THE HOTTEST PLANET YET IS TWICE THE SIZE OF JUPITER AND HOT AS A STAR

The gas giant is rapidly losing its atmosphere

NASA/ESA/G. Bacon

This new exoplanet is too hot to handle. At twice the size of Jupiter and with

 temperatures reaching 4600 kelvin (over 4300°C), KELT-9b is the warmest 

world ever discovered.

Finding exoplanets around stars much hotter than the sun is difficult. This is because

 the  blinding brightness and rapid spinning of stars like the one KELT-9b

 orbits can obscure the relatively minuscule changes to their light that passing

planets cause.

Furthermore, most worlds orbiting such hot stars have no chance of being habitable,

 so the search for exoplanets has largely focused on dimmer, cooler stars in recent

 years.

When B. Scott Gaudi at The Ohio State University and his colleagues started using

 the Kilodegree Extremely Little Telescope (KELT) to look for exoplanets passing

 in front of relatively cool stars, they kept seeing planets that had already been

 discovered.

 So instead, they decided to focus on hotter, brighter stars with more extreme planets.

“In some sense, it’s a race to the bottom: finding the smallest planets around the

 smallest stars because those are the ones that might be habitable,” says Gaudi.

 “What our collaboration likes to say is that there’s room at the top.”

Gaudi and his team detected KELT-9b by observing the dip in light as the

 planet passed between us and its star, which is about 650 light years away

and almost twice as hot as the sun. It is by far the hottest, brightest and most 

massive star around which we’ve ever found a planet in this way.

Short orbit

Given the star’s temperature, it is unsurprising that its planet is the hottest we’ve

discovered. With an orbit of merely an Earth day-and-a-half or so, KELT-9b is very

 close to its star and likely to be tidally locked, with a dayside always facing the star

 and a cooler nightside always pointing away.

As a result, its dayside is constantly bombarded by heat and bright, high-energy blue

 and ultraviolet light. This makes the dayside surface as hot as a relatively warm star, 

while the nightside is still hotter than the most tepid stars, like our cosmic neighbour

 Proxima Centauri (which has at least one exoplanet of it

KELT-9b is heated from the outside rather than from internal atomic fusion, 

so it will never become a star, but it probably looks like a bright ember.

“The planet’s dayside, which is facing the star, is going to glow a bright orange, 

and then on the nightside, it’s going to be deep, dark red,” says Gaudi.

“This planet is much hotter than most stars in the universe,” says David Charbonneau

 at Harvard University. “It is intriguing to consider how it formed and what its

 ultimate

 fate will be.”

Vanishing atmosphere

The star’s intense light and heat are enough to evaporate the gas giant’s atmosphere

 at a rate of up to 10 million tonnes per second, which may render the planet a naked 

core by the time its star expands and envelops it.

One of the greatest mysteries in the study of exoplanets are the atmospheres of hot

 gas giants. Luckily, KELT-9b has a towering atmosphere that absorbs some of the light

 from its particularly bright star, making it a prime candidate for additional observations by

 both ground and space-based telescopes.

“Finding a wider range of these planets, especially ones that are amenable to further

 study like this one, is really important to trying to understand these types of worlds,”

says Gaudi.

And although KELT-9b is the hottest exoplanet found so far, Gaudi hopes its discovery 

will encourage researchers to examine more big, hot stars in the hunt for even stranger

 worlds.

“The diversity of planets orbiting other stars is humbling,” says Charbonneau.

 “Whenever I think, ‘No, there couldn’t possibly be a planet there’, sure enough one turns up.”

(New Scientist)