Classifying Exoplanets: Size & Surface Temperature

[lpetrich]

http://phl.upr.edu/projects/habitable-exoplanets-catalog/media/pte has an interesting classification, though it's a general grid rather than a wrap-around table.

Here are the planet sizes:

Mercurian /
Miniterran ... 10^(-5) - 0.1 Me ... 0.03 - 0.4 Re
Subterran ... 0.1 - 0.5 Me ... 0.4 - 0.8 Re
Terran ... 0.5 - 2 Me ... 0.8 - 1.25 Re
Superterran ... 2 - 10 Me ... 1.25 - 2.5 Re
Neptunian ... 10 - 50 Me ... 2.5 - 6 Re
Jovian ... > 50 Me ... > 6 Re

These look rather arbitrary, but it would be nice if someone could propose boundaries associated with changes in planets' features. Like having an atmosphere or having plate tectonics or being rocky or watery as opposed to gassy.

Here are the surface temperatures, by whether the planet's surface has a habitable range of temperatures:
Hot -- too hot
Warm -- in the right range
Cold -- too cold

I'm not going to repeat the numbers for known exoplanets and Kepler candidates, but I'll do so for the Solar System, so one can see how this system works:

Hot Miniterrans ... 1 ... Mercury
Warm Miniterrans ... 1 ... Moon
Cold Miniterrans ... (numerous) ... the larger asteroids, outer-planet moons, Kuiper-belt objects
Hot Subterrans ... 0
Warm Subterrans ... 1 ... Mars
Cold Subterrans ... 0
Hot Terrans ... 1 ... Venus
Warm Terrans ... 1 ... Earth
Cold Terrans ... 0
Hot, Warm, Cold Superterrans ... 0
Hot, Warm Neptunians ... 0
Cold Neptunians ... 2 ... Uranus, Neptune
Hot, Warm Jovians ... 0
Cold Jovians ... 2 ... Jupiter, Saturn

The Solar System does not fit the statistics of known exoplanets or Kepler candidates very well, but that is due to observational selection. Most of the Solar System's larger objects are invisible to our exoplanet-search efforts to date, or at best borderline visible, like Jupiter.

It is also evident that the Solar System has a gap in the superterran part of the classification.

 

[slatts]

Anyone looking at this would probably really dig a book called Rare Earth. When it appeared in the library a couple of years ago, I figured it was either a particularly thick religious tract or a granola-oriented paen to how wonderfully gooey everything is here. It turned out to be a summary, written by a couple of NASA scientists, of fascinating biological, geological, climatological, and (above all) astronomical reasons for the (probably) phenomenal rarity of multi-cellular life in our galaxy, complete with several pages of tables.

 

[lpetrich]

That's a different sort of issue.

Here are some exoplanet catalogs: http://exoplanets.org and http://exoplanets.eu However, they don't seem to have any way of overlaying the Solar System on to their graphs, and they don't try to estimate surface temperatures. A History Of Planet Detection in 60 Seconds | Lost in Transits has a nice animated GIF of that, plotting by orbit period and estimated mass. It also includes the Solar System, and one can get an idea of how detectable it is. Jupiter is the only planet that overlaps the known exoplanets, and Saturn, Venus, and the Earth are close to overlapping.

 

[slatts]

It's an interesting subject. In the short term, like the next century or so, I figure the book I mentioned will tend to stimulate manned space exploration, because I believe asteroid hits were projected to be much the largest factor in the rarity of life beyond the one-celled or one-celled-in-the-process-of-dividing level. Reaction to the approach of an unexpected asteroid could require very rapid and manned intervention, given the complexity of the three-body problem and the tendency of that stuff to disintegrate. The surface temperatures of the planets involved would have virtually nothing to do with it, but correlations between planet and star masses might be projected down to the asteroid level, so it's good to see the data base as substantial as the sites you've mentioned are showing it to be.

 

[Drakkith]

These look rather arbitrary, but it would be nice if someone could propose boundaries associated with changes in planets' features. Like having an atmosphere or having plate tectonics or being rocky or watery as opposed to gassy.

Considering our limited ability to observe these kinds of properties and the infancy of exoplanet observation I'm not sure we need a set classification system yet.

 

[lpetrich]

Back to the main subject, I've found Albedo and Reflection Spectra of Extrasolar Giant Planets - Abstract - The Astrophysical Journal - IOPscience The classification:

• I: Ammonia clouds (Jupiter, Saturn) -- 0.63, 0.59, 0.57, 0.55, 0.48, 0.38
• 150 K
• II: Water clouds -- 0.88, 0.84, 0.81, 0.79, 0.70, 0.56
• 350 K
• III: No clouds, sodium and potassium absorption -- 0.17, 0.14, 0.12, 0.10, 0.05, 0.01
• 900 K
• IV: Silicate clouds, sodium and potassium absorption -- 0.04, 0.03, 0.03, 0.02, <0.01, <0.01
• 1500 K
• V: Silicate clouds -- 0.57, 0.56, 0.55, 0.55, 0.53, 0.51

Albedos are Bond albedos for a fiducial or reference case, for primary stars having spectral types A8 V, F7 V, G2 V, G7 V, K4 V, M4 V

Cases I, II, and V are bright with a slight bluish tinge, while cases III and IV are dark blue.

Bond albedos for the Solar System (Wikipedia article): Mercury: 0.068, Venus: 0.90, Earth: 0.306, Moon: 0.11, Mars: 0.25, Jupiter: 0.343, Saturn: 0.342, Enceladus: 0.99, Uranus: 0.300, Neptune: 0.290

Jupiter and Saturn are both darker than what this calculation indicates, because their clouds are colored by "tholins", products of prebiotic organic synthesis. However, Venus's value is about right for water clouds, though its clouds are concentrated sulfuric acid.

 

[lpetrich]

I've found this: an Earth Similarity Index (ESI) - Planetary Habitability Laboratory @ UPR Arecibo, calculating how much like the Earth a planet is.

• Interior: mean radius, bulk density
• Exterior: escape velocity, surface temperature

That page also plots several exoplanets with those two axes, including the Solar System's planets and planetlike objects, like large moons. For the exoplanets' surface temperatures, they used a radiative-equilibrium value with a plausible albedo, ignoring greenhouse effects. So if one treated Venus as an exoplanet in this calculation, ignoring its very high surface temperature, it would be the most Earthlike planet, instead of being less Earthlike than Mars, the Moon, and Mercury.