Scorching Alien World Discovered That's Nearly as Hot as Our Own Sun

The planet, which orbits a star that's twice as big as our sun, is 2.8 times more massive than Jupiter — and is a study in exoplanet extremes.

Astronomers at the Ohio State University and Vanderbilt University have discovered a planet that is so hot, its temperature rivals most stars. | Robert Hurt, NASA/JPL-Caltech
Astronomers at the Ohio State University and Vanderbilt University have discovered a planet that is so hot, its temperature rivals most stars. | Robert Hurt, NASA/JPL-Caltech

An extremely little telescope has found an extremely large and hot extrasolar planet.  And the extremes don’t stop there.

The planet, located about 650 light years from Earth, is so close to its hot and massive star that it is constantly bombarded with extreme-ultraviolet irradiation. This makes the planet hotter than most stars and it has a giant, glowing gas tail like a comet.

“It’s a planet by any of the typical definitions based on mass, but its atmosphere is almost certainly unlike any other planet we’ve ever seen just because of the temperature of its day side,” said Scott Gaudi, an astronomy professor at the Ohio State University and one of the lead authors of the study that identified the planet, in a statement. The researchers shared their discovery this week at the meeting of American Astronomical Society in Austin, Texas.

The planet is named KELT-9b, named for the telescope used to make the discovery, the KELT, or Kilodegree Extremely Little Telescope.

According to data from KELT, the planet has a day-side temperature of 4,600 Kelvin (more than 7,800 degrees Fahrenheit), which is hotter than most stars, and only 1,200 Kelvin (about 2,000 degrees Fahrenheit) cooler than our own sun. The researchers say the ultraviolet radiation from the star it orbits is so brutal that the planet may be literally evaporating away under the intense glare, producing a glowing gas tail.

And the extremes keep going. The planet is a gas giant 2.8 times more massive than Jupiter but only half as dense, because the extreme radiation from its host star has caused its atmosphere to puff up like a balloon. It is so close to its star, it completes an orbit every day and a half.

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The planet is tidally locked to its star, meaning the same side faces the star at all times, so that side is continually bombarded by stellar radiation. Consequently, it is so hot that molecules such as water, carbon dioxide, and methane can’t form there.

The star is more than twice as large and nearly twice as hot as our sun, which is why the planet is so scorching.

“KELT-9 radiates so much ultraviolet radiation that it may completely evaporate the planet,” said co-author Keivan Stassun, from Vanderbilt University. “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.”

This means the prospects for life — as we know it, anyway — on KELT-9b are not promising.

“KELT-9 [the star] will swell to become a red giant star in about a billion years,” said Stassun. “The long-term prospects for life, or real estate for that matter, on KELT-9b are not looking good.”

The KELT telescopes are a joint project between Ohio State, Vanderbilt, and Lehigh universities. The system consists of two telescopes, one in the northern hemisphere (KELT-North) in Arizona in the United States, and KELT-South at the SAAO observing station near Sutherland, South Africa.

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The telescopes were built using mostly off-the-shelf technology to provide a low-cost means of planet hunting. While traditional astronomical telescopes costs millions of dollars to build, the hardware for a KELT telescope runs less than $75,000. The KELTs are survey telescopes, meaning where other telescopes are designed to look at very faint stars in small sections of the sky at very high resolution, KELTs look at millions of very bright stars at once, over broad sections of sky, at relatively low resolution.

Astronomers say the KELTs fill a large gap in the available technologies for finding extrasolar planets, plus they specifically look for large planets orbiting close to their stars. While these type of planets aren’t “hot” right now – meaning many astronomers are looking for Earthlike planets around small, cooler stars like our sun, the team says getting information about the larger, more extreme exoplanets is important too.

“As we seek to develop a complete picture of the variety of other worlds out there, it’s important to know not only how planets form and evolve, but also when and under what conditions they are destroyed,” Stassun said.

Gaudi added that the KELT telescopes help provide a touchstone for understanding how planetary systems form around hot, massive stars. Others involved with the project say that this finding demonstrates that even ‘extremely little’ telescopes can play an important role in discovering exoplanets.

But the researchers would also like to find out more about this extreme world and hope to take a closer look at KELT-9b with other, bigger telescopes, including Spitzer, the Hubble Space Telescope (HST) and eventually the James Webb Space Telescope after it launches in 2018. Observations with HST would enable them to see if the planet really does have a cometary tail and allow them to estimate how much longer the planet will survive its current hellish environment.