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3. List the three defining characteristics of a planet. The presence of an atmosphere is not a requirement for an object to be considered a planet, according to the IAU definition. Use the Internet to cite at least two examples in our solar system that illustrate why the existence/non- existence of an atmosphere does not qualify an object for planetary status. -the three defining characteristics of a planet by the IAU standards are orbits the sun sufficient mass for gravity to shape it cleared its orbit , Mercury and Pluto are perfect examples that the presence of an atmosphere is not a requirement fo be considered a planet. Pluto isn't considered a but its because it did not clear its orbit. Mercury is a planet without an atmosphere but it does meet all three characteristics of a planet.