Published: Mon, March 20, 2017
Research | By Jo Caldwell

Whether Pluto is a planet or not: all worlds are worth investigating

Whether Pluto is a planet or not: all worlds are worth investigating

That definition change meant Pluto was no longer considered the "ninth" planet, but instead a dwarf planet and Kuiper Belt Object.

The debate of whether Pluto is a planet or not has caught fire again with an astronomer from Johns Hopkins University gunning for Pluto so as to reinstate its lost planetary status.

The debate over Pluto's planetary status started after the International Astronomical Union in 2006 demoted Pluto to "non-planet", thus dropping the consensus number of planets in our solar system from nine to eight. He said that Pluto is basically being a planet, Phys.org reported.

Still, Pluto "as everything going on on its surface that you associate with a planet".

That expansion is part of the appeal of the new definition, Runyon says. All the authors are science team members on the New Horizons mission to Pluto, operated for NASA by the John Hopkins University Applied Physics Laboratory. In 2015, the New Horizons spacecraft became the first to fly by Pluto, some 4.67 billion miles from Earth, passing within 8,000 miles and sending back the first close-up images ever made of Pluto.

The IAU definition had two factors that essentially eliminated Pluto in 2006: the planet must be able to gravitationally hold its own in orbit, and clear the zone around itself, and it must be big enough to do so at progressive distances outward from the sun.

Their definition of a planet requires only that a celestial body to have never undergone nuclear fusion and that it have enough gravity to be more or less round. That portion - which required that a planet and its satellites move alone through their orbit - excluded Pluto. The new geophysical definition does not include stars, asteroids, black holes and meteorites but everything else in solar system is included.

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The new definition, which willing researchers can adopt without approval from a central governing body, is also more useful to planetary scientists, Runyon says.

Would having a 102 "new" planets stimulate interest in Solar System exploration? "It drives home the point of continued exploration". Most planetary scientists are also generally trained as geoscientists more than astrologists, so a geophysical definition might suit them better than the old classification.

The other authors are Kelsi Singer of the Southwest Research Institute in Boulder, Colorado; Tod Lauer of the National Optical Astronomy Observatory in Tucson, Arizona; Will Grundy of the Lowell Observatory in Flagstaff, Arizona; Michael Summers of George Mason University in Fairfax, Virginia.

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