Last year, scientists found the "hottest" planet to date, with a surface temperature of 4,000 degrees Celsius. Although not as good as the sun, it also surpassed most stars. In what way does the material exist in this magical form? On the morning of August 16th, Beijing time, a Swiss team published a paper in the world's top academic journal Nature, which found iron vapor and titanium vapor in the atmosphere of the hottest planet.

The planet is located in Cygnus, about 650 light-years from Earth, and was found by the North KELT Telescope in Arizona, USA, and was named KELT-9b. The temperature of the star KELT-9 is more than 10,000 degrees Celsius, almost twice the sun, and the KELT-9b is only one-thirtieth of the distance from the Earth. This results in a surface temperature of 4,000 degrees Celsius for KELT-9b, which is hotter than most stars.

The "hottest planet" has attracted the interest of scientists. What constitutes its atmosphere? Under such extreme conditions, what kind of evolution has it experienced?

Researchers at the University of Berne, Switzerland, have recently completed a theoretical analysis that shows extreme temperature and radiation conditions that most molecules on the planet cannot exist and can only exist as atoms. This is because, at such high temperatures, collisions occur between the particles, breaking the chemical bonds of the constituent molecules. The team also theoretically predicted the human telescope conditions and was able to observe the gaseous metal atoms in the KELT-9b atmosphere.

On the night of July 31-August 1, 2017, scientists from the Department of Astronomy at the University of Rib, Switzerland, used the Galileo National Telescope in La Palma, Spain to observe the transit of the KELT-9b. The so-called transit day refers to the phenomenon that when the planet glides through the surface of the star in the field of view of the telescope, it causes the brightness of the star to drop. The KELT-9b is very close to the star and will revolve every 36 hours. During the transit, a small portion of the light from the star is screened by the planet's atmosphere, and scientists can use spectrometers to analyze the composition of the planet's atmosphere. Just as a prism splits the sun into seven colors, the spectrometer breaks down the light that stars pass through the planet's atmosphere into a spectrum. Substances in the planet's atmosphere, thus leaving a unique "fingerprint" in the spectrum.

Iron is one of the most abundant transitional metal elements in the universe. However, most of the iron elements on exoplanets are mixed in gaseous oxides or fine dust particles, which is difficult to detect directly. This time, the experimental scientists found the neutral iron atoms and single-ion iron in the atmosphere of KELT-9b according to the predictions of theoretical scientists.

Jens Hoeijmakers, who led the study, said that after digging deep into the data, they also discovered that another metal vapor leaves a fingerprint in the spectrum: titanium.

KELT-9b represents a new class of planets called "super hot Jupiter." In the same environment, most of the planets will evaporate, and KELT-9b is one of the few cases to survive. This time, humans have seen a glimpse of the atmosphere of this type of planet, confirming that the high temperature will decompose most of the molecules. In the future, it will continue to play a unique "laboratory" for scientists to study the evolution of planets under high temperature and high radiation.