Where does the boundary of the Solar System lie? Does it conclude immediately beyond Neptune’s orbit, or beyond the orbit of the most remote dwarf planet in the Kuiper Belt? Is it situated at the heliopause, or does it extend approximately halfway to the nearest star?
The eight giant planets
How do you envision the Solar System? It is likely depicted as the Sun accompanied by eight planets revolving around it. This is precisely the type of image that a search engine typically provides in response to this query. However, is this depiction entirely accurate? The accuracy of the representation depends on the desired level of precision and the definition employed for the term “Solar System.”
For instance, when examining mass distribution, nearly 100% of the Solar System’s mass is attributed to the Sun and these eight planets. To be more precise, the proportion is approximately 99.99945%. An additional roughly 0.00005% comprises the Main Asteroid Belt and all small celestial bodies with aphelia situated within Neptune’s orbit.
Therefore, the assertion that the outer boundary of the Solar System is positioned at an approximate distance of 4.498 billion kilometers, or 30.07 astronomical units (AU), cannot be regarded as entirely erroneous, given that 99.9995% of all matter orbiting the Sun is confined within this boundary.
Kuiper belt
However, if we respond in that manner, it would imply that Pluto resides outside the Solar System, despite the fact that it — together with Eris, Makemake, and other trans-Neptunian dwarf planets — also orbits the Sun. In reality, collectively, these bodies are approximately ten times more massive than the Main Asteroid Belt, although they still constitute a very small proportion of the mass of the major planets.
Nevertheless, this presents a challenging inquiry: where, then, ought the outer boundary of the Solar System to be defined? The reality is that numerous objects within the Kuiper Belt possess nearly circular orbits, thereby residing not far beyond Neptune’s orbit. Nonetheless, some dwarf planets exhibit relatively distant aphelia — Sedna at 76 AU, Leleākūhonua at 65 AU, and 2012 VP 113 at 80 AU. All of these celestial bodies are also encompassed within the Solar System.
Nevertheless, a significant challenge exists in establishing the boundaries of the Solar System based on their aphelia. On one hand, it appears that extensive regions of near-vacant space have been encompassed within the Solar System solely due to the presence of a few relatively minor celestial bodies orbiting in those regions.
Conversely, it remains conceivable that the most remote objects within the Kuiper Belt have yet to be observed. Should these objects reside near their respective aphelia, hundreds of astronomical units distant from Earth, detection may be significantly delayed. Consequently, this would necessitate a reevaluation of the Solar System’s boundaries.
Oort Cloud
However, even the Kuiper Belt is not truly the most remote region of our Solar System from the Sun. Beyond it lies the Oort Cloud, which is thought to contain millions of comets. These comets occasionally enter the inner Solar System and are characterized by extremely long orbital periods.
However, it is important to note that no observations have been made of any comets within the Oort Cloud itself. Furthermore, even if such comets do exist in that region, the average matter density per unit volume would need to be even lower than that observed in the Kuiper Belt.
The outer diameter of the Oort Cloud continues to be a subject of debate. Some estimates suggest 50,000 AU, others propose one light-year, and still others cite 100,000 AU. All of these differing figures serve only to further complicate understanding among the public.
Ultimately, in all models that consider the presence of this structure, it is invariably represented as a massive cloud. However, in reality, even when the quantity of objects within it is estimated at its maximum, it remains practically devoid of content, containing only a few bodies — minute in comparison to Earth — distributed sporadically throughout.
Heliosphere
Nonetheless, besides estimating the mass of the celestial bodies orbiting the Sun, there exists an entirely alternative approach. Our Sun functions as a source of high-energy particles that emanate outward in all directions. These particles are referred to as the solar wind.
Consequently, particles originating from other stars migrate toward Earth from outer space. Their exerted pressure is significantly weaker than that of the solar wind; however, as the solar wind’s intensity decreases with increasing distance from the Sun, there exists a certain point at which these two forces reach equilibrium.
This region is designated as the heliosphere. As our Sun traverses space, it does not constitute a perfect sphere; rather, it is compressed at the front and elongated at the rear. In 2004, the Voyager spacecraft encountered this boundary at approximately 90 astronomical units (AU); however, both entities were moving toward the leading shock wave, where this distance is minimized.
Generally, it is considered that the heliosphere has an approximate diameter of 100 AU. It can likewise be regarded as the perimeter of the Solar System.
