If we start from the Sun and move away from it , in any direction, we will go through different regions. First the asteroid belt , later the Kuiper belt . Perhaps we will pass a planet on the way, although given the scales of the solar system, it is unlikely. If we continue to move away we will leave the heliosphere . Much further, we would cross the so -called Oort cloud , until a point was reached where we would enter another solar system , dominated by some of the stars closest to the Sun. However, at what exact moment would we have left the solar system?
The truth is that we do not know . Or, put another way, there are different definitions of what constitutes leaving the solar system. One of them would be from the heliopause , the region where the heliosphere ends. This would be the region in which the solar wind is capable of overcoming the interstellar medium . Imagine this interstellar medium as a very diffuse cloud of gas that fills the entire galaxy, or at least the region in which the Sun is located. This cloud will tend to expand and occupy every corner , unless something prevents it. In the vicinity of a star, the solar wind may exert enough pressure to keep the gas cloud at bay , creating a blanket around the star.
However, due to the movement of the Sun through this interstellar medium, the heliosphere will not be perfectly spherical , but will be compressed in the direction of movement of the Sun and elongated in the opposite direction. Therefore the heliopause is located at a distance between 80 and 100 astronomical units in the direction of movement of the Sun and about 200 astronomical units in the opposite direction, one astronomical unit being the average distance between the Earth and the Sun. The Voyager 1 probes and 2 from NASA we think went through the heliopause a few years ago . Voyager 1 did so at 94 astronomical units from the Sun, while Voyager 2 did so at 84 astronomical units . These probes were of course not traveling in the same direction, hence the difference between these distances.
Beyond the heliopause it is considered that the interstellar medium has been entered, so following this criterion the solar system would have been abandoned. However, at these distances, the Sun remains the gravitationally dominant object, capable of influencing the orbits of objects orbiting there. In fact, we believe that the influence of the Sun would dominate up to distances of about two light years , or 125,000 astronomical units, up to the limit of what we call the Hill sphere. Inside this sphere would be the hypothetical Oort cloud , a region that would range from a few thousand astronomical units to 50,000 AU from the Sun. This region has not been observed directly (due to the tremendous distances that separate us from any object that inhabits it) although we believe that all known long-period comets come from it. These comets can take thousands of years to complete one orbit around our star.
The Oort cloud could be populated by trillions (millions of millions) of small objects composed mainly of different ices. Although some reach sizes of tens of kilometers, most would be much smaller . These objects orbit the Sun very slowly, but can be affected by the close passage of a star.
But between the Kuiper belt and the Oort cloud and beyond the heliopause, we also find interesting objects. One of them is Sedna , a dwarf planet about a thousand kilometers in diameter that is almost 3 times farther from the Sun than Neptune , at about 84 AU, but whose orbit takes it up to 940 AU . This object has one of the largest orbits known, apart from those corresponding to long-period comets. It is estimated that it takes more than eleven thousand years to complete an orbit and that it is currently very close to perihelion, the closest point to the Sun in its orbit. Were it not for this coincidence, we probably would not have detected it. This suggests to us that there are probably objects with similar characteristics (enough mass to have a spherical shape, specifically) that are too far away to be detected with our current instruments.
However, the record for the maximum distance reached from the Sun is not held by Sedna, but by Comet West , which passed through the inner solar system between 1975 and 1976 , passing even closer to the Sun than Mercury, but which is currently heading to the farthest point of its orbit, which is estimated to be about 70,000 astronomical units , the largest known aphelion for any solar system object. This distance is greater than a light year away and leads Comet West, in its journey of several hundred thousand years around the Sun, to cross the Oort cloud and reach the very ends of the solar system , whatever these may be. .
Referencias:
M. J. Owens et al, 2013, The Heliospheric Magnetic Field, Living Reviews in Solar Physics. 10, doi:10.12942/lrsp-2013-5
V. V. Emelyanenko, 2007, The fundamental role of the Oort Cloud in determining the flux of comets through the planetary system, Monthly Notices of the Royal Astronomical Society. 381, doi:10.1111/j.1365-2966.2007.12269.x
