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If you want to know more about any particular topic, just hit one of the questions marks. These asides will give you more in-depth information about synestias and hopefully give you a little more food for thought.
Synestias are formed when a planetary body exceeds the corotation limit. The corotation limit is reached when the equator of a body is rotating as fast as a satellite would if it were in orbit. When this happens, the outer edge of the body cannot stay rotating at the same rate as the rest and must rotate at slower, orbital velocities.
The transition from a planet to a synestia is illustrated in the figure below. Each column shows an Earth-mass body with the same angular momentum (same amount of rotational inertia, learn more here) but with the bodies getting hotter from left to right. The top row shows pressure contours through each body, so that the rotation axis is along the vertical axis on the page. Yellows are lower pressures and blues are higher pressures but the important thing to notice is the shape of the body. As you go from left to right the body changes shape dramatically. The planet on the left is a flattened sphere, an ellipsoid, but the two bodies on the right are much more extended and look a little like flying saucers. This is because the body on the left is below the corotation limit and the two on the right are above the limit and are synestias. But what happens at the corotation limit that changes the shape?
The second row shows the angular velocity of each parcel of rock in a slice through the equator of the body, with each parcel a black dot. Angular velocity is the revolutions a body does in a certain period of time - the same as revolutions per minute for your car engine. The body on the left is below the corotation limit, so the whole planet is rotating at the same rate. However, the body in the middle is hotter and so it is more expanded. As the body expands, the outermost edge begins to rotate at the same rate as satellites in orbit, given by the red line. The parcels of mass in the outer edge can't keep rotating with the rest of the body and adopt near-orbital velocities. The region that is not rotating with the rest of the body we call the disk-like region.
The outer edge is now rotating slower than the rest of the body and so has a lower angular momentum than an equivalent parcel rotating at the same rate as the rest of the body. The lower panel shows the specific angular momentum of each of the parcels and shows the angular momentum of orbiting (red solid line) and co-rotating particles (red dashed line). Angular momentum needs to be conserved and so the outer edge of the body must spread out. This spreading out leads to the extended shapes of synestias. To learn more about the range of different synestias and their shapes see the next section.
We name structures beyond the corotation limit synestias. The name is derived from combining the Greek prefix syn, meaning together, with Hestia, the Greek goddess of architecture. Hestia means hearth and was closely associated to home life as well as buildings and structures. The name synestia therefore loosely means 'together structure' to emphasize the continuous nature of synestias.
Synestias can have a wide variety of shapes from ellipsoids with fins of material extending from the equator to giant red blood cell-shaped structures. The figure below shows a range of the possible shapes and sizes of a body of the same mass but varying thermal and rotational states. Shown are pressure contours in cross sections through each body with the rotation axis parallel to the vertical axis on the paper. Hotter colors are lower pressures and colder colors are higher pressure, but the most important thing to notice is the shape of the bodies. The bodies are getting hotter from left to right and the angular momentum (amount of rotational inertia, learn more here) of each body is increasing from top to bottom. Bodies to the left and above the red line are all rotating as solid bodies and are therefore are all squished spheres, ellipsoids. In contrast, bodies to the right and below the red line are synestias and can adopt a wide range of shapes and have a large range of sizes.
Terrestrial planets grow by collisions between different bodies. The most energetic of these collisions are called giant impacts. How likely are giant impacts to generate a synestia? We have analyzed the results of simulations of growth of terrestrial planets (see here or here). We expect most planets to be rapidly rotating at the end of formation, but we need to know how many impacts have sufficient energy to push a body above the corotation limit to form a synestia.
We find that most bodies that form large planets (with a mass greater than half the mass of the Earth) experience many impacts that could form synestias. The figure below shows the fraction of bodies that experienced a given number of impacts that had enough energy to possibly produce a synestia (black line) and to produce a high-energy synestia (red line) in a suite of planet formation simulations. Almost all the bodies experienced an impact with enough energy to form a synestia.