A couple of years ago, astronomers marveled at the discovery of the hottest planet in the known universe. And just when you thought KELT-9b couldn’t get even crazier, it goes and outdoes it. New research shows that not only is this planet hotter than most stars, with vaporized iron and rare earth metals in its atmosphere, its heat is so intense that it undergoes collapses all over the planet that tear the molecules in that atmosphere. No other exoplanet we have discovered comes close to this metal. The discovery of KELT-9b, a gas giant neatly stuck about three times the mass of Jupiter orbiting a burning star 670 light years away, was announced in 2016. But it was not until the following year that its extreme temperature was revealed. It reached a staggering 4,300 degrees Celsius (7,800 degrees Fahrenheit) on the day side – a temperature that is “at least 80% warmer than all known stars,” said astronomer Jonti Horner of the University of Southern Queensland ScienceAlert. It is classified as an ultra-hot Jupiter, and is the hottest planet that astronomers have ever identified. It is orbiting its star, a blue supergiant called HD 195689 that burns at temperatures around 9,900 degrees Celsius (17,850 degrees Fahrenheit). And the more we look at it, the wilder it becomes.
The latest discovery, made using observations from the newly retired Spitzer space telescope, is staggering. Astronomers have found that the day side is so hot that even hydrogen molecules can’t hold together. They are shattered into their constituent atoms and can only recombine in the relative coldness of about 2,300 degrees Celsius (4,170 degrees Fahrenheit) on the night side of the planet. This is called hydrogen dissociation and recombination, and is not unlike an atmospheric process previously discovered on ultra hot Jupiter. In 2018, astronomers found that intense heat on this class of planets also tears apart molecules of water, titanium oxide and aluminum oxide. And, although KELT-9b is in a class of its own, the dissociation and recombination of hydrogen should also occur on colder planets. “This type of planet has such an extreme temperature, it is somewhat separate from many other exoplanets,” said planetary scientist Megan Mansfield of the University of Chicago. “There are other hot Jupiters and ultra-hot Jupiters that are not hot enough but still warm enough for this effect to occur.”
Spitzer was an infrared instrument, which meant it could measure thermal radiation – heat. The team used this ability to carefully study the planet as it orbits the star, creating a temperature profile. The star’s orbit is extremely narrow – only 1.48 days. (Surprisingly, there are gas giants with even closer orbits, but none around hot stars like HD 195689.) In these neighborhoods, the planets are always blocked in an orderly way, with one side permanently facing the star and the other in the eternal night. But the atmosphere can still move. And this, the researchers found, is probably why the night side of KELT-9b isn’t colder than it is – because the atmosphere distributes temperature as it moves around the planet. There are other possible explanations; “Without taking hydrogen dissociation into account, very fast winds of 60 kilometers (37 miles) per second are obtained,” Mansfield said. “It’s probably not likely.” Furthermore, the dissociation and recombination of hydrogen may explain other ultra-hot Jupiter observations. For example, the process would likely produce negative hydrogen ions that are thought to give these planets their opacity. It’s an interesting result, but observations have also shown that we are far from being made with the mysteries of KELT-9b. The planets blocked in an orderly fashion have a hotspot, which in KELT-9b should have been located in the substellar point, directly in front of the star. Instead, it was compensated. Although in other hot Jupiters, this offset appears to have been produced by super-rotating atmospheric jets that transfer heat asymmetrically, the exact cause has yet to be determined for KELT-9b. Stay tuned.