Hot Planets are Out There!

Last week I did a “Skype-in” session with a group of students in Midland, Texas. They were doing a special science unit on space. As expected, I got a LOT of questions during the session about black holes, Pluto, and—interestingly enough, one about how much a SpaceX rocket costs to build. But, there was one about hot planets, and that got me thinking and searching to find out which is the hottest planet.

You’d think that’d be an easy answer. In one sense, it is, but only if you are thinking of our own solar system. Venus has the hottest surface temperatures of any world circling the Sun. They get up to nearly 500°C or 800° F. That’s pretty darned hot, and coupled with the very high atmospheric pressure from all those clouds, make the planet a very inhospitable place.

Another Way to Look at Hot Planets

If you broaden the definition of “hot planets” and use our galaxy as a search region, then there are hotter planets out there. They’re called “hot Jupiters” and they can get up to nearly 4,300°C (7,800°F). I’ve written about them before, including an intriguing report about water at these planets. Hot Jupiters are at least as massive as our own Jupiter and are often many times more than that. Astronomers recently reported on one that they discovered called KELT-9B.

The research team, which included Megan Mansfield of the University of Chicago, used the Spitzer Space Telescope to study the star and try to figure out what’s happening with it. KELT-9b is an “ultra-hot” Jupiter orbiting a star about 670 light-years away. It was first discovered by the Kilodegree Extremely Little Telescope (KELT) system based in Arizona and South Africa. The planet is not just like our Jupiter—it’s even weirder. It’s among the hottest worlds ever found. While it orbits its star, KELT-9b goes through these strange meltdowns. Things get so hectic during those events that gas molecules in the atmosphere get torn apart.

Why is a Hot Planet So Chaotic?

That’s a good question. The best models about hot planets, and this one, in particular, suggest that extreme heat plays a role. It’s at least partly to blame for the chaos that rips apart the atmospheric gases. How does it happen? The planet spins on its axis (just as Earth does). It takes 1.5 days to make one trip around its star (which basically gives it a 1.5-day “year”). That arrangement means that the planet shows the same “face” to its star all the time. It’s “locked” into doing so. However, the gases in the atmosphere do flow around the planet.

So, what happens? When the gases on the “day” side of the planet get superheated, the hydrogen molecules get shredded. Then, as that material flows around to the night side, everything cools down, and the gas reassembles itself. That is, at least, the working hypothesis that the scientists who study this star are using to explain its strangeness.

So, to give a more accurate answer to the students from Midland, Venus is truly the hottest planet in our solar system. BUT, our galaxy has even hotter ones out there. KELT-9B may be just one of many of the thousands of planets astronomers have found so far.

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