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Astronomers find exoplanet hotter than most stars

This artist's concept shows planet KELT-9b orbiting its host star, KELT-9. It is the hottest gas giant planet discovered so far. Credit: NASA/JPL-Caltech

This artist’s concept shows planet KELT-9b orbiting its host star, KELT-9. It is the hottest gas giant planet discovered so far. Credit: NASA/JPL-Caltech

Six hundred and twenty light-years from Earth, in the constellation Cygnus, a bright, young, Type-A, blue, main-sequence star designated KELT-9 burns brightly. More than twice as massive as the Sun and nearly twice as hot, KELT-9 is a rare star – one of a group of stars making up less than one percent of the total stars in the universe. According to a paper published this week in Nature, this unusual star hosts an equally unusual exoplanet.

Using data from the Kilodegree Extremely Little Telescope (KELT) administered by Ohio State University in Columbus, Ohio; Vanderbilt University in Nashville, Tennesee; Lehigh University of Bethlehem, Pennsylvania; and South African Astronomical Observatory (SAAO), scientists determined there to be a very strange, and very hot, exoplanet orbiting the bright KELT-9 star. The exoplanet was discovered after they noted a repeated dimming of the star approximately every 36 hours.

KELT is made up of two robotic wide-field telescopes. KELT-North at the Winer Observatory about an hour outside of Tucson, Arizona, and KELT-South at the Sutherland astronomical observation station about 230 miles (370 kilometers) north of Cape Town, South Africa.


Photo Credit: NASA / JPL-Caltech

The exoplanet it found, KELT-9b, is a “hot Jupiter” gas giant. It was expected to be roughly the same size as Jupiter, but further study proved it to be 2.8 times more massive and half as dense. It’s thought that the large size of KELT-9b is due to both its proximity to the star KELT-9 and the radiation KELT-9 gives off. This combination has caused the planet to become superheated and to “puff up”.

Scott Gaudi, an Ohio State University astronomy professor, worked on the study at the Jet Propulsion Lab, in Pasadena, California, while on sabbatical. Gaudi has spent the last two decades searching for exoplanets, and, according to him, “KELT-9b is one of the strangest exoplanets I’ve ever seen.”

With this discovery, KELT-9 became the seventh Type A star located to host an exoplanet. In addition, it is also the brightest star to host an exoplanet thus far.

Up until this discovery, WASP-33b was believed to be the hottest exoplanet, but KELT-9b is nearly 20 percent hotter. In addition, it receives nearly 700 times as much radiation from its host star than WASP-33b does. The ultraviolet radiation within the orbit of KELT-9b is beyond extreme. Because it is so close to its host star, it is presumed to be tidally locked – the same face of the planet is always facing the star.

The temperatures on KELT-9b on the side of the planet facing away from the star are estimated to be 6,830 degrees Fahrenheit (3,777 Celsius), whereas the star-facing side reaches temperatures approximately 7,820 Fahrenheit (4,327 Celsius). At this temperature, combined with the extreme UV radiation and stellar proximity, the surface of the planet is quite likely a molecular pandemonium with its surface atmosphere literally being evaporated.

Molecules such as water, carbon dioxide, and methane can’t form at all on the star-facing side of the planet. On the side facing away from the star, some molecules may be able to recombine, but probably only temporarily. The completely inhospitable environment of KELT-9b has made it a searing hot, puffy planet writhing in ions that it is possibly shedding its atmosphere much like a comet’s tail but on a more massive level.

As if the sheer heat of KELT-9b wasn’t enough of an oddity, there’s its orbit. Instead of orbiting its host star along its axial plane, KELT-9b is orbiting its star nearly perpendicular to it in a pole to pole orbit.

With the mass of a planet and the atmosphere of a star, all indications are that KELT-9b could very well be some type of hybrid planet-star or, at the very least, a new class of planet. Scientists are looking forward to studying KELT-9b in depth, with both the Hubble and Spitzer space telescopes as well as with continued observations with the KELT North and South.

This discovery has raised many new questions about the evolution of stellar systems like this. Especially considering what might happen when KELT-9 reaches the end of its life. After a 500-million-year sequence lifetime, KELT-9 will exhaust its hydrogen and become a red giant star, swelling to three times its current size. Scientists are already hypothesizing about what might become of the exoplanet KELT-9b at that time. It might be swallowed by the red giant or, perhaps, just remain as a scorched remnant of a planet with its atmosphere and volatiles completely stripped away. There is a possibility that there exists a population of close-in super-Earth remnant core planets orbiting subgiant stars.

It is hoped that with the launch of the Transiting Exoplanet Survey Satellite (TESS) and the James Webb Space Telescope (JWST) in March and October 2018, respectively, some of these questions may be answered.

CGI animation of planet KELT-9b orbiting star KELT-9

CGI animation of planet KELT-9b orbiting its host star, KELT-9. Credits: NASA / JPL-Caltech



A native of the Greater Los Angeles area, Ocean McIntyre's writing is focused primarily on science (STEM and STEAM) education and public outreach. McIntyre is a NASA/JPL Solar System Ambassador as well as holding memberships with The Planetary Society, Los Angeles Astronomical Society, and is a founding member of McIntyre is currently studying astrophysics and planetary science with additional interests in astrobiology, cosmology and directed energy propulsion technology. With SpaceFlight Insider seeking to expand the amount of science articles it produces, McIntyre was a welcomed addition to our growing team.

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