# How many times can iron be detected for the first time in an exoplanetary atmosphere?

I see today that there's a story about the first detection of iron in an exoplanetary atmosphere, in this case in the atmosphere of the ultra-hot Jupiter KELT-9b.

I remember there being a story a couple of months ago about the detection of iron rain in an exoplanetary atmosphere, the example being another ultra-hot Jupiter, WASP-76b.

It doesn't seem plausible for both of these to be the first detection of iron in an exoplanetary atmosphere. So which one is it?

Neither of these, interestingly enough, is the first time iron has been detected in an exoplanetary atmosphere. Other groups (Hoeijmakers et al. 2018, cited by both papers) have detected absorption lines of iron and other metals in the day-night transition zone of exoplanets, during transit. Ehrenreich et al. 2020, the new ESO paper, did something similar.

Now, Pino et al. 2020 are referred to in the spaceref.com article as having "directly demonstrated" the presence of iron in an exoplanet atmosphere for the first time. What I believe "directly" in this case refers to is the detection of iron emission lines, rather than iron absorption lines. This is what makes Pino et al.'s observations different: they observed KELT-9b shortly before it passed behind the star, allowing them to view the hot, day side of the planet and thus iron emission.

In other words, I think the spaceref.com article is playing fast and loose with its wording - Pino et al.'s results don't constitute the first time iron has been detected in an exoplanet atmosphere, just the first time iron emission lines have been detected. They got around that by characterizing emission lines as "direct"$$^{\dagger}$$, implying absorption lines are "indirect". Whether you agree with that is up to you, I think.

If you had to write an article labeling one study as the first detection of iron in an exoplanetary atmosphere, you could make a good argument in favor of Hoeijmakers et al., but I think the better option might be to simply not try to describe any of these in that manner. Some nuance here wouldn't be a bad thing.

$$^{\dagger}$$Hoeijmakers et al. refer to their observations as the first "direct detection" of iron, adding a layer of complexity and irony.