Extrasolar planet vs. Exoplanet: 300 words about a trifling choice

After pausing for a bit  when, at the prodding of a friend, I couldn’t remember whether I used “modeling” or “modelling” in my writing, I thought about another choice I face often. I often find myself using the terms “exoplanet” and “extrasolar planet” interchangeably to denote any planet outside the Solar System. I definitely use “extrasolar planet” more often during talks, even though it is a mouthful, and “exoplanet” in writing — especially in communicating with colleagues, where the meaning of the word does not need explaining, versus communicating with the general public. 

Let’s first get this out of the way: the two terms are synonyms. That said…

A cursory Google search of “extrasolar planets” (~380,000 results) vs. “exoplanet” (~830,000 results) reveals a definite 2:1 preponderance of the latter term.  A search of Greg’s oklo.org blog reveals a very similar ratio of blog posts using the two terms. Wikipedia prefers exoplanet, while Encyclopedia Britannica goes for extrasolar planet.

What has the scientific community at large settled on? I used a tool (only for private use, for now) that does simple ADS queries across years to track the popularity of keywords across article abstracts. This chart was the result:

Exoplanet vs. Extrasolar Planet: the community has spoken.
Exoplanet vs. Extrasolar Planet: the community has spoken.

The chart shows the number of articles published (on a log scale), per year, containing the keyword in the abstract. [ref]Any “holes” in the curve are due to 0 papers being published that year.[/ref]

The first mention of “extrasolar planet” appears to be in 1971, from this Icarus paper (Photometric Color Indices of Extrasolar Planets), while the word exoplanet appears in ADS in 1992. [ref]Note that these queries are open to non-refereed (e.g. proceedings) sources as well – I am using default ADS settings.[/ref]

Interestingly, it appears that the fortunes of the two keywords rapidly reversed: around 2003, the usage of “extrasolar planet” started flattening out, while “exoplanet” continued its meteoric rise. Around 2007, “exoplanet” caught up and quickly started surpassing “extrasolar planet”. (The most cited paper in 2007 was Dan Fabrycky’s paper Shrinking Binary and Planetary Orbits by Kozai Cycles with Tidal Friction, by the way… which used “extrasolar planet”).

In 2013, “exoplanet” beat “extrasolar planet” more than 4:1!  Paper containing both words made up about .8% of all indexed astronomy papers (down from a high of ~1% in 2011).

So, if in doubt, go with the majority and use exoplanet. (Or not!)

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