As NASA's New Horizons probe closes in on its rendezvous with Pluto, astronomers are revisiting the dwarf planet — not physically, but conceptually. Experts will discuss the International Astronomical Union's decision in 2006 to take the icy world out of the traditional nine-planet lineup during a debate starting at 7:30 p.m. ET Thursday. The debate will be presented in Cambridge, Massachusetts, by the Harvard-Smithsonian Center for Astrophysics — and it'll be webcast via YouTube. Science historian Owen Gingerich, who chaired the IAU's planet definition committee and unsuccessfully tried to push through a compromise plan on Pluto, will present the historical viewpoint on planethood. Gareth Williams, associate director of the IAU's Minor Planet Center, will present the IAU's viewpoint. And Harvard's Dimitar Sasselov will provide perspectives from exoplanet research. After the presentations, the audience at Phillips Auditorium will vote on what a planet is or isn't, and whether Pluto is in or out. "And just like the IAU, you have to be present to cast your ballot," the Center for Astrophysics says. Results will be announced Friday.