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Solar System Weirdness

Do you know how big is our Solar system? I can't be sure of course, but there's a strong possibility that common knowledge about our planetary neighborhood ends with enumerating most of the planets, one dwarf planet and couple of named moons, asteroids and comets. Amazingly, the truth is far, far beyond that and believe it or not, if we include Oort cloud, the Solar system, with us representing its only living residents, is approximately 3 light years in diameter. This is, more or less, equal to 3e+13 kilometers or km. The distance about 100 million times bigger than the distance to the Moon. It is tremendously huge and just about one and a half light years shorter than the distance from our Sun to the nearest star!

The layout of the solar system*

So next time when you, through your polluted sky, look up and see the Moon, Venus, Mars, Jupiter and occasionally some comet tail or shooting star, remember that what you see is just a fraction of all the weirdness of everything that is gravitationally bonded to the Sun and to each other. So let's see what we don't see with our eyes and check out some weird places with some of them not so far away from our own Earth. And just to be clear, the words 'weird' and 'weirdness' I added in title and throughout the post are here more for theatrical reasons. Surely, the fact is that, what's weird to me and you is only natural behavior and property of the physics of Solar system. We are just trying to understand it.

In such a way, let's start with first and probably the oldest mystery of the orbiting laws around the Sun. Back then in 19th century French mathematician Urbain Le Verrier tried to study Mercury's orbital motion around the Sun, in order to post an orbital model based on Isaac Newton's laws of motion. It happened almost a century before Einstein's theory of relativity, which is current, state of the art, mathematical model of the gravity and orbital physics, but back then, Verrier's model simply failed to match the observations. In short, Mercury refused to spot itself on predicted spots on the skies and in every orbit its perihelion (orbital spot where planet is closest to the Sun) moved away from predicted places by a small amount. Unfortunately, instead of doubting the equations, like many times before and after in the history, Verrier posted a theory of a new planet or a large orbital body 'inside' Mercury orbit that might be responsible for Mercury's misbehavior. He even proposed a name 'Vulcan', because of it's potentially very hot orbit so near to the Sun. This triggered series of searches for the Vulcan and until Einstein came with theory of relativity (and it's predictions of heavily banded space and time continuum near the heavy objects), that perfectly explained all the observations of one system so close to the massive Sun observed from the distance, many professional and amateur astronomers claimed that they found the Vulcan and spotted its transit over the main star. Perhaps, the final dots to the mystery posted SOHO and STEREO solar missions and neither of them found anything planetoid-ish inside Mercury orbit. Recent calculations go even further and rule out any asteroid, revolving the Sun inside Mercury orbit, that is bigger than 6 km in diameter.

Lagrange points *2

Next weirdness of the gravitational three-dimensional geometry of the Solar system (and all the other star systems out there) are called Lagrange points. Physics is observed and defined by great Italian mathematician and astronomer Joseph-Louis Lagrange in 18th century. He identified five points in the orbital system of two massive bodies from the perspective of third small mass. In short, if we consider, for example Sun and Earth, there are three points on the connecting line between the star and the planet (L1, L2 and L3) and two more, L4 and L5, positioned on the top of equilateral triangles where two other vertices are occupied by Sun and Earth. Now, what is special with this places is that small objects positioned in those points would be able to maintain a stable position relative to the large masses. If you check the image to the left, small rock positioned in point L1 would be able to revolve the Sun with same orbital period as the Earth. The same goes with other four points. However, first three points are pretty unstable and object positioned there would tend to fall out of orbit due to gravitational potential energy shown on the image as well with red and blue arrows. L4 and L5 on the other hand are completely different story, very stable and while spaceship parked in first three points would need to fire engines constantly in order to stay put, the same spaceship in L4 and L5 would be able to shut the engines down and park it there for eternity. Think of it like with 'egg vs equinox' myth - even though you can balance the egg on short or narrow ends (and not just on equinox), this position is pretty unstable and even a little vibration would knock the egg out of balance. Similarly, L4/5 points would be like putting the egg in the eggcup. Scientifically speaking within Earth-Sun system, L1 is very interesting as the point of monitoring the Sun without any orbital interruptions (SOHO is located there), L2 is great place for orbital telescopes (Planck and James Webb Space Telescope) and L3 is pretty useless as it is always hidden by the Sun and therefore origin of all science fiction stories with counter-Earth located in that very point, sharing the orbit with us while we would always be unable to see it. Of course, there is no planet on other side of the Sun, otherwise we would detect it's gravitational influence. However if some aliens exist on the mission of monitoring human kind they would pretty much choose this place to hide their mother ship.

Of course, Solar system is crowded with plenty of large orbiting objects and Lagrange points of i.e. Sun-Earth system are not really points per se and due to gravitational influences of other planets they vary in position depending on current positions of other planets in their orbits. Same goes for Lagrangian system of Earth-Moon with their L4/5 points for example suffering additional complications due to influence of the Sun. But still this points are ideal for some futuristic space-cities orbiting the Earth and some 40 years ago Carolyn Meinel and Keith Henson founded 'The L5 Society' around the idea of Gerard K. O'Neill to build a colony that would be positioned in tiny orbit around L5 point in the Earth-Moon system. In addition, there are also plans to use L1 and L2 points in the system, to build Lunar elevators with appropriate counterweights and 'cables' with use of materials that already exist in production today since they don't require a lot of strength in the process.

Jupiter and inner-solar system asteroids *3

Lastly and the absolute winner in the weirdness competition of the Solar system, related to Lagrange points is Jupiter and it's L4 and L5 points, or in this case regions. Due to these points nature and stability of the orbits within, Jupiter is using them as a, well, sort of, garbage collector. Believe it or not, these two regions are the home for more than 6000 asteroids. They all travels around the Sun with the same speed as their father Jupiter. By astronomical convention, these asteroids are named after the Trojan war and therefore the entire regions are called 'Jupiter Trojans'. Surely, the three largest asteroids in there are conveniently named Agamemnon, Achilles and Hector and the region around L4 is called 'Greek camp', while all the others in L5 belong to the 'Trojan camp'. Other planets also collect junk, dust, small and big asteroids in their L4/5 points and even Earth owns one (discovered so far). It is a rock 300-meter-diameter orbiting the Sun along with Earth in L4. There are also space rocks detected in Saturn's moons and their L4/5 points as well as the dust detected in the Moon's. It will be interesting what we will find in the (far) future when we start exploring Solar system for real. Lagrange points will surely be on the top of all lists to explore, study and use. I am more than positive that lots of L4 and L5 points throughout the Solar system will be used for various space lighthouses, radio beacons and wide variety of communication devices. Besides large number of asteroids caught by Lagrange, there is one more group of 1000+ asteroids gravitationally bonded with Jupiter. Their name is Hildian asteroids and they are in so called orbital resonance with Solar system's biggest planet. In this case it means that Hilda's aphelion point (farthest distance form the elliptical center) is in resonance with the planet and on every third orbit it is positioned directly opposite from Jupiter. The story with inner system asteroids doesn't end here and if we travel little bit inside the Jupiter orbit from Trojans and Hildas, soon enough we would stumble into famous asteroid belt with more than million of rocks larger than 1 km in diameter. At the beginning of 19th century, among certain group of astronomers, including Heinrich Olbers, was very popular so called Bode's law, stating that each planet in any star system would be approximately twice as far from as the one before. Remarkably, it fits nicely in Solar system with exception of Neptune and planet between Mars and Jupiter. Bode initiated a search for the planet to confirm the theory and when during the years 1801 and 1802 Ceres and Pallas are found in the more or less the same orbit, Olbers suggested that they might be remnants of a large planet named Phaeton. The theory flourished in later years, especially after discovery of other belt's large and small asteroids. Today we know more about asteroids in belt and their composition and mass (which is around 4% of the mass of the Moon) and current theory is that Phaeton never existed and its more likely that it was never formed due to heavy attraction from nearby giant. Nevertheless, both, Vulcan and Phaeton continued to live in sci-fi realm and also couple of mythologies.

If we continue our travel toward outer edges of the system, and pass four gas giants, around 30 AU, starts another belt full of heavy objects. Actually, astronomers identified two separate sub-systems, one named 'Kuiper belt' and the other 'Scattered disc'. Just like the main 'inner' asteroid belt, they contain many rocky objects and dwarf planets, with Pluto as the most famous one, but also objects composed from methane, ammonia and water ice. Scattered disk can be described as an elongated subset of the Kuiper belt containing objects with highly eccentric orbits, like short-period comets that orbits the Sun in less than 200 years. The best known comet from this bucket is no doubt Halley's Comet. Kuiper belt is discovered only recently, in the late 20th century and its discovery needs to thank big time to conspiracy theorists and scifi writers. Actually, after the last gas giant Neptune is found by following the lead of the deviations in Uranus orbit that its caused by Neptune, the same lead is pursued further, following similar perturbations in Neptune's orbit. This directly led to the discovery of Pluto, but as soon as it was found that its mass wasn't enough, the search continues further and many was sure that there was another big planet further away, conveniently named Planet X. In the fiction its name was 'Nibiru' with connections to 'ancient astronauts' theorists who gave it orbit of 3600 years with pure doomsday scenario, as once in a while it crosses with Earth's orbit and creates a living hell and pretty much end of the life as we know it. Of course, this was just another nonsense and pseudo-science but eventually and most thankfully to astronomer and unofficial father of 'Kuiper belt', Mike Brown, who discovered lots of small Trans-Neptunian objects beyond Pluto, we today know great deal about Kuiper belt and in this regard I will just quote Mike Brown: 'Eris (biggest TNO along with Pluto so far) and Pluto and all of the rest of them have only a trivial impact on our solar system. You could get rid of any of them (I have a vote which ones, too) and nothing much would change.' Recently, with more precise measurements of Neptune's mass, new calculation of its orbit accounted for all observed perturbations and deviations. However, this didn't mean Planet X doesn't exist. New theory just pushed it more beyond toward the edge of Solar system and it earned new name. This time it is called Tyche and it's location might be somewhere in Oort cloud. But before we encounter this final system's weirdness, let's see first what happens just after Kuiper belt in the very region where couple of man-made robots are currently still flying!

Solar system Heliosphere *4

Gravity is of course the main property of any star systems, but from the 'weird' point of view, our path brings us to the region of Solar system, just outside the most eccentric orbit from the swarm of all objects within Scattered disk. And it has nothing to do with rocky objects, tidal forces and orbital physics. It's name is Heliosphere and it's a first boundary of our system we can positively identify. This is the real edge of the system where ultimately solar winds finish their travel. Solar wind represents ionized particles emitted by solar corona and they start traveling at around four times the speed of sound in the interstellar medium. Geometrically speaking, Heliosphere is actually a bubble around the Sun and all the planets and other objects and it starts from the point where solar winds, due to interaction with Solar system particles, slow down to the subsonic speed and end at the point when they fully stop, or more precisely, reach pressure balance with interstellar medium. What is interesting with Heliosphere bubble is that it is not really spherically shaped. Sun is traveling around Milky Way center and this bubble follows forming a comet-like shape with a tail, called Heliotail, composed of particles that escaped Heliosphere slowly evaporating because of charge exchange with interstellar media and particles from other stars. It was also speculated that throughout Solar system travel, the front edge might create a turbulence edge, a bow shock, similarly to the meteors or satellites that enters the Earth atmosphere and burn on front. The Bow shock is still not confirmed and perhaps it doesn't exists as the Sun might not traveling with enough speed to form it. But it is observed in the motion of a star system called Mira, red giant in the constellation Cetus by GALEX, an orbiting ultraviolet space telescope in previous decade. Thanks to both Voyagers, we today know more about composition and pressure of interstellar gases. Voyager 1 already 'crossed' Heliosphere edge while Voyager 2 is still inside in so called "Heliosheath" region.

However, if solar wind stops at the outer edge of Heliosphere, Sun's gravity goes on and influences much further. The proposal boundary where Sun's gravity weakens and lost its dominance is at about 1,5 light years from the Sun. This edge is also the edge of theoretical Oort cloud, a spherical disk filled with remnants of the original protoplanetary disc from around the Sun at the time of Solar system creation, about 4.6 billion years ago. Due to large distance it is suggested that it might contain objects captured from other stars from the time of 'birth cluster' or the beginning of Solar system and other systems while they were in process of departing from each other. Oort cloud, even not scientifically confirmed today, could start with its inner circle at about 2000 AU or so. One day when Voyager 1 reaches the region (in about 300 years) it would need another 30000 years to pass it through entirely. Unfortunately V'Ger will be not operational by then (unless something happens to it's power source, like in first Star Trek movie form 1979). Oort cloud is so big that it's outer circle is not only influenced by the Sun gravity alone, but also by gravity of nearby stars as well as all the influences by tidal forces of entire Milky Way.

Imagined view of the Oort cloud *5

In the nutshell, Oort cloud is one giant swarm of icy objects and potential source of all long-period comets. It is also suggested that many, if not them all, short-period comets originated also from Oort cloud and captured by gas giants, especially Jupiter. The story of long-period comets is the one responsible for the new planet X location or Tyche, I mentioned before. Some 15 years ago, astrophysicists John Matese, Patrick Whitman and Daniel Whitmire proposed a theory that long-period comets instead of coming from Oort cloud in random orbits, caused by gravitational perturbations originated in galaxy tidal forces, might be fully clustered and notably inclined to orbital pane of planets. As the solution to this clustering or grouping of long-period comets, they proposed existence of one giant planet inside Oort cloud that is either similar to Jupiter only 3-4 times bigger, or even a brown dwarf, a failed star that would count our Solar system as a, sort of, binary star system, which are the most common systems in the galaxy. However, this theory, even though the most plausible of them all encountered, to add more big planets into our Solar system, lacks enough data to spot cluster of long-period comets as their orbital periods are in the realm of thousands of years. Additionally, within the Wide-field Infrared Survey Explorer space telescope mission and its all-sky infrared survey data, no such dwarf or big planet was found. Even more, WISE ruled out the possibility of a Saturn-sized object at 10,000 AU, and a Jupiter-sized or larger object out to 26,000 AU. If it still exists, Tyche might be even further away which also might mean that it could also harbor large moons of its own. Another bold theory, but more likely is that it doesn't exist at all and we just need to learn more about Oort cloud complex physics to understand it fully.

I will be careful while concluding anything substantial out of this post. The fact is that I am not real scientist or astronomer and definitely not conspiracy theorist or pseudo-science admirer. To be on the safe side, I can say this - posting new theories in astronomy and cosmology from the surface of Earth is way easier than confirming them. We are talking about vast region of space and while astronomical instruments along with science itself are more sophisticate and better every year I have no doubts that real breakthrough in this realm will come only when we eventually rise up and approach closer 'and see' for ourselves. I also have doubts that this will not happen any time soon, especially not in my or your life span.

Until then, metaphorically speaking, we will continue peeking out of the window and doing math from the distance. And continue to dream about wonders and weirdness of the heavens, waiting for us to come, see and finally understand.

Image credits:
* Credit; Charles Carter/Keck Institute for Space Studies


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