Here’s an update on Pamela’s favorite exoplanet: Kelt-9b. This hottest of hot Jupiters orbits a blue star – roughly an A0 or B9.5 star, that may be the largest and hottest star found to have a planet. Because Kelt-9 is so big and bright, we only have the ability to spot very large planets on very small orbits. The planet Kelt-9b is 2.9 times the mass of Jupiter, and has an orbit 10% the size of Mercury’s orbit, and circles in under 1.5 days. It should also be tidally locked, such that the same face of the planet is always looking toward its star. 

This super-hot star is able to heat the day-side surface of this super close planet to 7800 degrees Fahrenheit (4300 degrees Celsius). This temperature is cooler than the surface of our Sun, but is still hotter than the surface of many different stars. The everyday material that planets are made of aren’t entirely solid at this kind of a temperature, and this has significant effects on Kelt-9b’s possible geology. 

In a new paper in the Astrophysical Journal, a multi-institutional research team has looked at how these kinds of temperatures can break molecular bonds and how this heat drives atmospheric circulation. This paper, with first author Megan Mansfield, uses Spitzer Space Telescope data to look at how the world’s temperature varies around the world. According to the paper, the hotter the planet, the faster the heat redistribution. Put a different way, there are massive winds and other processes that drive heat away from the noon position on the Kelt-9b. 

If there were no other effects at play, Kelt-9b’s massive winds could reach speeds of 37 miles or 60 km per second. Luckily there are other factors at play. Some of that energy goes into breaking up molecules instead of generating winds. On the planet’s day side, molecules with two Hydrogen atoms – H₂ – are disassociated, and those individual atoms can only come back together if they make it to the world’s other side – the nightside. Since these atoms are the lightest of all atoms, collisions with more massive atoms can send them flying at escape velocities. Were we close enough to study it better, we might see material streaming out behind this planet as gas is driven from its atmosphere. Also driven into excited states are neutral and ionized iron and ionized titanium in this planet’s atmosphere. 

Kelt-9b is a little less than 3 Jupiter Masses in size, and is puffed out to roughly 2 Jupiter diameters. This massive size means this world isn’t going to get blown apart anytime soon, but it is going to slowly shrink slowly. It’s host star, which is several solar masses in size, will eventually bloat up into a red giant and consume whatever is left of this world. In what is one of our favorite statements to appear in a press release, this broiled planet “will stay firmly categorized among the uninhabitable worlds.”