With the rise of the heliocentric design, objects that orbit the Sun were worlds, suggesting that Earth was a planet, but so was the Moon. When Galileo discovered four moons of Jupiter, he referred to them as the Medicean planets. When Cassini found Saturns moon Titan, he referred to it as a brand-new planet.
Titan has seas, mountains, and intricate geology. Credit: NASA/ JPL-Caltech/ University of Arizona
For Galileo, a moon is a planet that orbits another world, called after the very first of that name. Planet and moon were not exclusionary terms.
When astronomers found Ceres in 1801, it was clearly a world. After a lots worlds were found in between Jupiter and Mars, lots of astronomers argued that they should be not planets, but asteroids.
Moons and asteroids ought to not be considered planets. When Pluto was discovered in 1930, it was clearly a planet due to the fact that it was neither an asteroid nor a moon. None of these fulfill the IAU meaning of a classical world, however its hard to argue that they are not worlds as complex as Mars or Venus.
Venn diagram of world definitions. Credit: Metzger, et al
. What makes a world a planet? Based upon their work, the team argues that the IAU meaning is a bad one. The basic public dislikes it since the meaning excludes Pluto, but more significantly, many scientists disregard the definition and still refer to bodies such as Titan, Pluto, Ceres, and others as worlds. What seems to be the most consistent specifying element is that of complex geology and geophysics.
If we define worlds by their geophysical qualities, then the Galilean Moons are worlds, as is Pluto, as is Plutos moon Charon, as is our own Moon. Anything with a diameter larger than about 500km would be a planet, suggesting that our solar system alone has more than a hundred planets.
Recommendation: Metzger, Philip T., et al. “Moons are planets: Scientific effectiveness versus cultural teleology in the taxonomy of planetary science.” Icarus (2021 ): 114768.
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What makes a planet a planet? The answer turns out to be rather contentious. The main meaning of a planet, as specified by the International Astronomical Union (IAU) is that a world should satisfy three conditions:
It needs to orbit the Sun.It should remain in hydrostatic equilibrium.It must have cleared its orbital neighborhood.
By this meaning there are just eight planets in our solar system, most especially excluding Pluto. This has actually stirred all way of debate, even amongst astronomers. Several alternative meanings have been proposed, but a brand-new study argues we ought to seek to history for the option.
What makes a planet a planet? The main definition of a world, as specified by the International Astronomical Union (IAU) is that a planet should satisfy three conditions:
With the rise of the heliocentric model, objects that orbit the Sun were planets, implying that Earth was a planet, however so was the Moon. For Galileo, a moon is a planet that orbits another world, called after the very first of that name. What makes a planet a planet?
