Solar System
Boundary area and uncertainties

Much of the Solar System is still unknown. Areas beyond thousands of AU away are still virtually unmapped and learning about this region of space is difficult. Study in this region depends upon inferences from those few objects whose orbits happen to be perturbed such that they fall closer to the Sun, and even then, detecting these objects has often been possible only when they happened to become bright enough to register as comets. Many objects may yet be discovered in the Solar System's uncharted regions.

The Oort cloud is a theorized spherical shell of up to a trillion icy objects that is thought to be the source for all long-period comets. No direct observation of the Oort cloud is possible with present imaging technology. It is theorized to surround the Solar System at roughly 50,000 AU (~0.9 ly) from the Sun and possibly to as far as 100,000 AU (~1.8 ly). The Oort cloud is thought to be composed of comets that were ejected from the inner Solar System by gravitational interactions with the outer planets. Oort cloud objects move very slowly, and can be perturbed by infrequent events, such as collisions, the gravitational effects of a passing star, or the galactic tide, the tidal force exerted by the Milky Way.

As of the 2020s, a few astronomers have hypothesized that Planet Nine (a planet beyond Neptune) might exist, based on statistical variance in the orbit of extreme trans-Neptunian objects. Their closest approaches to the Sun are mostly clustered around one sector and their orbits are similarly tilted, suggesting that a large planet might be influencing their orbit over millions of years. However, some astronomers said that this observation might be credited to observational biases or just sheer coincidence.

The Sun's gravitational field is estimated to dominate the gravitational forces of surrounding stars out to about two light-years (125,000 AU). Lower estimates for the radius of the Oort cloud, by contrast, do not place it farther than 50,000 AU. Most of the mass is orbiting in the region between 3,000 and 100,000 AU. The furthest known objects, such as Comet West, have aphelia around 70,000 AU from the Sun. The Sun's Hill sphere with respect to the galactic nucleus, the effective range of its gravitational influence, is thought to extend up to a thousand times farther and encompasses the hypothetical Oort cloud. It was calculated by G. A. Chebotarev to be 230,000 AU.
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