Friday, October 1, 2021

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

 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 appeared.

 These objects would have formed closer to the Sun, but gravitational interactions with young giant worlds such as Jupiter have pushed them 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.

 Who first suggested the existence of the Oort Cloud seems to have been the Estonian astronomer Ernst Öpik, in 1932. The idea was to explain the long period comets, that is, that travel through space for decades, or centuries, before approaching again of the Sun. The proposal was carried out in 1950 by the Dutch astronomer Jan Oort to solve a paradox.

 What Oort noticed was that there was a peak in the number of long-period comets that could move away to about 20,000 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 made up of two parts: a disk-shaped inner part and a spherical outer part.

 In this external part, would be the gravitational limit of the Solar System, that is, the last place in the universe where the Sun exerts gravity enough to “hold” objects in its orbit.  Thus, the Oort Cloud is at an estimated distance between 5,000 and 100,000 astronomical units.  However, astronomers have not yet been able to obtain any images 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.

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

 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 forming).

 If Halley's Comet, which is one of those long-period objects, could be used as a parameter for the type of bodies that exist there, the total mass of the Oort Cloud could be 3x10²⁵, 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 300 years to reach this frontier and 30,000 years to completely leave the Solar System.  Yeah, there's a lot of space out there.

 heliosphere and heliopause

 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 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 usually 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 239 km/s.  Thus, the Sun completes a revolution in the galaxy in 219 million years.  This high speed causes the bubble that surrounds the Solar System to leave a trail behind.

 On the other hand, in the “front” part 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 passed by Voyager I in December 2004 when it was about 80 AU from the Sun. I reached it in 2012, when it was 121 AU from the Sun.

 Remember, however, that the Oort Cloud is 5,100,000 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, 100,000 astronomical units is 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 in 2025.

 (Image: Reproduction/NASA/JPL-Caltech)

 Source: NASA, Live Science

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