Apr 24, 2015
Posted by Benjamin T. Solomon in categories: astronomy, cosmology, human trajectories, innovation, science, space, space travel, transportation
Until 2006 our Solar System consisted essentially of a star, planets, moons, and very much smaller bodies known as asteroids and comets. In 2006 the International Astronomical Union’s (IAU) Division III Working Committee addressed scientific issues and the Planet Definition Committee address cultural and social issues with regard to planet classifications. They introduced the “pluton” for bodies similar to planets but much smaller.
The IAU set down three rules to differentiate between planets and dwarf planets. First, the object must be in orbit around a star, while not being itself a star. Second, the object must be large enough (or more technically correct, massive enough) for its own gravity to pull it into a nearly spherical shape. The shape of objects with mass above 5×1020 kg and diameter greater than 800 km would normally be determined by self-gravity, but all borderline cases would have to be established by observation.
Third, plutons or dwarf planets, are distinguished from classical planets in that they reside in orbits around the Sun that take longer than 200 years to complete (i.e. they orbit beyond Neptune). Plutons typically have orbits with a large orbital inclination and a large eccentricity (noncircular orbits). A planet should dominate its zone, either gravitationally, or in its size distribution. That is, the definition of “planet” should also include the requirement that it has cleared its orbital zone. Of course this third requirement automatically implies the second. Thus, one notes that planets and plutons are differentiated by the third requirement.
As we are soon to become a space faring civilization, we should rethink these cultural and social issues, differently, by subtraction or addition. By subtraction, if one breaks the other requirements? Comets and asteroids break the second requirement that the object must be large enough. Breaking the first requirement, which the IAU chose not address at the time, would have planet sized bodies not orbiting a star. From a socio-cultural perspective, one could suggest that these be named “darktons” (from dark + plutons). “Dark” because without orbiting a star, these objects would not be easily visible; “tons” because in deep space, without much matter, these bodies could not meet the third requirement of being able to dominate its zone.