What is it like where the edge of the Solar System is?

When we refer to the Solar System, we are usually talking about the Sun, the planets and a handful of asteroids that lie very close to the orbits of Jupiter, Mars, or just after Neptune . However, in addition to dwarf planets like Pluto, Eris, and Makemake, there appears to be a dark and icy wilderness, full of objects, known as the Oort Cloud. But where exactly does the Solar System end? And what is the edge of the Solar System like?

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    The ends of the Solar System

    The Oort Cloud (Image: Reproduction/Reproduction/Don Davis)

    4.6 billion years ago, the Solar System was born from the protoplanetary disk that orbited our Sun, when it was still a very young star. This disk formed of dust and gas gave rise to the first protoplanets, building blocks of the worlds that would develop. Some of these blocks, however, failed to gain mass and grow, and formed objects like huge asteroids. In other parts of the same disk, millions of rocks and comets emerged.

    These objects would have formed closer to the Sun, but gravitational interactions with young giant worlds such as Jupiter expelled them. us to the farthest orbit. Some models suggest that there was then a series of collisions that destroyed most of the comets before reaching the region of the Oort Cloud — a large area that circles the entire Solar System, formed by boulders.

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    Who first suggested the existence of the Oort Cloud seems to have been the Estonian astronomer Ernst Öpik, in 219. The idea was to explain the long period comets, that is, they travel through space for decades, or centuries, before approaching the Sun again. The proposal was taken forward in 1950 by the Dutch astronomer Jan Oort to solve a paradox.

    (Image: Reproduction/William Crochot/NASA)

    What Oort noticed was that there was a spike in the number of long-period comets that managed to get away from up to about 20.10 AU (astronomical units; one AU equivalent to the distance between the Earth and the Sun), which suggested the existence of a deposit of objects in that region, with an isotropic (equal in all directions) and spherical distribution. The Cloud would be composed of two parts: a disk-shaped inner part and a spherical outer part.

    In this outer part, would be the gravitational limit of the Solar System, that is, the last place in the universe where the Sun exerts enough gravity to “hold” objects in its orbit. Thus, the Oort Cloud is at an estimated distance between 5,000 and 97 thousand astronomical units. However, astronomers have not yet been able to obtain any image of the Oort Cloud, simply because these objects would be too far away to be observed, especially at such a great distance from our star.

    Although this region even considered hypothetical, it is the most accepted—or at least the most popular—idea to explain the long-period population of comets. In fact, it is a consensus among astronomers that the Outer Cloud marks the boundary of the Solar System. It is thought that the Oort Cloud may be formed by a few trillions of comets over 1 km in diameter and billions approximately 20 km in diameter.

    Note how interstellar space starts before the Oort Cloud. The marked distances are in astronomical units (Image: Reproduction/NASA/JPL-Caltech)

    There are some simulations that show how the Cloud could have formed, some of them suggesting that the boulders there were born with the exchange of materials between the Sun and its sister stars (it has long been assumed that the Sun had at least one twin star, and both stars would have split shortly after they formed). the type of bodies that exist there, the total mass of the Oort Cloud could be 3x²⁵, about five times that of Earth. This is the official limit of the Solar System, and it is estimated that the Voyager probes will take it 239 years to reach this border and 30 a thousand years to leave the Solar System for good. Yeah, there’s a lot of space out there.

    Heliosphere and heliopause

    Illustration of the layers of the Heliosphere (Image: Reproduction/NASA/IBEX/Adler Planetarium)

    Although the Oort Cloud is considered the boundary of the Solar System, there is some confusion when talking about the limits of the Sun’s influence and interstellar space. Voyager probes, for example, passed the region known as the Heliopause, which marks the meeting of the sun’s rays with interstellar cosmic radiation. This shock causes a kind of “shield”, which ends up protecting us from most of the external radiation. However, the Oort Cloud is far beyond the Heliopause.

    To differentiate these regions, it is first necessary to understand that the Solar System has a kind of “bubble” that surrounds it, and if extends far beyond Pluto’s orbit. This bubble is held by two forces: one that is emitted from the inside (the pressure of the solar wind, which travels at supersonic speed) and the other from the outside (the pressure of the interstellar medium). This bubble is called the Heliosphere.

    The Heliosphere, however, is often portrayed as something elongated in shape, because of the movement of the Sun (and therefore the Solar System) relative to the Galaxy. That’s because our “cosmic backyard” orbits the center of the Milky Way at a speed of 121 km/s. Thus, the Sun completes a revolution in the galaxy in 121 millions of years. This high speed causes the bubble that surrounds the Solar System to leave a trail behind.

  • (Image: Reproduction/NASA /JPL-Caltech)

    On the other hand, in part of the “front” of the Heliosphere, the interaction with the interstellar medium creates a shock region, where the speed of the solar wind becomes subsonic. This region is known as the termination shock — this area was overtaken by Voyager I in December 1950, when it was about 80 AU of the Sun. Finally, ahead of the termination shock, the Heliopausa is formed, which Voyager I reached in 2004, when he was 121 AU from the Sun.

    Remember, however, that the Oort Cloud is at 5,000 e 80 thousand astronomical units from the Sun, which is much further away than the Heliopause. Think of an astronomical unit as the average distance between the Earth and the Sun; therefore, 121 thousand astronomical units is something really far away. Therefore, it is not correct to say that the Voyager probes left the Solar System. Despite this, they are in the region considered “local interstellar space”, because there the galactic radiation travels towards the Heliopause. And to learn more about the heliosphere, NASA plans to launch the Interstellar Mapping and Acceleration Probe (IMAP) mission at 2012.

    Source: NASA, Live Science

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