question: If all planets were flat, including stars, moons, etc, how would that effect the solar system?
comment: They would all immediately start cracking and crushing toward their centers and end up roughly spherical. They're spheres because of gravity.
passages: Section::::Potential problems.
  
  Most of the accretion of Mars must have taken place outside the narrow annulus of material formed by the grand tack if Mars has a different composition than Earth and Venus. The planets that grow in the annulus created by the grand tack end with similar compositions. If the grand tack occurred early, while the embryo that became Mars was relatively small, a Mars with a differing composition could form if it was instead scattered outward then inward like the asteroids. The chance of this occurring is roughly 2%.
   Section::::Evolution and long-term stability of anti-aligned orbits.
   The roughly spherical shape of the Earth can be confirmed by many different types of observation from ground level, aircraft, and spacecraft. The shape causes a number of phenomena that a flat Earth would not. Some of these phenomena and observations would be possible on other shapes, such as a curved disc or torus, but no other shape would explain all of them.
  
  Section::::Effects and empirical evidence.:Spacecraft.
   Section::::Development of the Nice model.:Timing of the instability.
   For the WGS84 ellipsoid to model Earth, the "defining" values are 
  
  from which one derives
  
  so that the difference of the major and minor semi-axes is . (This is only  0.335% of the major axis, so a representation of Earth on a computer screen would be sized as 300px by 299px. Because this would be virtually indistinguishable from a sphere shown as 300px by 300px, illustrations typically greatly exaggerate the flattening in cases where the image needs to represent Earth's oblateness.)
   The solar eclipse of 21 August 2017 gave rise to numerous YouTube videos purporting to show how the details of the eclipse prove the Earth is flat. Also in 2017, "the Tunisian and Arab scientific and educational world" had a scandal when a PhD student submitted a thesis "declaring Earth to be flat, unmoving, young (only 13,500 years of age), and the centre of the universe".
   From these laws, we know the amount of mass the Earth contains, and that a non-spherical planet the size of the Earth would not be able to support itself against its own gravity. A flat disc the size of the Earth, for example, would likely crack, heat up, liquefy, and re-form into a roughly spherical shape. On a disc strong enough to maintain its shape, gravity would not pull downward with respect to the surface, but would pull toward the center of the disc, contrary to what is observed on level terrain (and which would create major problems with oceans flowing toward the center of the disk).
   Imagine for a moment what will happen to the lines of latitude and longitude when they are projected from the sphere onto the flat plane. The lines of latitude are all parallel to the equator, so they will become perfect circles centered on the origin . And the lines of longitude will become straight lines passing through the origin (and also through the "point at infinity", since they pass through both the north and south poles on the sphere).
   Section::::Development of the Nice model.:Ejected planet.
   The Vedic texts depict the cosmos in many ways. The earliest Indian cosmological texts picture the Earth as one of a stack of flat disks.
   On a perfectly spherical Earth, flat terrain or ocean, when viewed from the surface, blocks exactly half the sky - a hemisphere of 180°. Moving away from the surface of the Earth means that the ground blocks less and less of the sky. For example, when viewed from the Moon, the Earth blocks only a small portion of the sky, because it is so distant. This phenomenon of geometry means that when viewed from a high mountain, flat ground or ocean blocks less than 180° of the sky. The rate of change in the angle blocked by the sky as altitude increases is different for a disc than a sphere, and values observed show that the Earth is locally convex. (The angles blocked would also be different for a mountain close to the edge of a flat Earth compared to a mountain in the middle of a flat Earth, and this is not observed.) In theory, measurements of this type from all around the Earth would confirm that it is a complete sphere (as opposed to some other shape with convex areas) though actually taking all those measurements would be very expensive.
   Section::::Clustering of orbital poles (nodal alignment).
   Section::::Non-spherically symmetric objects.
  
  Most real astronomical objects are not absolutely spherically symmetric. One reason for this is that they are often rotating, which means that they are affected by the combined effects of gravitational force and centrifugal force. This causes stars and planets to be oblate, which means that their surface gravity is smaller at the equator than at the poles. This effect was exploited by Hal Clement in his SF novel "Mission of Gravity", dealing with a massive, fast-spinning planet where gravity was much higher at the poles than at the equator.
   Section::::Surface geology of inner solar planets.:Venus.
  
  The surface of Venus is comparatively very flat. When 93% of the topography was mapped by "Pioneer Venus", scientists found that the total distance from the lowest point to the highest point on the entire surface was about 13 kilometres (8 mi), while on the Earth the distance from the basins to the Himalayas is about 20 kilometres (12.4 mi).
   Section::::Solar System effects.
   Section::::Potential issues and an alternative.
  
  A study using a numerical simulation that included gravitational interactions among all objects revealed that a dynamical instability occurred in less than 70 million years. Interactions between planetesimals dynamically heated the disk and lead to earlier interactions between the planetesimals and giant planets. This study used a limited number of planetesimals due to computational constraints so it is as yet unknown whether this result would apply to a more complete disk.
   In the example to the right, Earth's Hill sphere extends between the Lagrangian points and , which lie along the line of centers of the two bodies. The region of influence of the second body is shortest in that direction, and so it acts as the limiting factor for the size of the Hill sphere. Beyond that distance, a third object in orbit around the second (e.g. the Moon) would spend at least part of its orbit outside the Hill sphere, and would be progressively perturbed by the tidal forces of the central body (e.g. the Sun), eventually ending up orbiting the latter.
   Section::::Development of the Nice model.:Initial conditions.
   When the fragmentation due to hit and run collisions are included in simulations with an early instability the orbits of the terrestrial planets are better produced. The larger numbers of small bodies resulting from these collisions reduce the eccentricities and inclinations of the growing planets orbits via additional collisions and dynamical friction. This also results in a larger fraction of the terrestrial planets mass being concentrated in Venus and Earth and extends their formation times relative to that of Mars.
   The ancients assumed the literal truth of stars attached to a celestial sphere, revolving about the Earth in one day, and a fixed Earth. 
  
  The Eudoxan planetary model, on which the Aristotelian and Ptolemaic models were based, was the first geometric explanation for the "wandering" of the classical planets. The outer most of these "crystal spheres" was thought to carry the fixed stars. Eudoxus used 27 concentric spherical solids to answer Plato's challenge: "By the assumption of what uniform and orderly motions can the apparent motions of the planets be accounted for?"
  
  Section::::Star globe.
   Section::::Notes.:Manual calculations (unless otherwise cited).
  
  (1-qp_{\nu})^{1/4}}{\sqrt{2}}\sqrt{52/r},\end{smallmatrix} where T =54.8 K at 52 AU, formula_1 is the geometric albedo, "q"=0.8 is the phase integral, and formula_2 is the distance from the Sun in AU. This formula is a simplified version of that in section 2.2 of Stansberry et al., 2007, where emissivity and beaming parameter were assumed equal unity, and formula_3 was replaced with 4 accounting for the difference between circle and sphere. All parameters mentioned above were taken from the same paper.
  
  Section::::Notes.:Individual calculations.
   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.
   Section::::History.:Alternate or mixed theories.:Greece: spherical Earth.
   BULLET::::- a planet might yet have some shred of sect dignity if it is in the hemisphere of the chart corresponding to its inherent sect--for example, if Jupiter is in the same hemisphere as the Sun, whether or not the Sun is above the horizon, or if Venus is in the hemisphere opposite the Sun, whether or not the Sun is below the horizon.
  
  Planets satisfying all three of these sect conditions were said to be "Hayz", but it is not clear how Hayz strength compares to strength from essential dignities.
  
  Section::::Further reading.
   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.
  