The largest exoplanets are typically called Jupiter-like planets, or hot/cold Jupiters. Since Jupiter is the largest most massive planet in our Solar system, it makes sense that it would be chosen as an analog. For intermediate-size exoplanets, Neptune is used as an analog (e.g. 1).

Neptune and Uranus are both roughly the same size and mass. So why is Neptune rather than Uranus used as the archetype of intermediate-mass planets?

Did someone call an exoplanet a Neptune-like planet, choosing Neptune over Uranus for no particular reason, and then everyone else followed suit? Or is there a fundamental difference between the two planets that makes Neptune a better analog?

  • $\begingroup$ Interestingly, Neptune seems to be more popular than Uranus, over most of the last centuries, see Google Ngram. I am aware that the words are not necessarily refering to planets, in particular as the discovery of Neptune was in September 1846. (Uranus was discovered in March 1781.) $\endgroup$ – B--rian Mar 16 at 12:39
  • $\begingroup$ Just guessing, Uranus is a bit of an oddball, with its extreme axial tilt, and low temperature. $\endgroup$ – PM 2Ring Mar 16 at 12:45
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    $\begingroup$ I'm 100% sure that a) it's because of the way Uranus sounds when said out loud, and b) the decision was made in the collective unconscious, we'll never find a smoking gun, or a... $\endgroup$ – uhoh Mar 16 at 12:48
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    $\begingroup$ @uhoh I totally agree with your explanation, that's why I was playing with Ngrams. My result is far from reproducible since Ngrams also depends heavily on the corpus used. Beside of the obvious puns on Uranus, Neptune is also the more-known Greek god than Uranus, I feel (without having a statistics on that). $\endgroup$ – B--rian Mar 16 at 12:53
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    $\begingroup$ @uhoh I suspect Urontosomething. $\endgroup$ – David Hammen Mar 16 at 17:30

NASA distinguishes four types of exoplanets: Gas Giant, Super-Earth, Neptune-like, and Terrestrial. The question asks why ice planets termed "Neptune-like" and not "Uranus-like".

Let's start with some statistics: What is curious, that (at least according to scholar.google.com), intially more icy exoplanets were termed Uranus like planet rather than Neptune like planet before eventually, the latter term started growing:

1980 to 1990

1990 to 1995

1995 to 2000

2000 to 2005

2005 to 2010

This supports the hypothesis that it was a "vote with the feet", rather than something coordinated, much like initial density fluctuations in a protoplanetary disk which might serve as the seed for a planet, or not.

@uhoh already suspected this in a comment, plus also mentions that the pronouncaction of Uranus might lead to unintended puns when talking about "planets like ...". It is sometimes these linguistic pecularities which make a brand successful and another one not.

I'm 100% sure that

  1. it's because of the way Uranus sounds when said out loud, and
  2. the decision was made in the collective unconscious, we'll never find a smoking gun, or a...

After all, there quite some similarities between Uranus and Neptune, see e.g. the concise comparison between the two planets at WolframAlpha, although there is also reasoning Why Neptune and Uranus are different.


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