KELT-9b is unlike any planet that exists in our Solar System. Described as an ultra-hot Jupiter, this gaseous giant lies so close to its host star that it completes a full orbit in just 36 hours. This speedy orbit, coupled with the incredible brightness of its star, KELT-9, has made complete observations of the planet, located 620 light-years away from Earth, rather complicated.
However, the efforts of an international team of researchers to curb these issues have made such observations possible. Using an Italian telescope, the Telescopio Nazionale Galileo, the group have directly detected iron in KELT-9b’s atmosphere – the first time the element’s presence in an exoplanet has been demonstrated from a planet’s own light.
A previous look at the starlight from KELT-9 while the gas giant passed in front of it, told the astronomers there had to be iron in the planet’s atmosphere. However isolating the light of the planet to confirm this observation was a tricky process. Not only is KELT-9b outshone by the host star, whose surface temperature is almost twice as hot as the Sun, but it is the planet’s dark night side that faces Earth in the short time-frame in which it is visible.
Their only chance was to try and pick up the light from the planet’s hotter and brighter day-side as it disappeared behind the star. This 8-hour window was captured by the Italian telescope, located on the Spanish island of La Palma, on the night of July 22, 2018. A spectrograph attached to the telescope, called HARPS-N, along with a technique called cross-correlation enabled the astronomers to extract the light from the planet and identify the presence of specific atoms and molecules.
“The star is stationary, but the planet is moving,” Lorenzo Pino, lead author of the study accepted for publication by The Astronomical Journal Letters and currently available on preprint server arXiv, said in a statement. “The cross-correlation is a kind of filter that moves with the planet. This allows us to isolate the planetary light.”
From this data, the team think that iron in the planet’s upper atmosphere absorbs the incoming starlight and therefore heats this top layer, whilst the lower sections remain colder. This process is similar to the ozone’s absorption of radiation in the Earth’s atmosphere.
Going forward the researchers hope to quantify the iron content in KELT-9b’s atmosphere and ultimately help to reveal how all hot Jupiters emerge.