Roughly 650 light-years from earth, a planet larger than Jupiter and hotter than most stars zips around its host star, KELT-9, once every one-and-a-half days.
Hannah Jang-Condell, University of Wyoming assistant professor of physics and astronomy, helped confirm the planet’s discovery with a team composed of graduate and undergraduate UW students.
“It’s basically the hottest exoplanet we’ve discovered to date,” Jang-Condell said. “And the reason it’s so hot is because it’s around a very, very hot star and also it’s really close.”
Called a “hot Jupiter,” the planet, KELT-9b, is actually 2.8 times more massive than Jupiter, but only half as dense because it is being constantly bombarded by radiation.
“Mercury is further out from the Sun than this planet is from its host star, so this planet’s absolutely roasting,” Jang-Condell said. “And, in fact, it’s so hot that it’s as hot as some stars actually are.”
At 7,800 degrees Fahrenheit, KELT-9b is the hottest planet humans have ever discovered, but its star is just as interesting, said David Kasper, graduate student and member of Jang-Condell’s team.
“What’s really interesting about it is not just that it’s a large and hot planet,” Kasper said. “The star that it orbits around is a type of star that’s never been seen to have planets orbit around it.”
Hotter stars live shorter lives. While our own very average sun will last billions of years, a massive star like KELT-9 burns hot and fast and will die out in mere millions of years, leaving less time for planet formation and decreasing our likelihood of observing its planets, if any, Kasper said.
“(This discovery) allows us to probe what kind of planets can form around stars (where) we previously didn’t have evidence that there were planets around them,” Kasper said. “That allows us to do things like understand planet formation more in general and planet characteristics more in general.”
The KELT (Kilodegree Extremely Little Telescope) program searches for possible exoplanets in wide sweeps, a process Kasper described as looking for a needle in a haystack. When it notices a possible exoplanet, teams from all around the world coordinate to take more detailed, independent observations of the system in question.
This is important because the original sweep notices only changes in a star’s light. The global teams — ranging from amateur astronomers to institutions such as UW — must rule out other possibilities for that change in starlight, such as the star in question being one half of a binary star system.
The Jang-Condell’s team utilized UW’s Red Buttes Observatory, operating its telescope remotely from campus to take measurements. The team then interpreted these measurements, which were used for a paper announcing the discovery of KELT-9b, “A giant planet undergoing extreme-ultraviolet irradiation by its hot massive-star host,” published in “Nature.”
Exoplanets are discovered by detecting dips in a star’s light. If a planet’s orbit is titled just right, the planet will pass between its star and earth during its orbit. If the system was near enough for us to see, this pass would cast a shadow over the star — a partial eclipse.
“If the orbit had been tilted a little bit, we would not have seen it at all,” Jang-Condell said. “But it’s aligned just right, so every time it goes around, it passes in front of the star.”
KELT-9 — and every other star around which astronomers search for planets — is too far away for people standing on earth to notice a shadow. But using powerful telescopes, astronomers can measure a star’s brightness. Planets orbiting the star, causing a tiny, partial eclipse as they do, will cause a dip in brightness.
“It’s less than a percent dip in brightness, but it’s enough that we can measure it,” Jang-Condell said.
Exoplanets are rarely discovered around stars as hot and massive as KELT-9, given the technical difficulties of measuring their brightness. With a big star, a dip in brightness caused by a planet in orbit will be smaller, percentage-wise, than it would be if that same planet were orbiting a smaller star.
“(Hot, massive stars) also tend to be fast rotators,” Jang-Condell said. “When you have something that’s rotating very fast, it’s often harder to characterize the star and so if you can’t characterize that star well, it’s harder to see planets going around them.”
Other members of Jang-Condell’s team included Tyler Ellis, who graduated UW in 2015, and undergraduates Rex Yeigh and Aman Kar.
Kasper said in addition to the contributions this discovery made to the field of astronomy, it also gave the undergraduates experience collecting and deciphering data — skills necessary in future careers as astronomers.
“We helped find a planet,” he said.