Pluto hasn't been a planet for almost 20 years. In the early 2000s, scientists discovered several objects of a similar size to Pluto. So, during the summer of 2006, members of the International Astronomical Union convened in Prague to reconsider what counts as a planet in our solar system. IAU members decided that there were three criteria to be a planet — and Pluto did meet all of them. But planetary scientist Wladymir Lyra says that even though it was downgraded to a dwarf planet, Pluto still has much to teach us about planet formation. This episode, he also lays out his case for Pluto — and many other objects in the solar system — to be considered a planet. This episode is part of Short Wave's space camp series about all the weird, wonderful things happening in the universe. Check out the full series. Questions about the state of our universe or smaller happenings here on planet Earth? Email us at shortwave@npr.org — we'd love to consider it for a future episode!

Joro spiders are spreading across the east coast. They are an invasive species that most likely arrived in shipping containers from eastern Asia. Today, we look into why some people find them scary, why to not panic about them and what their trajectory illustrates about the wider issue of invasive species. Questions? You can also email those to shortwave@npr.org.