Do you know how big our solar system is? 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 a 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 30.000.000.000.000 km. The distance is 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!
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 the 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 6,000 asteroids. They all travel 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 the 'Greek camp', while all the others in L5 belong to the 'Trojan camp'. Other planets also collect junk, dust, and 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 the 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 a wide variety of communication devices. Besides the 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 the solar system's biggest planet. In this case, it means that Hilda's aphelion point (the farthest distance from 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 a little bit inside the Jupiter orbit from Trojans and Hildas, soon enough we would stumble into a famous asteroid belt with more than a million rocks larger than 1 km in diameter. At the beginning of the 19th century, among certain groups 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 the solar system with the exception of Neptune and the 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 were found in 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 the discovery of other belt's large and small asteroids. Today we know more about asteroids in the belt and their composition and mass (which is around 4% of the mass of the Moon), and the current theory is that Phaeton never existed and that it was more likely that it was never formed due to heavy attraction from nearby giants. Nevertheless, both Vulcan and Phaeton continued to live in the sci-fi realm and also a couple of mythologies.
If we continue our travel toward the 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 orbit the Sun in less than 200 years. The best-known comet from this bucket is no doubt Halley's Comet. Kuiper Belt was 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 were 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 continued further, and many were 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 an orbit of 3600 years with a pure doomsday scenario, as once in a while it crosses with Earth's orbit and creates a living hell and pretty much the end of 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 a great deal about Kuiper Belt, and in this regard, I will just quote Mike Brown: 'Eris (the 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. The 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 first see what happens just after Kuiper belt in the very region where couple of man-made robots are currently still flying!
Gravity is of course the main property of any star system, but from the 'weird' point of view, our path brings us to the region of the solar system, just outside the most eccentric orbit from the swarm of all objects within the scattered disk. And it has nothing to do with rocky objects, tidal forces, or orbital physics. It's name is heliosphere, and it's the 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, the 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 the interstellar medium. What is interesting about the heliosphere bubble is that it is not really spherically shaped. The sun is traveling around the center of the Milky Way, and this bubble follows, forming a comet-like shape with a tail called a heliotail, composed of particles that escaped the 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 enter the Earth's atmosphere and burn on top. The bow shock is still not confirmed, and perhaps it doesn't exist as the sun might not travel 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 the previous decade. Thanks to both Voyagers, we today know more about the composition and pressure of interstellar gases. Voyager 1 already 'crossed' the heliosphere edge, while Voyager 2 is still inside in the so-called "Heliosheath" region.
Image credits:
* Credit; Charles Carter/Keck Institute for Space Studies
https://exoplanets.nasa.gov/news/1400/interstellar-crossing-the-cosmic-void/
http://www.universetoday.com/32522/oort-cloud
*2 http://map.gsfc.nasa.gov/mission/observatory_l2.html
*3 https://en.wikipedia.org/wiki/Jupiter_trojan
*4 http://sci.esa.int/ulysses/42898-the-heliosphere/
*5 http://www.sciencemag.org/mysterious-oort-cloud-objects
Refs:
http://motherboard.vice.com/blog/new-planets
http://www.scientificamerican.com/article/astronomers-skeptical-over-planet-x-claims/
http://www.universetoday.com/89901/pluto-or-eris-which-is-bigger/
http://news.discovery.com/space/alien-life-exoplanets/mike-brown-planetx-pluto.htm
http://voyager.jpl.nasa.gov/where/
http://physics.stackexchange.com/questions/36092/why-are-l4-and-l5-lagrangian-points-stable
http://www.astrosociety.org/edu/publications/tnl/62/equinox2.html
http://www.nss.org/settlement/L5news/L5history.htm
https://en.wikipedia.org/wiki/L5_Society
https://www.nasa.gov/content/nasa-s-ibex-provides-first-view-of-the-solar-system-s-tail
https://en.wikipedia.org/wiki/Michael_E._Brown
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, some of them not so far away from our own Earth. And just to be clear, the words 'weird' and 'weirdness' I added in the 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 the solar system. We are just trying to understand it.
In such a way, let's start with the first and probably the oldest mystery of the orbiting laws around the Sun. Back then, in the 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 a current, state-of-the-art mathematical model of 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 (or orbital spot where the 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's orbit that might be responsible for Mercury's misbehavior. He even proposed the name 'Vulcan' because of it's potentially very hot orbit so near the Sun. This triggered a series of searches for the Vulcan, and until Einstein came up with the 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 around the Sun inside Mercury's orbit, that is bigger than 6 km in diameter.
Lagrange points *2
The 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 was observed and defined by the great Italian mathematician and astronomer Joseph-Louis Lagrange in the 18th century. He identified five points in the orbital system of two massive bodies from the perspective of a 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 about these 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, a small rock positioned in point L1 would be able to revolve the sun with the same orbital period as the Earth. The same goes with the other four points. However, the first three points are pretty unstable, and objects 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 stories, very stable, and while a spaceship parked in the 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 the '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 the Earth-Sun system, L1 is very interesting as the point of monitoring the Sun without any orbital interruptions (SOHO is located there), L2 is a 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 the 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 the other side of the sun; otherwise, we would detect it's gravitational influence. However, if some aliens exist on the mission of monitoring humankind, they would pretty much choose this place to hide their mother ship.
Of course, the solar system is crowded with plenty of large orbiting objects, and Lagrange points, i.e., the Sun-Earth system, are not really points per se, and due to gravitational influences of other planets, they vary in position depending on the current positions of other planets in their orbits. Same goes for the Lagrangian system of Earth-Moon with their L4/5 points, for example, suffering additional complications due to influence of the Sun. But still, these 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 the 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 the 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 the 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 6,000 asteroids. They all travel 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 the 'Greek camp', while all the others in L5 belong to the 'Trojan camp'. Other planets also collect junk, dust, and 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 the 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 a wide variety of communication devices. Besides the 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 the solar system's biggest planet. In this case, it means that Hilda's aphelion point (the farthest distance from 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 a little bit inside the Jupiter orbit from Trojans and Hildas, soon enough we would stumble into a famous asteroid belt with more than a million rocks larger than 1 km in diameter. At the beginning of the 19th century, among certain groups 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 the solar system with the exception of Neptune and the 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 were found in 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 the discovery of other belt's large and small asteroids. Today we know more about asteroids in the belt and their composition and mass (which is around 4% of the mass of the Moon), and the current theory is that Phaeton never existed and that it was more likely that it was never formed due to heavy attraction from nearby giants. Nevertheless, both Vulcan and Phaeton continued to live in the sci-fi realm and also a couple of mythologies.
If we continue our travel toward the 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 orbit the Sun in less than 200 years. The best-known comet from this bucket is no doubt Halley's Comet. Kuiper Belt was 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 were 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 continued further, and many were 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 an orbit of 3600 years with a pure doomsday scenario, as once in a while it crosses with Earth's orbit and creates a living hell and pretty much the end of 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 a great deal about Kuiper Belt, and in this regard, I will just quote Mike Brown: 'Eris (the 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. The 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 first see 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 system, but from the 'weird' point of view, our path brings us to the region of the solar system, just outside the most eccentric orbit from the swarm of all objects within the scattered disk. And it has nothing to do with rocky objects, tidal forces, or orbital physics. It's name is heliosphere, and it's the 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, the 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 the interstellar medium. What is interesting about the heliosphere bubble is that it is not really spherically shaped. The sun is traveling around the center of the Milky Way, and this bubble follows, forming a comet-like shape with a tail called a heliotail, composed of particles that escaped the 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 enter the Earth's atmosphere and burn on top. The bow shock is still not confirmed, and perhaps it doesn't exist as the sun might not travel 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 the previous decade. Thanks to both Voyagers, we today know more about the composition and pressure of interstellar gases. Voyager 1 already 'crossed' the heliosphere edge, while Voyager 2 is still inside in the so-called "Heliosheath" region.
However, if solar wind stops at the outer edge of the heliosphere, the sun's gravity goes on and influences much further. The proposed boundary where the sun's gravity weakens and loses its dominance is at about 1.5 light years from the sun. This edge is also the edge of the 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 the large distance, it is suggested that it might contain objects captured from other stars from the time of the 'birth cluster' or the beginning of the solar system and other systems while they were in the 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 not be operational by then (unless something happens to it's power source, like in the first Star Trek movie from 1979). The Oort cloud is so big that it's outer circle is not only influenced by the sun's gravity alone but also by the gravity of nearby stars as well as all the influences by tidal forces of the entire Milky Way.
Imagined view of the Oort cloud *5
In a nutshell, the Oort cloud is one giant swarm of icy objects and the potential source of all long-period comets. It is also suggested that many, if not them all, short-period comets originated also from the Oort cloud and were 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 clouds in random orbits, caused by gravitational perturbations originated in galaxy tidal forces, might be fully clustered and notably inclined to orbital panes of planets. As the solution to this clustering or grouping of long-period comets, they proposed the existence of one giant planet inside the 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 clusters 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 a real scientist or astronomer and definitely not a 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 a vast region of space, and while astronomical instruments, along with science itself, are more sophisticated and better every year, I have no doubts that the 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
https://exoplanets.nasa.gov/news/1400/interstellar-crossing-the-cosmic-void/
http://www.universetoday.com/32522/oort-cloud
*2 http://map.gsfc.nasa.gov/mission/observatory_l2.html
*3 https://en.wikipedia.org/wiki/Jupiter_trojan
*4 http://sci.esa.int/ulysses/42898-the-heliosphere/
*5 http://www.sciencemag.org/mysterious-oort-cloud-objects
Refs:
http://motherboard.vice.com/blog/new-planets
http://www.scientificamerican.com/article/astronomers-skeptical-over-planet-x-claims/
http://www.universetoday.com/89901/pluto-or-eris-which-is-bigger/
http://news.discovery.com/space/alien-life-exoplanets/mike-brown-planetx-pluto.htm
http://voyager.jpl.nasa.gov/where/
http://physics.stackexchange.com/questions/36092/why-are-l4-and-l5-lagrangian-points-stable
http://www.astrosociety.org/edu/publications/tnl/62/equinox2.html
http://www.nss.org/settlement/L5news/L5history.htm
https://en.wikipedia.org/wiki/L5_Society
https://www.nasa.gov/content/nasa-s-ibex-provides-first-view-of-the-solar-system-s-tail
https://en.wikipedia.org/wiki/Michael_E._Brown