Outer Space

Meet KELT-9b: A Gas Giant Planet So Hot, It Rivals Our Sun

What's the difference between a star and a planet? It's not that stars are made of gas and planets are made of rock: Jupiter is gaseous, after all. It's not that stars are big and planets are small: neutron stars can be as small as your average city. Stars are at least hotter than planets, right? Well, not if the gas giant KELT-9b has anything to say about it.

How Hot Is It?

On a list of the universe's most extreme objects, KELT-9b is a real contender. It's nearly three times the size of Jupiter, the largest planet in our solar system, but half as dense. It orbits so close to its home star that it's tidally locked—meaning one side always faces its sun—and its "day" side blazes at more than 4,300º C (7,800º F). That not only makes it the hottest gas giant ever found, but makes it rival the heat of our own sun (5,500º C isn't far off). In fact, it's actually hotter than some stars: red dwarfs can be as cool as 2,300º C.

That searing heat means that its atmosphere is completely devoid of those life-giving molecules we know, like water and carbon dioxide. Instead, the atmosphere is full of free-floating metal atoms. It's so hot that astronomers suspect it may be evaporating into space. "[Its sun] KELT-9 radiates so much ultraviolet radiation that it may completely evaporate the planet. Or, if gas giant planets like KELT-9b possess solid rocky cores as some theories suggest, the planet may be boiled down to a barren rock, like Mercury," said Vanderbilt University astronomy professor Keivan Stassun.

Astronomers at The Ohio State University and Vanderbilt University have discovered a planet that is so hot, its temperature rivals most stars.

The Telescope Of Unusual Size

If KELT-9b is one of the most extreme things in the universe, the telescope that discovered it may be the most unassuming. That telescope is called KELT, for Kilodegree Extremely Little Telescope (if you think that's a funny name, you should see the other names astronomers give telescopes). As the name suggests it's, well, extremely little. Your average astronomy-grade telescope will run you millions of dollars, whereas KELT—built from mostly off-the-shelf hardware—costs less than $75,000 to build.

But that doesn't mean it's any less useful. There are actually two KELT telescopes, one in the northern hemisphere and one in the southern hemisphere, which lets them work together to cover the entire sky over the course of a year. While other larger and more powerful telescopes stare at tiny sections of the sky at very high resolution, KELT examines millions of stars over wide regions of the sky at low resolution. That means they can check out brighter stars than higher-resolution telescopes could, and thereby more easily discover the telltale dim that occurs when an exoplanet passes in front of its star, known as a "transit." That's how the KELT team discovered KELT-9b, and it's how they've discovered many other exoplanets in the past.

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Written by Ashley Hamer July 1, 2017

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