An orbit is the continuous, curved path an object takes around a massive central body, driven by gravity. In our Solar System, the Sun’s immense mass dictates the elliptical trajectories of every planet, asteroid, and comet. The gravitational boundary of the Solar System extends out to the Oort Cloud, a vast shell of icy bodies where the Sun’s gravity gives way to the pull of the rest of the galaxy. Almost every object within this boundary is gravitationally bound to the Sun and follows an orbit.
However, countless objects in the universe do not follow the Sun’s path, existing completely outside its gravitational dominance. These objects range from temporary visitors passing through our neighborhood to entire systems and structures located in the farthest reaches of space. Their motion is instead governed by other stars, the collective mass of the Milky Way, or the expansion of the cosmos itself.
Interstellar Objects Passing Through
Interstellar objects enter the Solar System but are not gravitationally bound to the Sun. They travel on an open, hyperbolic trajectory, briefly accelerating around the Sun before leaving the system forever. Their velocity is too high to be captured by the Sun’s gravity, differentiating them from comets or asteroids originating in the Oort Cloud. The discovery of these cosmic visitors confirms that material is exchanged between star systems throughout the Milky Way.
The first confirmed interstellar object was 1I/’Oumuamua, detected in 2017, which showed an orbital eccentricity greater than 1.0, a clear signature of an unbound path. The second was the comet 2I/Borisov, confirmed in 2019, which possessed an even higher eccentricity. These objects arrive from random directions, having been ejected from their original star systems. Their flybys offer scientists the first opportunities to study material from outside our stellar neighborhood before they continue their journey through interstellar space.
Objects Gravitationally Bound to Other Stars
Vast numbers of objects are strongly gravitationally bound, but their orbits are dominated by a different star, not the Sun. These include the planets, moons, asteroids, and comets that make up the exoplanetary systems throughout the Milky Way. The Sun’s gravity is negligible at these distances, even for the closest stellar systems like Proxima Centauri, which is over four light-years away. Every star possesses its own gravitational domain, or sphere of influence, within which its gravity is the primary force controlling orbital motion.
Exoplanets orbit their host stars following paths dictated by the mass of their parent star. Although these star systems move through the galaxy, their internal planetary orbits are completely independent of the Sun’s local gravitational field. Astronomers estimate there are billions of potentially habitable, Earth-sized planets in the Milky Way alone, all orbiting stars other than our own. Even brown dwarfs, often called “failed stars,” can host planets that orbit them.
Structures and Objects Orbiting the Galactic Center
The Sun is not stationary; it orbits the center of the Milky Way galaxy, carrying the entire Solar System along with it. This massive orbit takes approximately 230 million years to complete. Numerous other structures within the galaxy also orbit the Galactic Center, but their paths are independent of the Sun’s localized gravity.
Globular clusters are dense, spherical collections of stars that orbit the galaxy’s center, typically in the outer halo. Many of these clusters, such as Omega Centauri, have a retrograde orbit, moving opposite the Milky Way’s general rotation. Large molecular clouds and entire dwarf galaxies captured by the Milky Way’s gravitational pull also follow distinct, large-scale galactic orbits. Their orbital trajectories are determined by the collective mass of the Milky Way, not the Sun’s comparatively tiny gravitational tug.
Objects Residing in Intergalactic Space
The most distant objects that do not orbit the Sun are located entirely outside the Milky Way galaxy, in the immense void of intergalactic space. These objects are not gravitationally tied to our galaxy and are completely beyond the Sun’s sphere of influence. The universe contains billions of galaxies, and their movement is dictated by the gravity of their local galaxy cluster or the overall expansion of the universe.
This category includes distant spiral and elliptical galaxies, as well as smaller, isolated dwarf galaxies. Astronomers also theorize the existence of truly isolated objects ejected from their home galaxies, such as rogue stars and rogue planets. Rogue planets, also called free-floating planets, drift through space without orbiting any star and are not gravitationally bound to any galaxy. Their existence in intergalactic space represents the ultimate independence from the Sun’s gravitational pull.