Newly Discovered Planet Is Hotter than Most Stars

Astronomers have discovered a giant exoplanet that’s hotter than most known stars. Clocking in a 7,820˚ F, the planet known as KELT-9b orbits a star almost twice as hot as the Sun.

The planet, which is almost three times the size of Jupiter, is “tidally locked,” meaning that the same side is always facing its host star. It’s also extremely close to its host, taking only one and a half Earth days to orbit the giant blue star.

A Jupiter-like planet and its hot "A-type" host star.

The team that made the discovery, led by astronomers at The Ohio State University and Vanderbilt University, reported its findings in the journal Nature. Scott Gaudi, an astronomer at Ohio State and lead author on the study points out that KELT-9b’s extreme temperature affects it in unusual ways.

“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,” Gaudi said in a statement.

Here’s Ben Guarino, reporting for the Washington Post:

Though scientists had previously discovered “hot Jupiter” planets, so named for their extreme temperatures and gas giant nature, all fell about a thousand degrees Celsius short of KELT-9b. KELT-9b is at the “extreme end of the population” of known gassy, hot giants, said Jonathan Fortney, a University of California at Santa Cruz professor who creates computer models to determine what’s in an exoplanet atmosphere.

Fortney, who was not directly involved with this paper, was asked to model KELT-9b’s atmosphere. He declined. “I can’t possibly do that. The planet’s way, way too hot,” he said.

A planet’s atmosphere is typically made up of molecules, which are made up of atoms bound together. Molecular hydrogen gas is common on Jupiter, for instance, as are water vapor and other gases on Earth. KELT-9b’s dayside is an exception. The heat breaks molecules into its constituent atoms. “Almost all the elements are in atoms, not in molecules,” Fortney said. “It’s essentially too hot for molecules to exist.”

The team made the discovery using relatively inexpensive off-the-shelf technology in the form of a KELT, or Kilodegree Extremely Little Telescope. Gaudi said their discovery is unusual because astronomers typically focus on smaller, cooler stars whose planets might support life. By contrast, KELT-9b’s host star is so hot that the exoplanet may be trailing a comet-like tail of its own evaporated material.