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    Laurel Kornfeld said:

    Our System Has More Than Eight Planets

    Nine planets is NOT more than our own solar system has; it’s less. Dwarf planets are planets too, both structurally and compositionally. They are in hydrostatic equilibrium and orbit a star. In fact, Dr. Alan Stern coined the term “dwarf planet” in 1991 to designate a third class of planets, small planets that are very much akin to the larger ones except for their size.

    According to the geophysical planet definition, a planet is any non-self-luminous spheroidal body in orbit around a star (thank you, astronomer extraordinaire Al Witzgall for this great definition). An object does not have to gravitationally dominate its orbit to be considered a planet, according to this equally scientific and legitimate definition. It would be a great service if you could acknowledge that the IAU planet definition is just one side in an ongoing debate and not fact.

    As Dr. Stern said, our solar system made many more planets than we learned about in school. At minimum, the number of planets in our solar system is 16: Mercury, Venus, Earth, Mars, Ceres, Jupiter, Saturn, Uranus, Neptune, Pluto, Haumea, Makemake, Eris, and three unnamed dwarf planets discovered in 2011.

    As far as exoplanets, we have no way of knowing if any of the more than 700 confirmed gravitationally dominate their orbits.

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      Torbjorn Larsson said:

      This is important since it confirm that systems are individuals but that our system isn’t exceptional in every way.

      @ Laurel Kornfield:

      Your commentary is totally besides the point of this awesome result. They have potentially observed is more planets than a corresponding observer looking at our system would see, excluding Mercury most likely they would see 6-7 of our 8 planets.

      The problem with exoplanet studies referring back to our system is that we are trying to compare observational apples with pears. But most exoplanets will fulfill the planethood dynamic index criteria, which means a planet will be able to eventually clear its neighborhood and up its chances to remain in a stable orbit for long times, since those bodies are the ones we will tend to see.

      One can debate whether such a dynamical criteria is useful for our system, but it seems useful elsewhere and in comparison everywhere. Other planet criteria, not so much.


      We do have ways of knowing this absolutely as well as relatively statistically, as I noted above, because a) eventual clearing derives from a dynamical criteria based on orbital and planetary characteristics b) “domination” can be directly observed by some of the same methods used to find the planets in the first place – the “tug-of-war” is how the Kepler team could confirm some dense systems right away, and it is also potentially seen in cases of shepherding (say, Fomalhaut, though that example isn’t decided as of yet).

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