Astronomers Discover Iron In Exoplanet's Atmosphere For The First Time

Artist's impression of Exoplanet KELT-9b's atmosphere and its star. Denis Bajram, denis@bajram.com.

Roughly 620 light-years from Earth, there is KELT-9b, a record exoplanet. This is the hottest gas giant exoplanet discovered so far, with a temperature of over 4,000 Kelvin (3,750°C/6,800°F) – hotter than many stars. Now astronomers have been able to study its atmosphere and have detected iron and titanium. 

As reported in Nature, this discovery is an important first in the studies of worlds outside the Solar System. Iron has never been detected in the atmosphere of an exoplanet before, although the element is the most abundant transition metal.

Finding the metal in the atmosphere wasn’t straightforward. Iron is usually highly refractive, so it doesn’t leave a strong fingerprint in the light we see coming from the planet. However, the researchers performed detailed simulations of what an extreme atmosphere like this should be like. This gave them confidence that it might be possible to see iron in the atmosphere of KELT-9b with current instruments.

"The results of these simulations show that most of the molecules found there should be in atomic form, because the bonds that hold them together are broken by collisions between particles that occur at these extremely high temperatures," co-author Kevin Heng, a professor at the University of Bern, said in a statement.

The team observed the planet as it passed in front of its star. During this transit, the light from the star is filtered through the planet's atmosphere, which astronomers can use to work out its chemical composition based on the light. The simulations suggested that iron vapor, if present, would leave a clear signature.

"With the theoretical predictions in hand, it was like following a treasure map," added lead author Jens Hoeijmakers, a researcher at the Universities of Geneva and Bern, "and when we dug deeper into the data, we found even more." The observation revealed that the planet also had titanium vapor in its atmosphere.

KELT-9b is an ultra-hot Jupiter, a class of incredibly high-temperature gas giant planets. These objects have complex atmospheres, partly like stars and partly like planets, so objects like KELT-9b are unique laboratories to study the evolution of atmospheres exposed to the extremes of the cosmos.

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