Solar System

How far does the Solar System extend?

Voyager 1 and 2 have crossed the heliopause, the boundary of the region shaped by the solar wind. Beyond it, plasma—matter made of electrically charged particles—behaves more like the material between the stars than plasma from the Sun, so this region is called interstellar space. Yet if the Solar System includes objects that orbit the Sun, the unseen Oort Cloud may extend hundreds of times farther than this boundary. Entering interstellar space is not the same as travelling beyond every object in the Solar System.

The Sun's influence does not switch off at one point. Sunlight weakens with distance, the solar wind changes direction where it meets interstellar matter, and gravity decreases with the square of the distance yet still shapes the orbits of extremely remote comets. Before asking where the Solar System ends, we therefore need to decide whether we mean the range of the planets, the plasma emitted by the Sun, or objects that can orbit the Sun.

• About 30 AU Neptune's average distance from the Sun, the farthest of the eight planets
• About 122 and 119 AU Where Voyager 1 and 2 met the heliopause in different directions
• About 2,000–5,000 AU Model estimates for the Oort Cloud's inner boundary
• About 10,000–100,000 AU Widely varying model estimates for the Oort Cloud's outer boundary

What lies beyond Neptune?

If we count only the eight planets, Neptune is the outermost, orbiting at about 30 AU. But beyond it, the Kuiper Belt contains Pluto and many other bodies across roughly 30–50 AU. Scattered-disc objects that exchanged gravitational energy with Neptune in the past can travel hundreds of AU away on much longer, more steeply tilted orbits. The “last planet” is merely the end of the planet list, not the end of all material orbiting the Sun.

Nor are there sharp dividing lines between these groups of small bodies. Resonant Kuiper Belt objects, the scattered disc, and candidates for the inner Oort Cloud are classified by the shapes and motions of their orbits, so the categories partly overlap. Textbook diagrams often stop at Neptune to show the planets clearly. There is no wall around the Solar System at that location.

What boundary did the Voyager probes cross?

The Sun sends electrically charged particles outward at hundreds of kilometres per second. This stream is the solar wind. Far from the Sun, pressure from the material between the stars abruptly slows the wind. Beyond that point lies a region where the slowed particles are compressed and deflected. The heliopause is the outer boundary where solar-wind plasma meets interstellar plasma.

In 2012, at about 121.6 AU, Voyager 1 detected a steep fall in low-energy particles from the Sun and a rise in cosmic-ray particles from the Galaxy. Its instrument for directly measuring plasma was no longer working, however, so researchers could not immediately confirm that it had crossed the boundary. They later measured how quickly nearby plasma oscillated in response to waves from solar eruptions. The resulting particle density showed that Voyager 1 had entered interstellar plasma, which is much denser than plasma inside the heliosphere.

Voyager 2 crossed the boundary in 2018 at about 119 AU. Its plasma instrument was still working, so it directly measured the solar particle flow disappearing and giving way to cooler, denser plasma. Its magnetic-field, high-energy-particle, and low-energy-particle instruments recorded the same transition. The two probes identified the same kind of boundary in different directions, but two crossing points are not a photograph of the heliopause's full shape.

The line Voyager crossed is not where the Sun's gravity ends. It is where plasma emitted by the Sun can no longer dominate the surrounding space.

Is the boundary of the heliosphere always in the same place?

Voyager 1 and 2 encountered the boundary about 3 AU apart. That does not prove that one side is always closer to the Sun. The probes crossed in different directions and at different times. The strength of the solar wind changes with the Sun's roughly 11-year activity cycle and its eruptions, while the interstellar plasma and magnetic field pressing from outside also vary by direction. The location of the boundary therefore keeps moving.

Sunlight and the Sun's gravity do not suddenly vanish beyond the heliopause. This region is called interstellar space because the surrounding matter displays the properties of interstellar material more strongly than those of the solar wind. Interstellar atoms also enter the heliosphere. The boundary does not perfectly separate pure solar matter on the inside from a region with no solar influence on the outside.

Some illustrations show the heliosphere as a droplet or as a shape with a long tail, but no spacecraft has photographed the whole structure at once. These shapes are estimates combining Voyager's two crossing points, remote observations by missions such as IBEX, and plasma calculations.

How did we discover the unseen Oort Cloud?

Long-period comets that take hundreds of thousands of years or more to circle the Sun arrive from many directions rather than clustering in the plane of the planets. When astronomers calculate their orbits backward, the results imply a large population of icy bodies thousands to tens of thousands of AU from the Sun. The leading explanation is that the giant planets scattered small icy bodies outward early in Solar System history, after which the Galaxy's gravity and passing stars altered their orbits and spread them into a roughly spherical swarm around the Sun.

NASA commonly places the inner boundary at about 2,000–5,000 AU and the outer boundary at roughly 10,000–100,000 AU. The range is so broad because no one has directly imaged and catalogued Oort Cloud bodies at those distances. Their distribution is inferred from known comet orbits and simulations of Solar System formation, and models also differ on the division between the inner and outer cloud.

At such great distances the Sun's pull is weak, so even the small forces exerted by the Galaxy and passing stars can have large effects over long periods. Some comets are nudged inward toward the Sun, while other bodies escape the Sun's gravity entirely. The outermost range of Sun-orbiting bodies cannot be fixed at one exact radius; it can differ with each object's orbit and with the stars that have passed nearby.

1. About 30 AU If only planets are counted, the list ends with Neptune.
2. About 30–50 AU The main Kuiper Belt contains many icy bodies orbiting the Sun.
3. About 119–122 AU The two Voyager probes crossed the heliopause and measured interstellar plasma.
4. About 2,000–5,000 AU The estimated inner boundary of the still-unseen Oort Cloud may begin.
5. Up to about 100,000 AU Models allow the distant reservoir of very slowly orbiting comets to extend this far.

Have the Voyager probes completely left the Solar System?

Neither Voyager is on an orbit that will carry it around the Sun and back. Close passes by planets gave the probes enough speed to follow escape trajectories out of the Solar System. That still does not make their present location the boundary of the entire Solar System. Whether one object is gravitationally bound to the Sun and whether other Sun-orbiting objects occupy the same region are different questions.

The Voyagers entered interstellar plasma near 120 AU. At their speed, however, reaching the distance where the Oort Cloud may begin would take about 300 years, and passing its farthest estimated extent could take about 30,000 years. These times do not mean the probes will remain in orbit around the Sun. They simply show the enormous difference between the boundary of the solar wind and the range of distant bodies.

What should count as the edge of the Solar System?

If we mean the planets, the answer is Neptune. If we mean the disc holding many icy bodies, it is the Kuiper Belt. If we mean the solar wind, it is the heliopause. If we include the most distant bodies orbiting the Sun, we must consider the estimated Oort Cloud tens of thousands of AU away. These are not competing answers in which only one can be correct; the boundary changes with what we choose to measure.

Voyager's entry into interstellar space did not reveal a single point where the Solar System ends. It was the first passage through the plasma boundary where the Sun meets the environment between the stars, measured in two directions. Any precise statement about the Solar System's edge should place the words “the edge of what” beside the number.

Sources

• NASA — Voyager 2 Instruments at the Heliopause
• Nature Astronomy — Voyager 2 Plasma Observations of the Heliopause
• Nature Astronomy — Plasma Densities Near and Beyond the Heliopause
• NASA Science — Oort Cloud Facts and Distance Ranges
• NASA Science — Kuiper Belt Facts
• NASA Science — Criteria for the Edge of the Solar System
• Celestial Mechanics and Dynamical Astronomy — Stellar and Galactic Perturbations of Oort-Cloud Comets