Blistering Planet Hotter Than Many Stars

Given its stats, the recently discovered planet KELT-9b probably deserves its own baseball card. The planet and its host star, KELT-9, compose a unique system among the exoplanets discovered so far. KELT-9 is a relatively young, very hot star and its scorching heat warms the near side of the planet to a blistering 7,800 degrees Fahrenheit. KELT-9b isn’t just the hottest gas giant so far discovered, it’s hotter than many stars.

The dynamics of the system are pretty eye-catching too.:

  • KELT-9b orbits its sun once every 1.5 days. If you lived on the planet you’d have a birthday every day-and-a-half.
  • The same side of KELT-9b faces its sun all of the time due to tidal locking.
  • KELT-9b orbits its sun perpendicular to the direction in which the star spins. This is in contrast to our solar system, in which the sun spins and objects orbit in basically the same direction.
  • Intense radiation from the star is likely stripping away the atmosphere of KELT-9b.
Astronomers at The Ohio State University and Vanderbilt University have discovered a planet that is so hot, its temperature rivals that of most stars.
Credit: Robert Hurt, NASA/JPL-Caltech.

An international team of scientists led by researchers at The Ohio State University and Vanderbilt University introduced the world to KELT-9b this week, through a journal article in Nature and at the 230th Meeting of the American Astronomical Society in Austin, Texas. Although it is clearly not among the list of potentially habitable planets we’ve discovered, KELT-9b is both intriguing and scientifically valuable.

Like Jupiter and the other outer planets of our solar system, KELT-9b is a gas giant. The planet is nearly three times as massive as Jupiter, but half as dense. By volume the planet is significantly bigger than scientists expected: although astronomers aren’t sure exactly how, the intense radiation from KELT-9 causes the atmosphere to puff up kind of like a balloon.

KELT-9b was discovered using the transit method, in which astronomers see regular dips in the brightness of a star indicative of a planet passing between us and the star. It was found by its namesake telescope, KELT-North (Kilodegree Extremely Little Telescope), at Winer Observatory in Arizona. KELT-North and its partner KELT-South, located in the southern hemisphere, together search for exoplanets passing in front of bright 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.
Image Credit: NASA/JPL-Caltech.

More than 3600 exoplanets have been discovered in the last 22 years. Of these, only six others have been seen passing in front of stars approaching the temperature of KELT-9 (an intense 18,000 degrees Fahrenheit). None have been seen transiting hotter stars. This doesn’t mean such planets are rare, but current methods for detecting and confirming the existence of exoplanets are more tuned for studying cooler, lighter stars.

As stars like KELT-9 age, they cool off, slow down, and expand. Astronomers have looked for short-orbit, giant planets around stars that might be considered the “retired version” of KELT-9, but have come up shorthanded. Some theories suggest that intense radiation from the such stars evaporates nearby planets. Others suggest that the stars engulf nearby planets as they evolve and expand. Through studying KELT-9b and similar systems, astronomers hope to get a better understanding these massive, bright stars and how planets form and disappear around them.

“The future evolution of the KELT-9 system is uncertain but interesting,” write the researchers. Will the planet evaporate into space from the intense radiation? Burn up as the star expands later in life? Although we won’t be around to see firsthand, the information astronomers have gained and hope to gain from future studies of this sweltering system will help us better understand its fate.

Kendra Redmond

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