New oddities discovered in the exoplanet KELT-9b and its star

Artist's concept of Kelt-9b and its star in the background (Image NASA's Goddard Space Flight Center/Chris Smith (USRA))
Artist’s concept of Kelt-9b and its star in the background (Image NASA’s Goddard Space Flight Center/Chris Smith (USRA))

An article published in “The Astronomical Journal” reports a study on the characteristics of the orbit of the exoplanet KELT-9b, an ultra-hot Jupiter very close to its star. A team of researchers led by John Ahlers of the Exoplanets and Stellar Astrophysics Laboratory at NASA’s Goddard Space Flight Center used data collected by NASA’s TESS space telescope to create a model of the interaction between star and exoplanet that allowed to understand better the peculiar characteristics of the star and the extreme ones of KELT-9b. For example, it turned out that the star spins so fast that its poles are flattened making them hotter, and the exoplanet orbits around those poles with the consequence that it has two summers when it passes over them while it has two winters when it passes over the star’s equator.

The exoplanet KELT-9b became famous in 2017 after a study showed that it’s hotter than most stars. That’s because it’s an ultra-hot Jupiter, an extreme case of a gas giant planet that orbits very close to its star with the consequence that it receives an enormous amount of heat from it. The star has a size that is approximately twice the Sun and the heat it emits is on average 56% higher than the Sun’s, with the result that KELT-9b receives an amount of heat that is on average 44,000 times what the Earth receives from Sun.

The interest in the exoplanet KELT-9b led to more studies and this new one was possible thanks also to the new 27 transits in front of its star detected between July 18 and September 11, 2019, by the TESS space telescope, launched on April 18, 2018. The new observations made it possible to better understand the characteristics of the star, which turned out quite extreme as well. It spins in 16 hours, 38 times faster than the Sun, so fast that the centrifugal force distorts its shape by widening it at the equator. The consequence is the phenomenon called gravitational darkening, which means that the star’s poles are hotter and brighter than the equator with a surface temperature difference between these areas of almost 800° Celsius.

The orbit of the exoplanet KELT-9b is another anomaly because it’s polar, which means that it’s so misaligned from its star’s rotation that, at an angle of about 87°, it’s almost perpendicular to it passing over its poles. The consequence is that KELT-9b passes over one pole of the star, over its equator, over the other pole, and again over the equator. Because of the temperature difference in the various areas of the star, on this exoplanet, it’s summer twice a year, when it passes over the poles, and it’s winter twice a year, when it passes over the equator. Each season lasts a few hours as its year lasts around 36 hours.

Jason Barnes, a professor of physics at the University of Idaho, one of the authors of this research, stated that of the planetary systems studied by him and his colleagues through gravitational darkening, the effects on KELT-9b are by far the more spectacular. He added that this work brings an advancement towards the unification of gravitational darkening with other techniques that measure planetary alignment, which in the end they hope it will reveal the secrets of the formation and evolution of planets around high-mass stars.

The exoplanet KELT-9b is confirmed as an extreme case, together with its star. The progressive discovery of exoplanets, which include many hot Jupiters, is bringing a lot of useful information to study the migrations of planets that form far from their star and then get closer. In the case of KELT-9b, the age of the star is estimated at around 300 million years, therefore the strange configuration with its exoplanet was reached very quickly in astronomical terms. It will certainly be at the center of more research.

This NASA video shows the configuration of the exoplanet KELT-9b and its star.

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