question: Why is our solar system flat?
comment: This comes down to the fact that things that are spinning like to keep spinning in that same direction. When a solar system (or galaxy, for that matter) is forming, there is a whole bunch of particles going every which way. Assuming they are all bound (they can't escape the system) you can add up the spinning of each particle. When you do this, you find that there is a *net* spinning of the system as a whole, even if all of the individual particles are going in different directions. This net spinning has to be around a single axis (since we live in 3 spatial dimensions), and so once all of these particles start to collide, they transfer momentum to each other. After a very long time, the random directions start to cancel out due to these collisions and *everything* starts to spin about this one axis. Another way to say that things are spinning around an axis is to say that they are spinning in a plane. So the end result is a bunch of stuff spinning the same direction in the same plane.
passages: Section::::Galactic context.:Neighbourhood.
   Section::::Solar System features.:Scattered disc and Oort cloud.
   Section::::The upper disc.
  
  The upper disc contains a "horizon", that defines the visible part of the sky at any given moment, which is naturally half of the total starry sky. That horizon line is most of the time also distorted, for the same reason the constellations are distorted.
  
  The horizon line on a stereographic projection is a perfect circle.
  
  The horizon line on other projections is a kind of "collapsed" oval.
   Section::::Trans-Neptunian region.:Scattered disc.
   The Solar System's location in the Milky Way is a factor in the evolutionary history of life on Earth. Its orbit is close to circular, and orbits near the Sun are at roughly the same speed as that of the spiral arms. Therefore, the Sun passes through arms only rarely. Because spiral arms are home to a far larger concentration of supernovae, gravitational instabilities, and radiation that could disrupt the Solar System, this has given Earth long periods of stability for life to evolve. The Solar System also lies well outside the star-crowded environs of the galactic centre. Near the centre, gravitational tugs from nearby stars could perturb bodies in the Oort cloud and send many comets into the inner Solar System, producing collisions with potentially catastrophic implications for life on Earth. The intense radiation of the galactic centre could also interfere with the development of complex life. Even at the Solar System's current location, some scientists have speculated that recent supernovae may have adversely affected life in the last 35,000 years, by flinging pieces of expelled stellar core towards the Sun, as radioactive dust grains and larger, comet-like bodies.
   Section::::Scenarios.:Chaos from geological processes.
  
  Another example is Earth's axial tilt which, due to friction raised within Earth's mantle by tidal interactions with the Moon (see below), will be rendered chaotic at some point between 1.5 and 4.5 billion years from now.
  
  Section::::Scenarios.:External influences.
   Section::::Features and interactions.:Gravitational interactions.:Perturbation and instability.
   The Solar System orbits within the Milky Way, a barred spiral galaxy that is a prominent member of the Local Group of galaxies. It is a rotating mass of gas, dust, stars and other objects, held together by mutual gravitational attraction. As the Earth is located within the dusty outer arms, there are large portions of the Milky Way that are obscured from view.
   Section::::Trans-Neptunian region.:Kuiper belt.
   Smaller effects also contribute to nutation. These are caused by the monthly motion of the Moon around the Earth and its orbital eccentricity, and similar terms caused by the annual motion of the Earth around the Sun.
  
  Section::::Nutation of the Earth's axis.:Effect on position of astronomical objects.
   Section::::Alternative hypotheses.:Inclination instability in a massive disk.
   Section::::Comets.
  
  The Kuiper belt was initially thought to be the source of the Solar System's ecliptic comets. However, studies of the region since 1992 have shown that the orbits within the Kuiper belt are relatively stable, and that ecliptic comets originate from the scattered disc, where orbits are generally less stable.
   If the average density of the universe exactly equals the critical density so that formula_8, then the geometry of the universe is flat: as in Euclidean geometry, the sum of the angles of a triangle is 180 degrees and parallel lines continuously maintain the same distance. Measurements from Wilkinson Microwave Anisotropy Probe have confirmed the universe is flat with only a 0.4% margin of error.
   Section::::The Asteroid Belt.:87 Sylvia and its moons Romulus and Remus.
   Section::::Farthest regions.:Boundaries.
   Stability of the Solar System
  
  The stability of the Solar System is a subject of much inquiry in astronomy. Though the planets have been stable when historically observed, and will be in the short term, their weak gravitational effects on one another can add up in unpredictable ways.
  
  For this reason (among others) the Solar System is chaotic in the technical sense of mathematical chaos theory, 
  
  and even the most precise long-term models for the orbital motion of the Solar System are not valid over more than a few tens of millions of years.
   In the inner Solar System the formation of streaming instabilities requires a larger enhancement of the solid to gas ratio than beyond the ice line. The growth of silicate particles is limited by the bouncing barrier to ~1 mm, roughly the size of the chondrules found in meteorites. In the inner Solar System particles this small have Stokes numbers of ~0.001. At these Stokes numbers a vertically integrated solid to gas ratio greater than 0.04, roughly four times that of the overall gas disk, is required to form streaming instabilities. The required concentration may be reduced by half if the particles are able to grow to roughly cm-size. This growth, possibly aided by dusty rims that absorb impacts, may occur over a period of 10^5 years if a fraction of collisions result in sticking due to a broad distribution of collision velocities. Or, if turbulence and the collision velocities are reduced inside initial weak clumps, a runaway process may occur in which clumping aids the growth of solids and their growth strengthens clumping. A radial pile-up of solids may also lead to conditions that support streaming instabilities in a narrow annulus at roughly 1 AU. This would requires a shallow initial disk profile and that the growth of solids be limited by fragmentation instead of bouncing allowing cm-sized solids to form, however. The growth of particles may be further limited at high temperatures, possibly leading to an inner boundary of planetesimal formation where temperatures reaches 1000K.
   If Pluto's orbit is chaotic, then technically the whole Solar System is chaotic, because each body, even one as small as Pluto, affects the others to some extent through gravitational interactions.
  
  Section::::Studies.:Laskar #1.
   Section::::History.
   Over the course of an orbital period, the obliquity usually does not change considerably, and the orientation of the axis remains the same relative to the background of stars. This causes one pole to be directed more toward the Sun on one side of the orbit, and the other pole on the other side—the cause of the seasons on Earth.
  
  Section::::Standards.
   Section::::Solar System features.:Outer-system satellites.
   Several sets of geocaching caches have been laid out as solar system models.
  
  Section::::A model based on a classroom globe.
   However, others argue that the Sun is currently close to the galactic plane, and yet the last great extinction event was 15 million years ago. Therefore, the Sun's vertical position cannot alone explain such periodic extinctions, and that extinctions instead occur when the Sun passes through the galaxy's spiral arms. Spiral arms are home not only to larger numbers of molecular clouds, whose gravity may distort the Oort cloud, but also to higher concentrations of bright blue giants, which live for relatively short periods and then explode violently as supernovae.
  
  Section::::Galactic interaction.:Galactic collision and planetary disruption.
   The right hand side of the last expression above contains constants only and therefore the left hand side must remain constant throughout the evolution of the universe.
   On a medium-sized, low-gravity planet with a very slow rotational period, the side that is farthest from the sun is always a very hard mixture of frozen carbon dioxide, rock hard ammonia, and solid water ice, and the side that is closest, a sea of turbulent liquids and icebergs. The planet rotates so slowly that the part that is half ice and half ocean, is constantly beset with troubling weather conditions ranging from blizzards to unanticipated ice melts and earthquakes. It takes centuries if not thousands of years for the wobbling of the planet on the ecliptic to melt certain parts of the planet. Even still, it is possible for the planet to have become locked in orbit, so the farthest side never melts. The tidal locking of the planet may have occurred hundreds of millions of years ago, and seasons appear to be limited to the slight wobbling of the planet on the ecliptic. Far from being an uninteresting planet, the ecliptic along which the planet travels is also tilted. Most of the colonies are located along the "coast" - the place in a murky half-shadow of constant night and constant day.
  