One of the hottest known exoplanets appears to be even stranger than previously thought. As NASA has determined with measurements by its new exoplanet hunter TESS, the celestial body called KELT-9 b has two “winters” and “summers” in each of its only 36 Earth-hours years.

This is due to the peculiarities of the star KELT-9 and the unusual orbit of the exoplanet, which was discovered in 2017. TESS was able to observe 27 transits of the exoplanet in 2019, enabling more accurate models of the system.

According to astronomers, KELT-9 b is about 80 percent larger than Jupiter, but is nearly three times the mass. It orbits around its star in a bound rotation, so it shows the same side to its sun all the time, which receives more than44,000 times as much energy as our Earth receives from the sun. On the day side of KELT-9 b, temperatures reach 4300 degrees Celsius, which is almost twice as hot as on WASP-76b, where it rains iron and even more than on many stars. A KELT-9 b orbit takes only 36 Earth hours, which is why the telescope TESS could see so many transits of the distant exoplanet within a few weeks.

KELT-9 is unusual in that the star is about twice the size of the sun, but only about 56 percent hotter. At the same time, however, it rotates 38 times faster, making its poles flatter. KELT-9 b, on the other hand, circles extremely close to the star. Over the Poles there are therefore two “summers” and two “winters”, each for 9 hours.

The researchers were able to determine all this from the peculiarities of the blackouts that TESS analyzes. If the exoplanet flies past its star from our perspective, it temporarily blocks the light. The fluctuation is detected by TESS.