KELT-9b has a dayside temperature hotter than that of most stars
Radiation is causing the planet to evaporate, creating a cloud of hydrogen and helium
The newest-discovered exoplanet doesn’t act like any planet you’ve ever heard of. The bizarre find is 650 light-years from Earth, in the constellation Cygnus.
KELT-9b is a giant planet nearly twice the size of Jupiter, with a dayside temperature hotter than that of most stars and thousands of degrees warmer than any known exoplanet, according to a study published Monday in the journal Nature.
It’s so close to its host star, KELT-9, that it orbits every 1½ days, unlike the year it takes Earth to orbit our sun. This proximity isn’t exactly kind to it.
The star is twice as hot and 2½ times more massive than our sun. It’s also rotating 50 times faster than our sun – so fast that its poles have flattened and the equator bulges out. This makes it the hottest host star of an exoplanet that we know of, said Scott Gaudi, study author and professor of astronomy at the Ohio State University.
The temperature and size would make the star appear light blue to our eyes. Its temperature puts it in the hottest class of A- and B-type stars.
Due to its incredibly hot, luminous and close star, KELT-9b is being blasted by ultraviolet and high-energy radiation.
This radiation is causing the planet to evaporate at an unknown rate, creating a puffy, balloon-like cloud of charged hydrogen and helium around it. The researchers also believe a comet-like tail trails the planet as it orbits.
The planet is tidally locked to its star, like the moon always shows the same face to Earth. Its dayside would look orangeish, so hot that complex molecules can’t stay together and only 2,000 degrees cooler than our sun. If you poured water on the surface, it would immediately disassociate into oxygen and hydrogen, Gaudi said.
The nightside would be the dark red of a red dwarf star, mostly because it wouldn’t be able to properly redistribute the energy from the dayside.
As if this planet weren’t strange enough, it also orbits perpendicular to its star, traveling around the star’s poles rather than the equator. The researchers also believe that the orbit is like that of a top, getting closer and closer.
KELT-9b isn’t on a kind trajectory. Its star, already a couple hundred million years old, will run out of the hydrogen energy source at its core in 300 million more years. After that, it will expand to three times its size, cool and slow down.
If the planet is evaporating at a high rate, all that may be left after the star cools is a rocky core, which the researchers believe to be at the heart of all giant planets. This is the less likely outcome, Gaudi said.
More likely is that as the star grows in size, it will swallow the planet and become enhanced by its lithium content.
Either way, whether it evaporates or is eaten whole, it isn’t a pleasant end for KELT-9b. “At no point did this planet have a nice life,” Gaudi said.
Because KELT-9 is so bright, Gaudi is confident that observations from space telescopes like Hubble, Spitzer, TESS and, eventually, the James Webb Space Telescope could reveal more about this strange system.
The researchers want to learn more about how atmospheres of planets work under such incredible conditions, as well as the true nature of its odd orbit.
They also believe that something else, whether a planet or a star, is in the system. KELT-9b most likely formed farther out from its star, but a violent interaction with something else threw it into a close orbit.
KELT-9b was discovered by one of two KELTs, or Kilodegree Extremely Little Telescopes. Jointly operated by Ohio State, Vanderbilt and Lehigh universities, they are placed in the north and south hemispheres and designed to look at millions of bright stars at once.
Gaudi and the team of researchers think this highlights an important aspect of exoplanet discovery, one that has nothing to do with habitable planets.
“I think it’s important that we don’t lose sight of the larger context in the search for potentially habitable planets,” he said. “That is an important endeavor but should take place in the context of understanding how planetary systems form. KELT-9 isn’t telling us anything about life.
“We’re trying to see what’s up there at the very hottest massive stars that are the most rapidly rotating and see what planets are like around there. All of this goes to show that Mother Nature is way more imaginative than we are. And anytime you find something this weird, it broadens your horizons of what nature can possibly be like.”