Feeling Hot, Hot, Hot: Astronomers Discover a Giant Planet Hotter Than Most Stars

After a decade of searching and verification, a research team revealed a new exoplanet, and you could say it’s coming in hot. The orb is the hottest giant exoplanet ever found, according to a study published today in Nature. “KELT-9b” intrigues astronomers not just because of the unusual chemistry the heat is bound to create in its atmosphere, but also because of the opportunities it offers for more in-depth studies.

 “We’re fascinated with the weirdness that nature hands us,” says B. Scott Gaudi, an astronomer at the Ohio State University who helmed the study, as he explains why his academic circle is ablaze with KELT-9b curiosity. First and foremost, there is the planet’s impressive heat. Gaudi and his team calculated KELT-9b to be roughly 3,777 degrees Celsius (6,830 degrees Fahrenheit) on the dark side and 4327 degrees C (7,820 degrees F) on the star-exposed side. This makes KELT-9b almost 20 percent hotter than the next-most-scorching exoplanet, WASP-33b. The new planet’s blistering temperature is thanks to its parent star, KELT-9. The star falls in the A-type bracket of stars, the third-hottest category. It is the seventh star of its classification to be found with an exoplanet, and at roughly 300 million years old, it is particularly young and toasty—9,897 degrees C (17,846 degrees F), to be exact. That is nearly twice the temperature of our own sun.

With extreme conditions come extreme attributes. KELT-9b receives 700 times as much solar radiation as WASP-33b. That amount is similar to the glow of an entire K-type star, the second coolest type, and is more than enough to break down molecules to their constituent atoms. Gaudi predicts, then, that the atmosphere of KELT-9b is writhing with ions in a star-like pandemonium. Drake Deming, an astronomer at the University of Maryland, agrees. “KELT-9b is a hybrid planet-star. It has the mass of a planet, and [yet] an atmosphere like a star,” he says.

The group’s finding could also influence astronomy research in a slightly less splashy but equally important way. “By and large, the population of planets around higher-mass stars has been relatively unexplored,” Gaudi says. Early exoplanet discoveries happened around low-mass stars. These findings inspired a trend of researchers pursuing stars of lower and lower masses. In an effort to carve out their own niche in the KELT, or Kilodegree Extremely Little Telescope, project, Gaudi and his team targeted higher-mass stars. The discovery of KELT-9b might encourage more researchers to dig into planets on the opposite end of the typically researched spectrum, Gaudi explains. Deming also thinks the find has opened the door for a new approach to exoplanets. “This is a new class of planet,” says Deming. “I think we’ll find a lot more of these.”

Even if the hunt for companions to massive star grows, researchers will continue their study of KELT-9b. The planet is a good candidate for high-quality, precise characterization by a variety of telescopes, says Adam Burrows, an astrophysicist at Princeton University who was not involved in the study, because its level of brightness is just right for follow-up inspection across the electromagnetic spectrum. “It’s bright enough for the instruments, but not too bright,” he says. The James Webb Space Telescope, an infrared telescope set to launch next year, might be particularly suited to scope out KELT-9b. Gaudi is eager to see additional instruments take a crack at KELT-9b, such as the aging Hubble Space Telescope and NASA’s Transiting Exoplanet Survey Satellite, which is also expected to launch in 2018.  “This planet represents one of the best targets for characterizing any individual planet that we have to date,” he says.

In Deming’s mind, KELT-9b’s accessibility is really what sets the planet apart from other wild finds. He laughs as he confesses he is glad KELT-9b is not habitable. “I’m interested in what I can measure,” he explains. “Those habitable worlds are difficult to measure, and this will be measurable.”

Gaudi and his team are intrigued by far more than just precise measurements, however. They have a whole list of KELT-9b attributes they would like to understand further. In fact, the planet is so strange, one of Gaudi’s graduate students bet him a bottle of single-malt scotch that they had not even found a planet (clearly Gaudi won). KELT-9b is surprisingly puffy, given that its mass is almost three times Jupiter’s: most ultraheavy worlds are several times more massive than Jupiter yet about the same diameter, compressed into Jupiter-sized orbs by their immense gravitational fields. KELT-9b, by contrast, is almost twice Jupiter’s diameter. For reasons no one fully understands yet, the planet also orbits its star from pole to pole, rather than around the equator as would be expected. Gaudi assumes KELT-9b’s atmosphere must be boiling off into space given how much heat it is receiving from the star, so he would like to know at what rate that is happening. One of Gaudi’s colleagues, Thomas G. Beatty, an astronomer at the Pennsylvania State University, is most curious about what molecular mayhem is brewing on the dark side of the planet. The temperature drop outside the star’s direct light is enough to tease hydrogen atoms into recombining, he says. The process is probably similar to the hottest types of welding on Earth. “That’s where the real fun begins,” Beatty says.

KELT-9b is also unique in its proximity to its giant, tempestuous A-type star. Previous research on older, cooler A-type stars found few large and tightly orbiting planets like KELT-9b—perhaps because as they cool and expand over time, the stars consume any unlucky closely held planets. Having found one very tightly orbiting planet around a young A-star, Gaudi thinks the hunt for other duos experiencing intermediary steps in star-planet evolution will help astronomers solve the mystery of why most older A-types lack companions.

Outer space does not give up its gems easily. So when something as wondrous as KELT-9b can also be thoroughly poked and prodded with today’s technology, astronomers will struggle to resist its pull. While research groups submitted their project ideas for the James Webb Space Telescope months ago, Deming has a feeling a few teams will want to change course and pore over KELT-9b instead. If that is the case, the plethora of discoveries from the scorching planet are bound to expand what astronomers deemed possible. “Every time we lift up a rock, we find a new exoplanet, and we have to go back to the drawing board,” says Gaudi. Until then, Gaudi has a few glasses of scotch to enjoy.


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