Kelt 9b: The Hottest Alien World

A vast chest of awe-inspiring and bewildering planets was discovered orbiting remote stars outside our own. Of these bizarre worlds, there is a class of massive gas giants called hot Jupiters, who stand out as some of the strangest planetary beasts of all in the crowd. Hot Jupiters easily and tightly surround their roiling parent-stars in roasting orbits and, as such, they are too heavy to support life. Such big, alien “oddballs” are both interesting and enigmatic, and inside our own solar system, there is nothing like them.

Astronomers revealed their latest discoveries in January 2020 showing that the best of all of them is indeed the strangest. Indeed, this hottest known hot Jupiter, dubbed Kelt-9b, is classified as an “ultra-hot Jupiter.” The broiling tormented giant world suffers so severe planet-wide meltdowns that it rips away the molecules that make up its exotic atmosphere containing ionized atomic iron and unique ionized titanium.

Kelt-9b is one of several known exoplanet varieties belonging to the faraway family of an alien star inhabiting our Milky Way Galaxy, as an ultra-hot Jupiter. It weighs-in nearly three times the mass of the banded behemoth Jupiter of our own Solar System, and circles some 670 light-years from Earth around its stellar parent. Sports a roasting 7,800 degrees Fahrenheit surface temperature, Kelt-9b is cooler than a million asteroids. This planet has the distinction of being the hottest yet to be discovered.

Now, a team of astronomers using NASA’s Infrared Spitzer Space Telescope — which ended its very successful mission in January 2020–announced they’ve found evidence that even molecules on this strange planet are too much heat to remain intact. On the day-side of this searing-hot distant world, molecules of hydrogen gas are probably torn apart, and they are unable to “glue” themselves back together again until the cut pieces of their disjointed atoms travel around to the cooler night-side of Kelt-9b. As an atomic species, normally sequestered refractory elements can survive, including neutral and unique ionized atomic iron ( Fe and Fe+) and single ionized titanium (Ti+).

While the night-side of Kelt-9b is already roasting, it’s marginally cooler than the day — so it’s cool enough to encourage the hydrogen gas molecules to meet so change. This pleasant state continues until the molecules of hydrogen migrate back to the crushing heat of the day-side, where they are torn apart — and the process starts again.

“This kind of planet is so extreme in temperature, it’s a bit separate from many other exoplanets. There are some other hot Jupiters and ultra-hot Jupiters that aren’t quite as hot but still warm enough to make that effect happen,” explained Megan Mansfield on January 24, 2020, NASA Jet Propulsion Laboratory (JPL) Press Release. Ms. Mansfield is a University of Chicago graduate student and lead author of a new paper that reveals those findings. JPL is located in Pasadena, California.

These new findings are published in Astrophysical Journal Letters, and they reveal the much-improved sophistication of the technology and analysis needed to investigate these very strange and mysterious distant worlds. Astronomers are just beginning to be able to look into exoplanet atmospheres, studying the brightest and hottest molecular meltdowns.

The Best And The Brightest–And Hottest

KELT-9b orbits the late B-type / early A-type star KELT-9, and astronomers with the Kilodegree Extremely Little Telescope (KELT) discovered the hot gas-giant in 2017.

KELT-9b ‘s parent star’s surface temperature is 10,170 K, which is unusually hot for a star hosting a transiting planet (a planet floating before its parent star’s glaring face as seen from Earth). Before KELT-9b was discovered, only half a dozen A-type stars were known to host planets, of which the warmest, WASP-33, was significantly cooler at 7.430 K. Previously no B-type stars were observed for a planet to circle. KELT-9b travels around its star by a circular but highly inclined orbit and roasts from its stellar parent at a mere 0.03462 astronomical units ( AU). One AU is equivalent to the mean distance of around 93,000,000 miles between Earth and Sun and its orbital period is less than 1.5 days.

KELT-9b is a massive gas-giant planet that weighs-in at an astounding 2.8-fold mass of the banded behemoth of our own Solar System, Jupiter. Yet the density of KELT-9b is less than half that of Jupiter. Like many others of its roasting kind, KELT-9b and its parent-star are tidally locked. The outer boundaries of its atmosphere nearly extend its lobe to Roche. This means that the melting earth experiences a fast air flight, propelled by the intense amount of radiation it gets from its roiling, broiling star.


KELT-9b will always maintain its uninhabitable categorization as a planet. Astronomers became aware in 2017, when it was first discovered, of its very hostile environment.

The team of planetary scientists used the Spitzer space telescope in the Astrophysical Journal Letters paper to collect temperature profiles from this hell-like, roasting earth. Spitzer was able to take measurements of slight changes in the heat of KELT-9b before it finished its flight. Observations, repeated over many hours, enabled Spitzer to detect changes in the exotic atmosphere of that world, as the planet showed itself in phases while circling its parent-star.

That allowed the astronomer’s team to gain a glimpse of the difference between the dayside of KELT-9b and its perpetual “night.” In this case, the planet hugs its star in such a close orbital embrace that it takes just 1 1/2 days for a “year”—one orbit around the star. That means the earth is tidally bound, giving its blinding celestial father forever only one ear. The night never ends, on the far side of KELT-9b. This is comparable with the way in which Earth’s Moon shows our planet only one face.

Gasses and heat travel from one side to another of the KELT-9b. For scientists trying to understand the atmosphere of alien planets, one major question is how radiation and flow balance each other out.

In such studies, computer models are major tools that reveal how these atmospheres are likely to behave at different temperatures. The best scenario for the data obtained from KELT-9b is one that includes the ripping of hydrogen molecules apart only for reassembly. This process is called the recombination and dissociation process.

“If you don’t account for hydrogen dissociation, you get really fast winds of [37 miles or] 60 kilometers per second. That’s not likely,” Ms. Mansfield commented in the JPL Press Release of January 24, 2020.

KELT-9b does not undergo significant temperature variations during daytime and nighttime. Which implies the transfer of heat from one hand to the other. In fact, the “hot spot” on the day-side, which is meant to be directly under the host star of this world, has been moved away from its predicted location. Astronomers aren’t aware of the reason. On this very weird, wonderful, and very alien distant world, it remains yet another enticing mystery to be resolved.

KELT 9b – Hottest Planet Discovered