If you're confused about what exactly a planet is, don't feel bad: Astronomers are still arguing over the term eight years after the International Astronomical Union (IAU) came up with a controversial new definition that demoted Pluto to "dwarf planet" status.

You can get an inside look at the ongoing debate tonight (Sept. 18) at 7:30 p.m. EDT (2330 GMT) during an event organized by the Harvard-Smithsonian Center for Astrophysics (CfA) called "What Is a Planet?" You can watch the lecture live here on Space.com, courtesy of the CfA, or on the CfA's "Observatory Nights" YouTube channel: http://www.youtube.com/user/ObsNights

Three experts will participate in tonight's event, CfA representatives said in a media advisory:

"Science historian Dr. Owen Gingerich, who chaired the IAU planet definition committee, will present the historical viewpoint. Dr. Gareth Williams, associate director of the Minor Planet Center, will present the IAU’s viewpoint. And Dr. Dimitar Sasselov, director of the Harvard Origins of Life Initiative, will present the exoplanet scientist’s viewpoint."

At the end of the debate, the audience will get to vote on its preferred definition of "planet," and whether or not Pluto makes the cut. Results will be announced Friday (Sept. 19), CfA representatives said.

Pluto, the most famous dwarf planet in our solar system, underwent a well-publicized (and somewhat controversial) reclassification that took away its title as the ninth and most distant planet from the sun. So, how well do you know this fascinating world?
Hubble Space Telescope photo of Pluto is most detailed ever seen.
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Pluto Quiz: How Well Do You Know the Dwarf Planet?
Pluto, the most famous dwarf planet in our solar system, underwent a well-publicized (and somewhat controversial) reclassification that took away its title as the ninth and most distant planet from the sun. So, how well do you know this fascinating world?
Hubble Space Telescope photo of Pluto is most detailed ever seen.
0 of questions complete

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