# What does "O I" mean in the context of detected molecules?

I've recently been analysing the Exoplanet Database, and have been looking at the 'molecules' field (molecules detected on the planet or in its atmosphere). They all make sense apart from "O I". It's not iodine oxide (which would be I2O), so I'm at a loss as to what it could be, especially since I'm very new to this field.

Any help is very much appreciated!

• Hint: Search for "ionization states of atomic oxygen;" a single atom molecule.
– uhoh
Mar 1 '18 at 10:17
• Please edit and gives us an example (text or link) of where we can find that abbreviation
– user1569
Mar 1 '18 at 10:34

This is the standard astronomer notation for neutral oxygen. Where chemists would write $\mathrm{O}$ (or $\mathrm{O}^0$), $\mathrm{O}^{+}$, $\mathrm{O}^{++}$, and $\mathrm{O}^{+++}$ (or $\mathrm{O}^{3+}$), for neutral, once-, twice-, and thrice-ionized oxygen, respectively, astronomers almost exclusively write $\mathrm{O}\,\mathrm{I}$, $\mathrm{O}\,\mathrm{II}$, $\mathrm{O}\,\mathrm{III}$, $\mathrm{O}\,\mathrm{IV}$, etc.
The same is of course the case for other elements. For instance, you may have an $\mathrm{H}\,\mathrm{II}$ region (i.e. ionized hydrogen) surrounding hot stars, in a larger $\mathrm{H}\,\mathrm{I}$ (i.e. neutral) envelope.
• @JThistle Atmospheres are far from my comfort zone, but in general to form molecules, you must have high densities and low temperatures. The former should be satisfied in a atmosphere, I suppose, but perhaps it's too close to its star so the temperature is too high, or it is exposed to a strong UV field from the star, so $\mathrm{O}_2$ is dissociated?