binary, and Zoe may also be more than it first appears. As the mutual events season approaches, astronomers will be watching closely, hoping to catch these rare and revealing interactions. The results could provide a much clearer picture of a system that, despite its distance and faintness, holds important clues about our cosmic neighborhood’s earliest days.
Source: Thirouin
