The search for the largest solar system outside our own requires astronomers to first agree on a definition of “largest.” Our own solar system, with its eight planets and countless smaller objects, is merely one example among billions in the galaxy. The sheer variety of exoplanetary arrangements complicates any simple comparison, meaning the concept of a “biggest” solar system depends entirely on the metric being used. This article focuses on the most dramatic measure of size: the maximum orbital distance, revealing the current record holder in this category.
Defining the Scope of Planetary System Size
The term “biggest” is ambiguous in astronomy, as planetary systems can be measured in several distinct ways. One metric is the sheer number of confirmed planets orbiting a single star, a record currently held by the Kepler-90 system, which is tied with our own Sun at eight known planets. Another measure could be the total mass of all orbiting components, where our own solar system is dominated by the Sun, which accounts for over 99.8% of the system’s mass.
Neither the planet count nor the total mass, however, truly captures the common understanding of a system’s physical scale. The most satisfying measure of size is the maximum orbital extent—the distance from the host star to the system’s outermost confirmed object. This metric describes the physical volume of space controlled by the star’s gravity, providing a direct answer to the question of which system is the most physically spread out.
The Largest Known Exoplanetary Orbit
The current record-holder for the widest known planetary orbit is the gas giant 2MASS J2126–8140, which is in a tenuous gravitational relationship with the star TYC 9486–927–1. This system features an orbital separation estimated to be around 6,900 Astronomical Units (AU), where one AU is the average distance between the Earth and the Sun. This separation represents approximately one trillion kilometers between the planet and its star.
The time required for 2MASS J2126–8140 to complete a single circuit around its star is staggering, with an orbital period of approximately 900,000 Earth years. The star itself is estimated to be between 10 million and 45 million years old, meaning the planet has completed fewer than 50 orbits since the system formed. At this immense distance, the star would appear in the planet’s sky as merely a bright, distant point of light, indistinguishable from other stars.
The scale of this system can be visualized by comparing it to our own neighborhood. Pluto’s average orbit is only about 40 AU from the Sun, making the orbit of 2MASS J2126–8140 about 170 times wider than Pluto’s path. Even the hypothesized outer boundary of the Sun’s Oort Cloud, a distant shell of icy bodies, is thought to extend only to about 100,000 AU. The 6,900 AU separation places the record-holder well beyond the orbits of every major planet, demonstrating the vast gravitational reach of its host star.
Why Extreme Systems Are Hard to Detect
The methods that have discovered most exoplanets are ill-suited for finding systems with such extreme separations. The transit method, which detects a planet by monitoring the slight dip in a star’s brightness as the planet crosses in front of it, is ineffective for wide orbits. A planet orbiting at 6,900 AU would take nearly a million years to complete one orbit, making the likelihood of observing a transit virtually zero.
The radial velocity method, which relies on detecting the tiny “wobble” of a star caused by a planet’s gravitational pull, is also largely useless at these distances. Since gravitational force diminishes rapidly with distance, the star’s reflex motion is exceptionally small and slow for a wide orbit. This movement is generally undetectable with current technology.
For these far-flung systems, direct imaging is the only viable detection technique. This method captures the light emitted or reflected by the planet itself, typically in the infrared spectrum, which filters out the overwhelming glare of the host star. Direct imaging is most effective for young, massive gas giants in wide orbits because they retain enough heat to glow brightly in the infrared, making them easier to spot.