Atmosphere, moons, geologic activity… Why isn’t Pluto a planet? Mike Brown doesn’t hate Pluto. As it happens, a Caltech astronomer killed Pluto — by discovering Eris, a small, icy world about the size of Pluto. Brown’s work led to both worlds being classified as 'dwarf planets' in 2006. Still, he devoured news of the probe’s flyby of the former planet. For Brown, the Pluto data sheds light on Eris and the other worlds he’s discovered at the edge of the solar system. 'With Pluto and Charon, you get a whole bunch of ideas about how the big Kuiper Belt objects behave,' he says. 'I look at Pluto, but I also have a little bit of Eris and Makemake in my head. Quaoar is probably somewhere between Pluto and Charon, with these old impact zones and a reddish tint.' If Pluto were a planet, then many of the worlds Brown discovered would be planets, meaning he would have discovered more planets in the solar system than any human in human history. That would be cool, he says, 'but also a bit of a scam.' The flood of new worlds on the outskirts of the solar system is what led to Pluto’s demotion. There are at least four potential planets in the outer reaches of the solar system; more are likely lurking in the rock-choked Kuiper Belt. Scientists still can’t agree on what to make of this mess of small, icy worlds. The New Horizons flyby of Pluto on July 14 has brought life to the former planet. High-resolution images have restored the face of this once-mysterious world. We’ve studied its atmosphere and photographed its moons for the first time. Ice peaks poke through the lower atmosphere, and nitrogen glaciers reveal a once-turbulent geologic history. 'What is it if it’s not a planet?' asks New Horizons mission director Alan Stern. Defiance of the International Astronomical Union, which names objects in space and actually changed Pluto’s status in 2006, Stern and his colleagues refer to Pluto as a planet during NASA televised events. This indulgence in Pluto does a disservice to the public, Brown says. 'As an educator, I want people to understand what our solar system is really like, not some cartoon. Calling Pluto a planet over and over again is just childish.' The irony is that the Pluto flyby has predictably resurrected a long-running debate about what a planet is. Size Matters On one side of the debate, we have scientists who say size matters. According to the IAU, a 'planet' is an object that a) orbits the Sun; b) has enough gravity to be round; and c) is large enough to scrape debris out of its orbit. Since Pluto lives in the crowded Kuiper Belt, it falls into the third category. So do Eris, Haumea, Makemake, and Ceres. So to keep the Solar System from becoming too crowded, the IAU changed its definition of planet in 2006. 'People who call Pluto a planet think of it as a big object in the Solar System,' Brown says. 'But it’s smaller than the Moon. If you were coming from another solar system, you’d immediately see eight big bodies and lots of little bodies in between.' And while Pluto has moons, an atmosphere, and a mobile surface, scientists on this side of the debate argue that that's not enough to qualify Pluto as a planet. Many of the largest bodies in the solar system have been geologically active in the past. Comets have atmospheres and complex surfaces, says planetary scientist Carolyn Porco, and asteroids have moons. Geological features don't figure into the IAU definition at all. Size Doesn't Matter Some planetary scientists, like the aforementioned Alan Stern and Mark Sykes of the Daw mission, think size doesn't matter. They say any object that orbits the sun and has enough gravity to flatten itself into a spherical shape is a planet. "When they get that big, they start to become geologically active," Sykes says. Scientists on this side of the debate don't like the IAU definition at all, because it's more about where an object is located than what it actually is. If Earth were in Pluto's position, it wouldn't even be able to clear its orbit, Stern says. And while Earth's diameter is five times yt3.ggpht.com