620 light-years away, astronomers have directly proved for the first time: the presence of iron in the atmosphere of an exoplanet

[Mobile software: Bo Ke Yuan] An international research team led by astronomers from the University of Amsterdam has directly proved the presence of iron in the atmosphere of an exoplanet for the first time. The researchers found emission lines from uncharged iron atoms in the spectrum of the exoplanet KELT-9b. The observations were complicated because the exoplanet is eclipsed by its bright host star. The exoplanet KELT-9b orbits its star KELT-9 every 36 hours. The star and planet are located in the constellation Cygnus, about 620 light-years from Earth.

The star is more than 10,000 degrees hot, almost twice as hot as our sun, and the planet KELT-9b is larger than Jupiter, but it is very close to the star, about one-thirtieth the distance from Earth to the sun. Astronomers already knew there must be iron in the planet's atmosphere; they saw signs of it years ago when studying starlight as the planet passed in front of it. In the new observations, astronomers looked directly at the planet's spectrum of light, a process complicated by the fact that the planet appears dimmed in the light of its star.

Additionally, because it is so far from its host star, a year on this planet is only about a day and a half on Earth time. For half of this short year, the night side of the planet is facing Earth, but that side is too dim to be seen. Therefore, the researchers began observing the planet eight hours before it disappears behind its star, observing its hotter, brighter day side. Lead author of the study, Lorenzo Pino (University of Amsterdam), compared searching for the light of an exoplanet against the glare of its host star to looking at a firefly near a lamppost:

Seeing the shadow of a firefly means, for researchers, the shadow of an exoplanet, and now an exoplanet has been seen directly. The researchers made the observations using the Italian telescope Telescopio Nazionale Galileo on the Spanish island of La Palma. The telescope features HARPS-N, a spectrometer that can split light and reveal the presence of specific atoms and molecules. The researchers extracted the atomic emission lines using a technique called cross-correlation. A series of images were compared: the star tends to be stationary, but the planet is moving.

The cross-correlation is a filter that moves with the planet, which allows astronomers to isolate the planet's light. Based on the data, the researchers now believe that iron in the atmosphere of the exoplanet KELT-9b heats the upper part of the atmosphere, making it warmer than the lower part. The idea is that iron absorbs starlight, which heats the atmosphere. On Earth, a similar process occurs in the atmosphere, however, in this case it is not iron that heats the top layer, but ozone. In the future, the researchers hope to conduct more in-depth observational studies by precisely measuring the iron content in the planet's atmosphere.

This could be achieved, for example, using the Hubble Space Telescope, to which Lorenzo Pino is assigned observing time. Ultimately, the astronomers hope to reveal how hot, gaseous giant exoplanets like KELT-9b emerge and why there are no similar examples in our own solar system.

Bo Ke Yuan | Research/From: Netherlands Academy for Astronomy

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