May 28, 2024


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Kelt 9b: The Hottest Alien World

A treasure trove of bewitching and bewildering planets has been found circling distant stars beyond our Sun. Of these bizarre worlds, there is a class of massive gas-giants termed hot-Jupiters, that stand out in the crowd as some of the strangest planetary beasts of all. Hot Jupiters circle their roiling parent-stars fast and close in roasting orbits and, as such, they are much too hot to sustain life. These enormous, exotic “oddballs” are fascinating, as well as mysterious, and nothing like them exists within our own Solar System. In January 2020, astronomers announced their new observations showing that the hottest of them all is also the weirdest. Indeed, this hottest known hot Jupiter, dubbed Kelt-9b, is classified as an “ultra-hot Jupiter.” The broiling tormented giant world suffers planetwide meltdowns that are so severe that they rip apart the molecules that compose its exotic atmosphere which contains ionized atomic iron and singly ionized titanium.

As an ultra-hot Jupiter, Kelt-9b, is one of several known varieties of exoplanets belonging to the faraway family of an alien star inhabiting our Milky Way Galaxy. It weighs-in at almost three times the mass of our own Solar System’s banded behemoth Jupiter, and it circles its stellar parent some 670 light-years from Earth. Sporting a surface temperature of a roasting 7,800 degrees Fahrenheit, Kelt-9b is hotter than some stars. This planet has the distinction of being the hottest discovered so far.

Now, a team of astronomers using NASA’s infrared Spitzer Space Telescope--that ended its very successful mission in January 2020–announced that they have found evidence that the heat is too much even for molecules on this weird planet to remain intact. Molecules of hydrogen gas are probably torn apart on the dayside of this searing-hot distant world, and they are unable to “glue” themselves back together again until the severed pieces of their disjointed atoms travel around to Kelt-9b’s cooler nightside. Normally sequestered refractory elements can survive as atomic species, including neutral and singly ionized atomic iron (Fe and Fe+) and singly ionized titanium (Ti+).

Even though Kelt-9b’s nightside is still roasting, it is slightly cooler than the dayside–and it is cool enough to enable the hydrogen gas molecules to meet up and reform. This happy situation lasts until the hydrogen molecules travel back to the intense heat of the dayside, where they are again ripped apart–and the cycle begins anew.

“This kind of planet is so extreme in temperature, it is a bit separate from a lot of other exoplanets. There are some other hot Jupiters and ultra-hot Jupiters that are not quite as hot but still warm enough that this effect should be taking place,” explained Megan Mansfield in a January 24, 2020 NASA Jet Propulsion Laboratory (JPL) Press Release. Ms. Mansfield is a graduate student at the University of Chicago and lead author of a new paper revealing these findings. The JPL is in Pasadena, California.

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

The Best And The Brightest–And Hottest

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

The surface temperature of KELT-9b’s parent-star is 10,170 K, This is unusually hot for a star hosting a transiting planet (a planet that floats in front of the glaring face of its parent star as seen from Earth). Before the discovery of KELT-9b, only a half-dozen A-type stars were known to host planets–out of which the warmest, WASP-33, is significantly cooler at 7,430 K. No B-type stars were previously observed to be circled by a planet. KELT-9b travels a circular but highly inclined orbit around its star, and it roasts at a mere 0.03462 astronomical units (AU) from its stellar parent. One AU is equal to the mean distance between Earth and Sun, which is about 93,000,000 miles, and its orbital period is less than 1.5 days.

KELT-9b is an enormous gas-giant world that weighs-in at an impressive 2.8 times the mass of our own Solar System’s banded behemoth, Jupiter. However, KELT-9b’s density is less than half that of Jupiter. Like many others of its roasting kind, KELT-9b is tidally locked with its parent-star. The outer boundary of its atmosphere almost extends its Roche lobe. This means that the melting planet is suffering rapid atmospheric escape that is driven by the extreme quantity of radiation it receives from its roiling, broiling star.


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

In the Astrophysical Journal Letters paper, the team of planetary scientists used the Spitzer space telescope to obtain temperature profiles from this hell-like, roasting world. Before it ended its mission, Spitzer was able to take measurements of subtle alterations in KELT-9b’s heat. Observations, repeated over many hours, enabled Spitzer to detect alterations in that world’s exotic atmosphere as the planet displayed itself in phases while circling its parent-star.

That allowed the team of astronomers to catch a glimpse of the difference between KELT-9b’s dayside and its perpetual “night”. In this case, the planet hugs its star in such a close orbital embrace that a “year”–one orbit around the star–takes a mere 1 1/2 days. This means that the planet is tidally locked, perpetually showing only one face to its glaring stellar parent. On the far side of KELT-9b, night never ends. This is comparable to the way that Earth’s Moon shows only one face to our planet.

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

Computer models are major tools in such studies, revealing how these atmospheres are likely to behave in different temperatures. The best scenario for the data obtained from KELT-9b is one that includes hydrogen molecules being ripped apart only to be reassembled. This process is termed dissociation and recombination.

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

KELT-9b does not experience big differences in temperature between its day- and nightsides. This suggests heat flow from one side to the other. Furthermore, the “hot spot” on the dayside, which is supposed to be directly under this planet’s host star, was shifted away from its expected position. Astronomers do not know the reason for this. It remains as yet another enticing mystery to be solved on this very weird, wonderful, and very alien distant world.