This is a place to put terms that may be unfamiliar to some, or that mean different things to different scientific communities. Please try to assemble this list in alphabetical order.


Alternating early/late transit


Closest approach planets

CMB (Astro)

Cosmic microwave background: thermal radiation left over from the Big Bang

CMB (Geo)

Core-mantle boundary: the interface between the metallic core and the rocky mantle.

Core (Astro)

The solid portion of a planet (below the atmosphere). For gas-giant planets, the most favored model is that this core (mostly) grows first, onto which gas accretes from the neblua.

Core (Geo)

The metal-rich interior layer of a rocky planet (below the silicate mantle.)



Type 1:

Motion of a planet while it is embedded in a gas disk, due to torques it launches in the disk. It is relevant for planets 1-50 M_Earth, or so -- they migrate faster for more massive planets, until...

Type 2:

A planet that is massive enough to open a gap in its disk (about Saturn's mass, depending on disk properties) then migrates slower, migrating along with the viscous evolution of the disk.
Key point --> Both types of migration are subject to many details, discussed by lots of papers.

Open cluster

A loosely-bound set of stars within a galaxy, which formed out of the same cloud (age spread of < a few Myr and small metallicity spread). Such clusters likely unbind, such that their members become "field stars" -- around which exoplanets are found in abundance.

Oxygen Fugacity

Oxygen fugacity is an equivalent of the partial pressure of oxygen in a particular environment (atmosphere, rocks, etc.) corrected for the nonideal character of the gas. (
In geology, a redox buffer is an assemblage of minerals or compounds that constrains oxygen fugacity as a function of temperature <and pressure>. ( <modified by Jie Li>.
<Note that oxygen fugacity is typically expressed in terms of the difference in log units with respect to a common redox buffer, e.g., Fe-FeO or iron-wüstite (IW). For the Earth, IW-4 is considered highly-reducing, IW-2 is the condition of core-mantle equilibrium, and IW is considered oxidizing>

Partition coefficient

The ratio of concentrations of two components in a mixture, at equilibrium.


The study of rocks (from the Greek πέτρα, petra = 'rock' and λόγος, logos = 'study' )

Refractory (materials)

Small planet

Depends on context, of course, but exoplanet observers often means one without a massive H/He envelope which makes it look much bigger.

synodic period

time between conjunctions


transit time variation


A rock fragment that is entrained in a larger rock during the formation of that larger rock. For example, a mantle xenolith is a sample of mantle rock that is brought up to the surface in a volcanic eruption.