Abstract
Technically speaking, there are loads of outer planets. We all know the gas giants Jupiter and Saturn, as well as the ice giants Uranus and Neptune, as well as the double planet Pluto-Charon, but the likelihood is that there are many more minor planets waiting to be discovered. One of the primary considerations for whether or not something is a planet comes down to its shape: planets (regardless of type) are large enough to compress themselves into a spherical shape, while asteroids are not.
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Notes
- 1.
The IAU definition of ‘planet’ requires a “spherical shape in hydrodynamic equilibrium”, which allows for things like a bulge at the equator due to rotation. For simplicity, we will simply refer to the shape of a planet as spherical.
- 2.
It was first the Voyager probes (1977) and later the Galileo orbiter (1992) that accurately measured the diameters of the Galilean moons and used their gravitational interaction with Jupiter to effectively ‘weigh’ them.
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Barth, D.E. (2022). Discovering the Outer Planets. In: Star Mentor: Hands-On Projects and Lessons in Observational Astronomy for Beginners. The Patrick Moore Practical Astronomy Series. Springer, Cham. https://doi.org/10.1007/978-3-030-98771-8_9
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DOI: https://doi.org/10.1007/978-3-030-98771-8_9
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