Let's dig into the definition and see some fundamental characteristics...
The planetary orbits around the Sun are almost coplanar ellipses which deviate only slightly from circles. This means that if you could look at them from above, you would notice something very similar to eight planets traveling along circles around the Sun.
The orbital planes of asteroids, minor bodies that circle the Sun mainly between the orbits of Mars and Jupiter, are often more tilted than the planes of the planetary orbits. Asteroids and distant Trans-Neptunian Objects revolve in the same direction as the major planets; comets, however, may move in the opposite direction. Cometary orbits can be very elongated, even hyperbolic, which means that part of their orbit lies outside of our Solar System!
Since the 1960’s a vast amount of data have been collected using spacecraft, either during a flyby, orbiting a body, or directly by landing on the surface. As a consequence, many methods traditionally used in various branches of geophysics can now be applied to planetary studies.
The shape and irregularities of the gravitation field generated by a planet reflect its shape, internal structure and mass distribution. Also the surface gives certain indications on internal structure and processes. The perturbations in the orbit of a satellite or spacecraft can be used to better understand the internal structure of a planet and any deviation from spherical symmetry is visible in the external gravitational field.
If we deep more into the technical side, the IAU planet definition states that planets are bodies in hydrostatic equilibrium. Hydrostatic equilibrium means that the surface of the body approximately follows an equipotential surface of gravity. This is true e.g. on the Earth, where the sea surface very closely follows the equipotential surface called the geoid. Due to internal strength of rocks, continents can deviate from the geoid surface by a few kilometers but compared to the diameter of the Earth the surface topography is negligible.
It can be surprising how different a planet can look like without water...
Beyond the gravity field, also the general shape of a planet is affected by its rotation around its axis: this phenomenon is called flattening. In general, a rotating planet is always flattened. The amount of flattening depends on the rotation rate and the strength of the material. Think about a liquid drop: it is more easily deformed than a rock.
The giant planets are in practice close to hydrostatic equilibrium, and their shape is determined by the rotation. Asteroids and other minor bodies are so small in mass that there is an upper limit for the rotation rate before they break apart due to centrifugal forces winning over gravity!
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Some has already been published about this topic, and more will be in the future!
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