KELT-9b

KELT-9 is an A-type star named for the survey that discovered it, the Kilodegree Extremely Little Telescope (KELT). Still in its main sequence, its surface temperature is a mind-boggling 10,000º C (our Sun, for context, is also in its main sequence and is a modest 5,000º C on its surface). In fact, KELT-9 is so hot that astronomers think it fits snugly in the transitional zone between A-type and the even-hotter B-type stars.

The hot star has been found to have an orbiting planet, named KELT-9b, with a surface temperature of of stunning 4,327º C. The planet itself is hotter than an M-type star – making it hotter than most stars we know. It is estimated to be twice the size, and approximately equal to the mass, of Jupiter. The reason this discovery is even more remarkable is that the newly discovered gas giant orbits extremely quick around its host star, completing its journey around the orb in 36 hours.

The hotter a star is, the more rapidly it rotates. And as it whizzes around, it also throws out incredible amounts of ultraviolet radiation. Because KELT-9b is so close, the planet ought to be tidally locked to the star such that the same side faces it at all times. This results in the planet’s dayside being bathed with a glow that can split molecules (if hopes of alien life persisted this far, it is finally time to leave). In fact, astronomers believe that elements like gases and metals found on and around KELT-9b can only exist as atoms, not as molecules. The stellar radiation is also quite likely assiduously stripping away the planet’s atmosphere. We have never directly observed a planet this large and this hot, and so its atmosphere makes for very compelling studies.

Click to expand...

A newly discovered Jupiter-like world is so hot, it's being vaporized by its own star.

With a dayside temperature of more than 7,800 degrees Fahrenheit (4,600 Kelvin), KELT-9b is a planet that is hotter than most stars. But its blue A-type star, called KELT-9, is even hotter -- in fact, it is probably unraveling the planet through evaporation.

"This is the hottest gas giant planet that has ever been discovered," said Scott Gaudi, astronomy professor at The Ohio State University in Columbus, who led a study on the topic. He worked on this study while on sabbatical at NASA's Jet Propulsion Laboratory, Pasadena, California. The unusual planet is described in the journal Nature and at a presentation at the American Astronomical Society summer meeting this week in Austin, Texas.

KELT-9b is 2.8 times more massive than Jupiter, but only half as dense. Scientists would expect the planet to have a smaller radius, but the extreme radiation from its host star has caused the planet's atmosphere to puff up like a balloon.

Because the planet is tidally locked to its star -- as the moon is to Earth -- one side of the planet is always facing toward the star, and one side is in perpetual darkness. Molecules such as water, carbon dioxide and methane can't form on the dayside because it is bombarded by too much ultraviolet radiation. The properties of the nightside are still mysterious -- molecules may be able to form there, but probably only temporarily.

"It's a planet by any of the typical definitions of mass, but its atmosphere is almost certainly unlike any other planet we've ever seen just because of the temperature of its dayside," Gaudi said.

Click to expand...