BIOLOGY | PART 8 | PLANT COLOR & SUNLIGHT - PART 3

PAART'S AUTUMNAL COLORS

VOTING RESULTS

Winning 2 to 1, for dominant plant color of Paart is...... Pastel Red, well, sorta, theory is beautiful, about it's feasibility - it gets complicated.

The starting conditions (early Cambrian) of Paartian soil is a pH 4,79 (wet soil), or similar to that of coffee left over to sit for 24h - places where the rain is minimum, ie, further inland of Sthalika, the acidity could be as low as pH 6,69 - equivalent to that of milk or Earth normal rainwater.

One way to do make reddish plants would be to use carotenoids such as β-carotene, licopen or lutein, or even use the low soil pH and give them red anthocyanin.

 

VIABILITY

On Earth, green algae wouldn't appear until ~1Gya, and red algae been around for at least 1,5Gya, however carotenoid biosynthesis wouldn't appear until ~580Mya (split between cilliates and diatoms), red anthocyanins until 450Mya.

So far, seems inevitable we would first have blue-green algae - but, while on Earth in it's analogue stages to where Paart is now, the oxygen levels were rising just above 10%, on Paart the oxygen levels have been decreasing from 14% to 10% in the last 310My, which means that the ozone layer of Paart during this period would have been in such high levels it would take another 200My for Earth to reach into the Devonian period.

One great limiting factor for life on land have been the amount of UV radiation incidence, with a thick ozone layer by 450~400Mya, mosses and land plants could finally arise make their way into the barren continents.

SOLUTIONS

Paart has already a great advantage compared to Earth, being so much far away from a low-mass star with a not so intense UV output, and receiving relatively little light - a thick ozone layer seems like the cherry in the cake, the bright lamp in a room full of moths - begging for an early plant-life explosion, short, but early explosion, at least, enough algae biodiversity that could have armed paartene life with the right chemicals for anthocyanin production.

Anthocyanin acts as plant sunscreen, regulating UV stress, the declining levels of oxygen after this small algal bloom would select ones that could produce anthocyanin more efficiently to deal with increasing UV radiation - mosses and terrestrial algae could have appeared as soon as 3950PMA.

MEET, THE CHRYSOPHYLL

Apart from early Earth plant-life, which would have a hard time trying to find magnesium in such acidic soils for the making of chlorophyll b, but would have no problem in finding iron, cobalt, copper or zinc - curiously enough, zinc and copper porphyrins are more efficient than conventional magnesium ones used by plants.

Zinc Protoporphyrin (ZPP) can be found in the bloodstream when by some reason the production of heme is inhibited (lead poisoning) or the body has a lack of iron - plants also could use ZPP as an anti-cancer agent in their body structure to deal with increased UV radiation.

Zinc Porphyrin 8 has a power conversion efficiency of 7,13%, with a major absorption line at ~425nm, 560nm and 610nm - magnesium porphyrin efficiency is around 5,73% - in practice Chlorophyll's efficiency is about 4%, I couldn't find papers on nature's use of Zn(II)P8, but on the same scale of fallibility, about ~4,98% efficiency, or +24,4% as efficient - such an efficiency boost would yield the same as using conventional chlorophyll a under 85% Earth's sunlight - instead of Paart's dull 68% Earth sunlight incidence.

On the other hand, by adding zinc acetate to porphyrin a in acidic medium, you are able to make zinc-chlorophyll, at room temperature (about 25°C), said to have similar potential as magnesium-chlorophyll - similar processes can be made to tweak bacteriochlorophyll to work with a zinc center metal.

Zn-Chl-1 has a strong absorption at 720nm, and slightly less at 420nm, mostly reflecting yellow-green light, but making far better use of these wavelengths than chlorophyll a (noticeable when comparing quantum efficiency yield), having an efficiency of 11,44% - turned down by the same previous fallibility value, maybe around ~8,0% efficiency, or +40% as efficient.

With relatively acid oceans (pH 6,6~7,0) Paart's zinc is mostly abundant in the form of ionic Zn+2 (~60%), and then in the form of citrates, phosphates and sulfates - in that scenario, despite being 40x less abundant than magnesium, it could have proportioned a great advantage over chlorophyll users - one example of this rare but efficient is vitamin B12, using cobalt which is 3x rarer than zinc in Earth's crust and is essential to DNA protection.

I will dub this form of photosynthetic pigment, which reflects yellow and red, Chrysophyll (Chy), or "golden-leaf" pigment.

When land plants appear, we would have a variety of yellow-orange mosses depending on their content of anthocyanin, purple and blue plants may appear the further away from acidic soil, and green plants may be a rarity, or, reduced to low-lying plants such as grassland and shrubland even in equatorial areas, whereas reddish plants occupy the dense forest spaces.

THE COLORS WE COULD HAVE

Honestly, I have to admit that I made the palettes before exploring the actual photosynthetic chemistry that's viable on the planet, so my original options doesn't include any of that material.

Potential 6 flavors of pigment plus different anthocyanin levels

I made an additional table for those extra 2 options that seem to be more viable than conventional chlorophyll.

If we assume every scenario is equally likely, red will appear 28,6% of the time, green 24%, yellow 19,5%, blue and purple 8,6% and 7,5%, respectively - the average of the values is a yellow-brown color (HEX 876150).]

I mapped out the distribution of acidity in soil colored by anthocyanin response to it, for the early Cambrian maps of Paart.

Regions A, B and C to track the mean plate movement of such regions

 

By the Paleocambrian period, 62,97% of the land area is neutral, 13% of the land have a pH around 6, 9,12% around pH 5, and 10,4% with pH of about 4,5.

In turn what this all mean, is that about 1/3rd of the land favors red/purple pigments, while the rest favors blue plants - what would not have happened if Paart's continents where smaller and tropical, what would have avoided a large neutral area to be protected by the polar cap.

 

OZONE LAYER AND ANTHOCYANIN RESPONSE

2,8% of the volar spectrum is UV radiation, on Earth the UV part of the solar spectrum is about 3,2%. Paart has proportionally half as much oxygen than Earth, but actually 2,52x more of the gas, being of very similar size to Earth, we can ignore distribution differences for now - Earth has an ozone layer that's ≤10ppm in ozone (surface levels are 0,3ppm).

Ozone is created when UV dissociates an Oxygen molecule, the subsequent reaction creates Ozone which absorbs UV light and dissociate as result.

Using the graphs 1 and 2, we are assuming a healthy ozone layer as ~300 Dobson Units thick.

Paart receives only 87,5% as much UV as Earth, and does have lower oxygen proportions than Earth's atmosphere, the total Ozone that would be naturally occurring is about 4,76ppm, or a layer that's about ~124,9DU thick (similar to ozone layer thickness above Antarctica in 2008), the global UV radiation levels would be ~1,2x that of UV with 300DU - weighted UVI for an Earth clear-sky is about 10,6, while a Paart clear-sky UVI is 11,1. With wet soil reflecting about 20% of UV light, the effective UVI for someone on the surface of the planet on a clear day is about 13,32, and 10,6 when it's cloudy.

Anthocyanin in plants increase with UV intensity, this may yield in some minor size/mass-losses in comparison with plants under lower UV levels (at least, with Earth-plants).

Exposure to UVI for UVB of 12,0 showed to increase Anthocyanin levels ~4x in certain plants, and exposures mixed with blue-light and UVB mixed (UVI ≤1,8), increases of 6~7x - gene expression for mixed UVB/Blue light are 400~5000x stronger. Wit,h our 330mW/m² UV incidence, we should expect the anthocyanin gene expressions to be at least 100~500x stronger with UVB/Blue light of 475 mW/m², in what case including blue light the exposure index is about 19,0.

Assume the "normal" anthocyanin expression is about 32ug/g of the fresh-weight (100th of the levels in peppers), if our expressions are around 200~250x times that, we should expect plants with 8mg/g of fresh-weight, or 1,3x more anthocyanin than in some grape peels, with the maximum expressions about 16mg/g, nearly as much as in black grapes (20mg/g).

Referent to color expressions, while my initial guesses where based on the argument that soil pH would directly influence plant anthocyanin color expression - it seems that the presence of aluminum and iron in the soil helps to stabilize the color expression, aluminum blue-shifts the anthocyanin expression while iron does red-shift it. Anthocyanin has also shown to turn yellow-ish when the plant pH is alkaline.

On Paart aluminum and iron are in identical levels and relatively low abundance when compared to Earth. However, iron is more resilient to acidity changes, inside the paartene soil pH range - the aluminum should be between 2,5ppm and non existent, whereas there could be about 75~100mg/Kg of iron in the soil depending if the soil is more acidic or near-neutral, respectively.

So, actually, the more neutral the soil gets - the purpler the plant, in some cases where the soil happens to be basic, there would be a variety of, yellow/green/blue plants depending on how basic and on iron deposit levels, whereas on neutral and acid soils, plants would be magenta towards the red, respectively.

 

THE TRUE COLORS OF PAART

Finally, after all this research, we get a color field that looks like this:

 Excel color map for reference

 

And then we get, 44% magenta plants, 27% red plants, 27% yellow plants, within variety, green and blue plants may appear where there is yellow plants. If we cluster colors together, 3/4 of plant-life are some tone of magenta/red - the average color of this distribution, ie, the color that would mainly apparent from space, is very close to Pantone 2342 C (HEX b65a65), that's Paart's official plant color from space, among other autumnal colors.

Paleocambrian Paart IF it was fully covered by plant life

Paartene plants would still be classified as photo-litho-autotrophs - but a rather special type, one that is very tolerant to UV radiation, and has learned to thrive without the magnesium and aluminum abundance Earth has - they are, the Ensarkophytes (meat-colored plants).

WHAT IF IT WASN'T RED?

Having in mind everything we've done, seems there wasn't any way we could have blue to start with, how would it go for blue/cyan plants?

If we had to use blue, we would use early algal phycocyanin with a zinc center and manganese ions to make the plant material basic enough to bump the anthocyanin towards blue - as mechanism of defense against the acidity, over time, as the soil incorporated these basic substances, the plants could resort to calcium ions as an evolutionary leap in combating acidity in internal fluids, they would still appear green and yellow depending on the iron content and anthocyanin expression - that's in a very, very summarized way of saying it.

- M.O. Valent, 25/09/2020

BIOLOGY | PART 8 | PLANT COLOR & SUNLIGHT - PART 2

UNDER THE BRIGHTEST SHADE, INTO THE DARKEST LIGHT

image: Freepik.com

There is a limit to how much or how little light can a plant feed on before dying on either end of the spectrum - people who have houseplants know well that matter to some extent - too little light and it won't trigger photosynthetic reactions, too much and it will overheat and dehydrate - eventually dying.

The point at with the photosynthetic reaction balances the plant's own respiration is called a balance point, bellow that point, the plant starts consuming it's own reserves in order to breathe, in nature this happens at night so it's a great concern, but plants can't stay bellow that level for extended periods of time, or else they die - that's why having an illuminated house is good for indoor plants.

Our eyes don't work with a linear light sensitivity, it works on logarithmic scale, so an object or ambient that looks slightly darker than its surroundings can actually be dozens to hundreds of times darker - or brighter if on the other end.

For an idea on the subject, a dark overcast day is over 100x brighter than the full moon at night, and direct sunlight is over 250x brighter than the sunrise lighting (wow I would have never guessed that the atmosphere filtered that much light).

It' safe to say - that despite all plants having evolved for Earth conditions, which is already a wide range of environments and energy levels - biochemistry has it's limit - no matter what planet you came from, you're astronomically more likely to be: Carbon-based, using RNA&DNA and need some kind of fluid medium - or posses your own stock of fluid.

This way, photosynthesis can only happen with so little and so much light, because of the weakness of it's reagents and pigments, and metabolic processes for animals can only happen above the freezing point of whatever fluid it's based on (could be more commonly water) and the boiling point of that liquid.

LIFE'S BIOCHEMISTRY BOUNDARIES

On Earth, the temperatures range from -89°C recorded in Antarctica to ~350°C in the deep sea hydrothermal vents - while some extremophiles can be found on the borderlines of -20°C and 120~122°C (highest temperatures for life-cycle completion belongs to Archea), these often are what we consider simple organisms such as bacteria and even as such, require specific conditions to thrive - more complex and active organisms live in the temperature range of 0°C to ~60°C.

An example on how these limits act - we have a temperature range x cyanobacteria richness by species, we see that little to now cyanobacteria will be found to grow on temperatures of 70°C or above (It's also reasonable to assume that it's null bellow the freezing point of water) - because that's how biochemistry works.

Some chemists out there reading these might point out that if would really be the case - then most life wouldn't be possible at all - for the values of such weak interactions such as the Van der Walls and Hydrogen bond forces are on the same energetic level as ambient temperature in most of the world.

The paper points the following:

These values are of the same order of magnitude as the mean thermal energy of molecules at 25 °C, and hence are likely to break more frequently at higher temperatures. The most common adaptations to allow macromolecules to function at high temperatures include changes in the number of residues influencing overall hydrophobicity, and an increase in the number of weak interactions to increase stability.

 

So - apart from establishing how little and how much lighting are we giving plant-life, we are also taking a step into determining sort of a temperature-based habitable zone.

Life possible at temperatures up to:

Archea - 122°C

Bacteria - 100°C

Terrestrial Invertebrates - from 60°C to 70°C

Angiosperms - 65°C

Unicellular algae and Yeasts - 60°C

Mosses - around 50°C 

Freshwater Invertebrates and Ectothermic Vertebrates - around 46°C

Lichens and Macroalgae - around 45°C 

Marine Invertebrates - from 42°C to 90°C

Endothermic Vertebrates - cells work between 30°C and 45°C

Plants (in direct sunlight) - up to 65°C of internal temperature

And with temperatures as low as:

Freshwater Invertebrates - 0°C

Marine Invertebrates and Ectothermic Vertebrates - minus 2°C

Lichens - minus 10°C

Archea - minus 16,5°C

Bacteria and Yeast - minus 20°C, but some were found to engage in respiration at minus 196°C

Mosses - minus 30°C 

Angiosperms - minus 70°C

Terrestrial Invertebrates (particularly the Tardigrade) - minus 196°C

INSOLATION IN PLANTS

Lower Boundary - about 0,16 W/m² or 0,00012x Earth's TOI

House plants which live in low light from 107 to 161 W/m² indoors, could live with as little as 0,16 W/m² of artificial lighting.

0,16 W/m² is the same lighting as a comet far into the Kuiper Belt receives - ie, sunlight at ~92AU.

Upper Boundary - about 4.110 W/m² or 3x Earth's TOI

Although the researchers found limits and records of ~65°C for a plant's leaf temperature, I'm backing it up for +5°C, to account for other strategies for heat dispersion or materials in cell make-up we might explore in the future, still not so far from reality, so we will be figuring out the limiting temperature for plants similar to that of cyanobacteria and invertebrates - around 70°C.

There is two ways we can do that:

1 - IMAGINE A SPHERICAL PLANT IN A VACUUM...

We use the planet equilibrium temperature equation to see at what insolation does our plant reach 70°C, that gives us 3,27x Earth's top of atmosphere insolation, or 4.480 W/m².

 

2 - PHYSICS

We figure out how much light we have to shine on a plant given it's physical properties to heat it up that much.

Most of plant products are 80~94% water by mass, the heat capacity of a plant that's 85% water and 15% wood would be around 3.900 J/kg*K, so we get that to push our model plant, that have a mass of 10kg, from 0°C to 70°C, we would need 2,73MJ, or over 2,73kW of power, of course, we are forgetting to take into account the time the plant takes to heat up to that point, and how much surface area the plant has for the intake of light.

So we will assume our little model plant - Steve, has a total leaf area of 1m², with a 0,7cm thickness, it's density of 0,6g/cm³, giving us 4,2kg of leaf material, we only need to deplete it's water content of the course of this one model-day, or about 3,57kg of water - which has a latent heat capacity of 2,5MJ per kg.

It's native planet has a rotation period of 24h, and it's average surface temperature is around 0°C, but it gets a bit hotter during day time, we will assume that it takes little over 3/4 of daytime for it to heat up, so our heating time will be little over 9h, or 32.400s.

An ideal plant would waste only 28,34% of the sun energy as heat, so whatever energy we get from our calculation, that much heat equates to those 28,34% of the spectrum, but we can work out the rest from that, dividing it by 0,2834.

Steve would need to exposed to 972 W/m² of sunlight to heat up that much. Which is 70,9% of Earth's top of atmosphere insolation - that may seem out of place but remember, the atmosphere does filter out a lot of radiation, so much that the actual lighting on Earth's surface is little over 340W/m², so this value is about 2,86x Earth's TOI.

Now, considering plant strategies to mitigate evaporation such as stomata behavior, wax layers or hairs for increased heat dispersal, and even wind heat transfer, which can even account for a 33K decrease in leaf temperature, we get between 1.004 W/m² to 1020 W/m², or 2,95x to 3,00x Earth's TOI.

A PRACTICAL STELLAR PHOTOSYNTHETIC ZONE

Using Earth and the Sun as a parameter, photosynthesis could occur as close as 0,57~0,6 AU from the Sun, under an Earth-like atmospheric filtering - of course, an Earth-like atmosphere at such distance from the Sun - would actually make the surface temperature over 104°C, and is more likely to be a Moist Venusian instead.

However, having the atmospheric mass down to 1/5th that of Earth's put's the average temperature just about 71°C, implying that temperatures on the tropics and poles may just lie bellow that point.

Speaking of atmospheric mass, plants are shown able to survive to pressures as low as 5~10Pa (about 0,000074 atm), WHICH DOESN'T BY ANY WAY means that you could have a tinfoil thin atmosphere on your planet that has a vulcanoid orbit - because plants descend from algae, and algae descend from cyanobacteria, and those need liquid water to appear in the first place.

Too little pressure and ice will start sublimating like on Mars, so keep your atmospheric pressure above the required for the Triple Point of water, or more than 0,006 atm.

That said, an Earth-sized planet with an atmospheric pressure of 0,006 atm - would need to orbit at least 0,54AU to have an average temperature of ~70°C, but at this pressure and temperature the water boils at -4,7°C, so the planet is actually a Steam-World - that's losing a lot of water to space also....

Having cyanobacteria in mind, the lowest pressure you could have is about 0,32atm - when water boils at 70°C.

This paper notes how different rotation rates even out the global heat.

An approximation to the model's pole-to-equator difference is:

ΔTₖ ~ 75 × r^(-0,175)

with r = planet rotation period in hours

With a rotational period of 24h, our pole to equator difference is about 43K, so we want to keep our Equator water temperature just bellow 70°C, so the oceans don't boil away and we don't end up with a Moist Venusian or Steam-World, so lets go back to that 65°C record for plant-life.

43 K bellow 65°C is about 22°C, so we're aiming for a planet which average temperature is 318 K, or about 43 ~ 44°C.

Which is not unreasonably hot, given Earth's average temperature back in the Archean Eon. 

So, under those conditions, the closest your planet could ever be to it's star is ~0,7x it's mean Habitable Zone, with little over 1,4x Earth's surface insolation in it's surface (about 480 W/m²) - that's it.

POSSIBILITY OF LIFE ON VENUSIAN PLANETS

Venus in false-color, Mariner 10 mission, 1974

 

Is rather inevitable that after this, that we note Venus lying on the bordeline of the so called Stellar Photosynthetic Zone we just calculated, at about 0,723 AU, as discussed before, at an altitude of 50km above Venus' surface, the pressures and temperatures are just right so simple life-forms could appear and thrive, if such a gaseous medium could really support a life-bearing event is still in doubt to this day - as one argument in favour of liquid water is that it has just the right density for biochemistry, "ice is too still, vapor is just too disperse for biochemistry to arise" thing.

LIFE ON COLD VENUSIANS

On the other side of the spectrum, where planets are further away from the star, but apart from Earth or Mars, have considerably thicker atmospheres, we have worlds like Aliens', Acheron.

In worlds far away from their stars, bordering the habitable zone, having a thick atmosphere may help - but in some cases, the atmosphere may be so thick it actually makes days hot and dark like the full-moon, and nights freezing cold and pitch-black.

Let's use a model planet that has a 75% cloud cover, a surface albedo of 0,20 and a cloud layer with albedo 0,70 - the planet's overall albedo is then about ~0,575.

Using 4x Earth's atmospheric mass, at Earth atmosphere composition, at the edge of he habitable zone, at 1,3AU, the planet has an average surface temperature of -2,7°C.

Spin it at a rate of 30h, and we would get a pole temperature of -43°C, and an equatorial temperature of 39°C.

However, we are ignoring the part that the clouds block light over the planet - thick clouds have a transmittance on the order of 0,18, within variance, our minimum value based of that is 0,15 transmittance.

The planet's TOI is ~810 W/m², so with cloud reflection we get only 42,5% of the TOI lighting into the clouds (344,25 W/m²), with a cloud 0,18 transmittance we get a surface insolation of ~62 W/m², with an atmospheric radiative power of 159 W/m² - the day on this planet is similar as a dense overcast day on Earth.

The average temperature is then -22°C, it makes 20°C in the equator, and -63°C in the poles - actually not too bad for plants.

Pushing the planet into Mars' orbit at 1,52 AU puts the mean global temperature around -42°C, -1°C in the equator and -85°C on the poles, the planet's polar circles are cold enough to have CO² snow, and daytime illumination is around 45,9W/m².

Planet 1 we've described in this part has an ESI between 0,806* and 0,819 - while Planet 2 has an ESI between 0,709* and 0,720.

*Values considering the planet's atmospheric greenhouse flux 

Planet 1 is borderline Earth-like as the standards require indexes greater than 0,8 to be classified as such.

We see that even though having a thick atmosphere, Planet 2 is too cold for most Earth life, it's rather unlikely to have liquid water on it's surface besides from nearby hydrothermal hotspots - what in theory is a good sign, but it would then be limited to these regions, unable to migrate outwards, and unable to grow much more complex than a microbial paste, if it even does so.

It does require a bit of a stretch to make atmospheric conditions suitable for life to even arise in martian-analogue orbits, and the ESI for Earth-like conditions is at it's lowest around our usual 1,3~1,37x the star's mean habitable zone. Unless by some reason the illumination conditions of a planet lower over the course of it's history, is unlikely plants or life would appear much further than that distance from the star, even if the illumination conditions are tolerable for plant-life.

- M.O. Valent, 25/09/2020

BIOLOGY | PART 8 | PLANT COLOR & SUNLIGHT - PART 1

NOW YOU CAN EAT SUNLIGHT!

history of the entire world, i guess - Bill Wurtz

 

PHOTOSYNTHESIS...

...A word derived from the Greek phos, "light", and sunthesis, "putting together", it is the name of a physical-chemical process, at the cellular level, carried out by chlorophyll living beings, which use carbon dioxide and water, to obtain glucose through the energy of sunlight - which is used to trigger the reaction, according to the following equation:

6 H₂O + 6 CO₂ + energy → C₆H₁₂O₆ + 6 O₂

Photosynthesis initiates most of the food chains on Earth. Without it, animals and many other heterotrophic beings would be unable to survive because the basis of their food will always be on the organic substances provided by green plants - their characteristic green color comes from the pigment chlorophyll.

Chlorophyll is a chlorinic pigment with four pyrrole rings linked by methyls, and a fifth ring absent in other porphyrins, a group of compounds to which it belongs and which includes compounds such as the heme group. At the center of the ring is a magnesium ion (Mg2+) coordinated by four nitrogen atoms. The compound is called Pheophytin when magnesium (or another metal ion) is not in its center.

Although green Chlorophyll (there are 6 types of chlorophyll) it's the most common pigment in Earth's photosynthetic life, it's not the only photosynthetic pigment, and some don't even produce oxygen, processing hydrogen sulfide instead.

Some known plant pigment groups and their colors (reflected wavelengths) include:
Carotene, orange
Xanthophyll, yellow
Phaeophytin a, gray-brown
Phaeophytin b, yellow-brown 

Chlorophyll a, blue-green
Chlorophyll b, yellow-green

Anthocyanins, vary from vivid red, pink, blue, to violet depending on how acid or basic the plant is.

 

Other pigments in microorganisms and some algae include:
Phycoerythrin, bright red

Phycocyanobilin, blue

Phycoerythrocyanin, magenta 

Phycourobilin, bright orange

Phycoerythrobilin, red

Bacteriochlorophylls a&b, purple

Bacteriochlorophylls c/d/e, green

 

DOMINANT COLOR DISCUSSION

The topic of determining a planet's main vegetation color is rather slippy, to start with, most discussions assume that only one type can prevail - even though this logic falls short when we look back at Earth because we have so many alternative pigments, of course, for some reason we are yet to fully understand, green chlorophyll ended up being the most used on Earth - it would be reasonable to look at our Sun and say - well, it's emission spectrum is pretty broad, it's understandable that we would have a broad absorption spectrum for photosynthetic life too - that may not be the case the further from a solar-analogue you get though.

One away to look at these cases is the Peak Spectrum theory, which assumes that plants would evolve to absorb the most abundant wavelengths from it's star, ie, planets around hotter and bluer stars would have reddish vegetation because they absorb blue-ish light, planets around cooler and redder stars would have anywhere from blue plants (both to reflect dangerous bursts of UV) to dark/black plants because they absorb most of the visible spectrum.

The Problem with this way of thinking, is that when you get to yellow stars like our sun, the plant color would have to be purple... Because the plants would absorb green-yellow light from the peak spectrum, and reflect blue and red, making purple to our eyes.

Using complementary colors to the peak, like our plants do, absorbing blue and red, reflecting yellow-green light, renders interesting arrangements.

Here is a simplification of the two models (peak reflection or complementary reflection):

Peak Emission colors and complementary colors to those, organized by star temperature, to the right - some common photosynthetic pigments and their colors

Bellow, a more complete table, and a luminance filter for CMYK coloring, accounts for the monochromatic effect in lighting, ie, even though blue plants could appear in a planet around a red dwarf, the lack of blue light photons would make the leaves actually look black - on the case for bluer stars, colors other than blue would look unsaturated.

An extension of the color table shown before

Having that in mind, and the range of human vision, here is my rendition of what plant-color gradients could look like:

Primary emission colors, variations on pigment concentration and mixtures could yield more saturation on different cases

Complementary colors, naturally range from dark blue to magenta to light yellow, the luminance difference would unsaturate the colors a little bit

While using the table, pick your star color temperature, and choose if you're either reflecting or absorbing the peak emissions, then with that color, pick the pigment that most resembles that color.

For example, for an F9V star, with 6000K temperature, the available colors are around light blue and rusty orange - we could then use a mix of neutral anthocyanin and chlorophyll-a, xanthophyll and petunidin (could be yellow-green, green, and cyan depending on the balance) phaeophytin-b and carotenoids, phaeophytin-a and acid anthocyanin, acid anthocyanin and carotenoids, the list could go on with different shades and proportions.

 

THE CASE FOR PAART

Following the so pre-established parameters, we have that plants on Paart could either have colors centered around pastel red, or turquoise-green.

That will be up for voting by the Project Paart crew, and until we decide - stay tuned, and good gardening!

- M.O. Valent, 22/09/2020

OTHER | PLANET CLASSIFICATION GUIDE

PLANET CLASSIFICATION GUIDE

We've for long referred to planets by their types here in this blog, using words like Terran or Terrestrial (derived from the Latin for Earth, Terra), or Gas Giant / Jovian (as relating to Jupiter), and so on, some terms are analogous, others rely on the knowledge of the reader to catch on, wouldn't it be good to have table with known, or, theoretically possible planet types?

Fans of the Star Trek series may recall have heard a couple times mr.Spok read on a planet's stats after scanning, coming up with planet classes as M, when referring to Earth-like worlds.

There isn't much of a real life parameter to pinpoint planet types as of today, it may happen in the future though, what we have are some base models applied to our planets and known exoplanets.

When I started this blog, I presented to you a couple planet types, but they come in even more flavours than that.

I'll build a planet classification chart along this post, more of as a baseline for future references.

THE FOLLOWING TEXT REFERS TO HOW I WOULD CLASSIFY PLANETS ACCORDING TO WHAT I HAVE READ ABOUT THEORETICAL AND OBSERVED PLANET TYPES

COMPLEMENTARY GRAPHS ARE FOUND AT THE END OF THE POST 

When we first look at our solar system we tend to classify it in rocky planets and gaseous planets, which is pretty fine, but we need to get more detailed.

Composition can vary a lot even between the 'uniform' gas giants, as they can be made of several different compounds such as light and heavy volatiles.

We can already expand our classification from 2 boxes (rocky and gaseous) to a lot more if we include mean composition, mass, and density alone - some planet types are also characterized by their temperatutes, orbits, or atmospheric conditions.

Me ~ Earth Mass

Mj ~ Jupiter Mass

Re ~ Earth Radius

Rj ~Jupiter Radius

I will use a concatenation system of Class-Subclass-Type/Subtype, or for clarity, UPPERCASE-Number-lowercase.

Potentially habitable for complex-life and microorganisms only types depending on atmospheric conditions color-coded GREEN and RED,  respectively, with an uncertainty midterm being YELLOW.

Class S - Subterran

A planet which mass and radius are substantially inferior to that of Earth and Venus.

A Subterran planetary mass range from 0,001 Me to 0,5 Me, and depending on composition, are usually smaller than Earth.

Subclass 1 - Dwarf Planet

A body which is massive enough to molded in a round form by it's own gravity, but not enough to cleanse it's orbital vicinity, and it's also not gravitationally bound to a particularly larger body than itself (like a main planet).
The mass of a Dwarf Planet is less than 0,01 Me, but more than most common large asteroids. Commonly made of rock and ice and can form beyond a star's terrestrial accretion limit.

Such a body around 0,01 Me in mass - made of Lunar regolith, would have a diameter of 0,254 Re.

Type a - Plutine
A body that's roughly 70% rock and 30% water ice by mass, may contain a thin atmosphere of methane and nitrogen, and some ammonia, the body most likely formed and still orbits beyond the Frost line.

Such world with 0,01 Me would be 3.058km wide - or 0,24 Re.

Type b - Cererian
A body that's 60~70% rock and 30~40% water ice by mass, the main difference between a Selenes and Hygieans is that those form before the Frost line, likely to inhabit the system's asteroid belt, it's tenuous atmosphere is composed of water vapour outgassing and sublimation and likely extends as a cloud around the object rather than a layer near the surface - which is composed of a silicate, piroxene, and ice mixture, and presents heavy createring.

Type b - Selene
A dwarf planet that is almost entirely rock, covered by piroxene, silicates and basaltic rock, having little to no water ice, much probably the leftovers of protoplanet formation of the system.


Type d - Hygiean
A dwarf planet that is not nearly as spherical as the others, but it's not an asteroid too, having some significant assimetry of it's axis for some reason - too little mass, low density and high rotation, or had a chunk of itself tore apart during impact events, like Haumea and Hygiea.

Subclass 2 - Minor Planet

A planet which mass is greater than 0,01 Me and less than 0,1 Me, can be made of metals, silicates, and ices depending on specific composition, due to low mass - these planets aren't suitable for holding on a long lasting atmosphere.

Type a - Mercurian

A planet which mass is within the minor planet subclass, however, it's found in the inner part of it's star system, atmosphere is nearly or completely absent and one remarkably characteristic is the extreme temperature difference between the night and day side of the planet.

A planet like Mercury would have a density similar to that of Earth's, about 5,4 g/cm³.

Type b - Super-Mercurian

A planet which mass is not quite within the minor planet subclass, however, it is made of dense materials such as metals, making the planet heavier than it appears to be by sheer size - as it would still be smaller than Earth, typically, these planets tend to have large core, surrounded by a thin mantle and rocky crust. Could also be more commonly found on the inner part of their systems.

A planet composed of 75% metals and 25% silicates would have density of ~7 g/cm³.

Type c - Martian

A planet which mass within the minor planet subclass, likely around the outer boundary of 0,1 Me, having a rocky composition and a metallic core, being substantially less dense than Earth, these planets tend to be rather large relative to their mass but still solid, though most probably geologically dead due lack of strong internal pressures.

Is probable that these worlds form with such a low density by either inhabiting the outer edge of the terrestrial accretion disk, or by having it's mass 'stolen' by a more massive planet nearby.

A planet of such type would have densities between 3,5 ~ 4,5 g/cm³, martian regolith density is about 3,93 g/cm³.

Type e - Vulcanoid

A rocky planet that is close to the star's innermost stable orbit, having no atmosphere at all, may be tidally-locked, if it's close enough to it's star, tidal forcing may heat up the planet to the point it's surface bears strong volcanism.

Classes T and TT - Terran and Superterran

A planet which mass is greater than 0,1 Me and less than 2 Me, or between 2 Me and 10 Me as a Superterran, can be made of metals, silicates, and ices depending on specific composition and location.

Planet's in these classes would have densities between 4,5 ~ and 7,0-ish g/cm³.

The term of this classification is mean atmospheric nature and distance from the star.

Subclass 1 - Dry

A planet with nearly or completely absent liquid water on it's surface.

Type a - Radiated

A planet is so close to it's star that is heavily radiated by it's emission, having no atmosphere at all, may be tidally-locked, if it's close enough to it's star, tidal forcing may heat up the planet to the point it's surface bears strong volcanism.

 

Type b - Barren

A planet that has little to no atmosphere, and has a rocky surface.

 

Type c - Dune

A planet that has some atmosphere, but little to no water, it surface is largely covered by dry deserts.

 

Type d - Desert
A planet that has some atmosphere, and little water, it surface is largely covered by deserts.

 

Type e - Carbonic

A planet that has a considerable atmosphere, largely composed of nitrogen and carbon dioxide, enough to maintain it's mean temperature above the melting point of water. Carbonic planets can be found on the outer habitable zone, but also between the Venus zone and the tropical habitable zone. Such planets would also require active volcanism to cycle the carbon in the atmosphere beyond the absorption capacity of it's soil. Water on Carbonic planets is rather acidic.

 

Type f - Venusian

A planet that has ran into a carbon-cycle runaway greenhouse effect, and has essentially lost all it's water, temperatures are evenly scorching throughout the planet, and pressures are up to hundreds of atmospheres.

Subclass 2 - Moist

A planet with little liquid water on it's surface.

Type a - Moisty Carbonic

A planet has enough water to be a global swamp, too close to it's star to favor other kinds of climates - think 20th century's view on Venus.

Type b - Steppe

A planet has enough water to bear sparse savannah or steppe environments, but too far away from it's star to favor other kinds of climates.

Subclass  3 - Wet

A planet that has an amount of water comparable or superior to that of Earth's.

Type a - Moisty Venusian

A planet that ran into a runaway moist greenhouse effect, has a dense water vapour and carbon dioxide atmosphere, has a rocky surface.

Type b - Steam World

A planet that has a considerable amount of it's mass formed by water, and it's close enough to it's star that a great portion of it is in vapour form, between it's solid surface and the dense water vapour atmosphere, lies an ocean either as a compresible or supercritical fluid depending on the exact pressures and temperatures.

Water may account for up to 5~15% of the planet's mass.

Type c - Water World

A planet that has a considerable amount of it's mass and own radii formed by water. A water world commonly has a hydrogen, water and carbon dioxide atmosphere, after some point into the water mantle, it's now a supercritical fluid, and further down, a layer of ice VI, VII or X depending on the mean temperature, bellow the ice layer, lies a rocky envelope and a metallic or ice core depending on where it formed in the inner system.

Type d - Oceanic

A planet that has a comparable amount of water to that of Earth's, however, don't have as much landmass, having more than 75% of the planet's surface covered by water.

Type e - Tropical
A planet that has a comparable amount of water to that of Earth's, and similar ratio of landmass to ocean ratio (about 1:3) of the planet's surface covered by water.

Type f - Snowball

A planet that has a comparable or superior amount of water to that of Earth's, however, most of it is in ice form, such a planet has either large polar caps, or is mostly frozen.

Some may have an atmosphere and temperature similar to Saturn's moon - Titan, ie, are cold enough to support a methane-cycle analogous to Earth's water-cycle, or other volatile that can exist in liquid form at these temperatures such as Chlorine, and other hydrocarbons.

Type g - Super-Sized Titan
A planet that's 30~50% water ice by volume, having an Earth-sized silicate lower-mantle and an iron core, the upper-mantle is composed mostly of pressurized water, methane, and ammonia ices, it's distance from the star and atmosphere allows a small portion of the surface to be liquid - forming an ocean that's hundreds of kilometers deep. The atmosphere is mostly water vapor, hydrogen, ammonia, methane. and carbon monoxide/dioxide.

Such worlds have densities around 3~4g/cm³, and topping masses comparable to those of Ice Giants, ~15% water and ~85% silicates and metals by weight.

Tectonic activity is locked by the sheer amount of water ice above the rocky material, so some form of cryovolcanism may be the predominant driving force of pseudo-tectonics with pressures over the ice and radiogenic heating involved.

Bodies like these could have formed early in the star system and due perturbations in those early days acquired eccentric orbits, so while they would have wandered far enough from the star to collect volatiles, they also wandered close enough to star so atmospheric escape take away most of free hydrogen.

Type h - Super-Sized Super-Mercury
A planet that has a metal-like density (more than 5 g/cm³ and less than 9 g/cm³), a Terran/Super-Terran size, a mass comparable to that of an Ice Giant - without having a thick atmospheric envelope as one, if it does have one at all depends o where exactly it did formed.

 

Type i - Cold Venusian
or Cold Primordial

A rocky planet with a dense atmosphere - mainly composed of nitrogen, carbon dioxide, methane, other volatiles and trace gases, can contain hydrogen.

This type of planet could actually be common in young star systems, with orbits on the outer regions of the habitable zone, the atmosphere is thick enough so the planet doesn't freeze but it's not hot enough to trigger runaway greenhouse effect.

Marked by extreme temperature differences between daytime and night-time on the order of ~100K, the thick cloud cover would make little to no light get through the atmosphere. I would say this world-type is an Acheron-analogue.

 

Class N - Mini-Neptune

A planet which is composed mostly of volatiles other than water, like ammonia and methane, are usually found to be short period planets around their stars, forming before the frost line.

A Mini-Neptune mass range from ≳1 Me to 20 Me, the major definition turns around it's size and composition. Mini-Neptunes are identical in composition to Ice Giants (about 20% Hydrogen/Helium and 80% Volatiles), however they are smaller than 3,9 Re and greater than 1,6~1,7 Re.

Type 1 - True Mini-Neptune

A planet that more or less attends the general rule of composition for an Ice Giant.

By mass, 60% mix of supercritical Water, Ammonia and Methane, ≲20% mix of Hydrogen, Helium and Methane gas, ~20% icy/rocky core.

Type 2 - Super-Sized Water World (or Wet-Neptune)

A planet that DOES NOT attends the general rule of composition for an Ice Giant, nor is as big as one, however, the amount of volatiles is so considerably large that it falls in between a TT3c planet and a N1-class planet, having a rocky/metallic bulk mass, a supercritical/liquid water envelope, and thick hydrogen/helium atmosphere - no pressurized Ice, being roughly 2~3 Re wide, with an atmospheric envelope no thicker than 0,5 Re.

Class J - Ice Giant

A planet which is composed of mostly of volatiles other than water, like ammonia and methane.

An Ice Giant mass range from ≳10 Me to 130 Me, the major definition turns around it's size and composition. Ice Giants are made of about 20% Hydrogen/Helium and 80% Volatiles, and attain to sizes greater than 3,9 Re and less than 8,4 Re, densities range on about 1~2 g/cm³.

An Ice Giant's general azure~cyan appearance is due to methane traces in it's atmosphere. Ice Giants aren't restricted to form beyond the frost line, however when they form before the frost line it is very close to their parent stars, those are called Hot-Neptunes then.

Type 1 - True Ice Giant

An Ice Giant that sits beyond the frost line, like Neptune and Uranus.

Type 2 - Hot-Neptune

A short-period (year is less than 10 days and longer than 1 day) planet rich in volatiles, with too much atmosphere to be a Steam World, too big to be Mini-Neptune, and too heavy to be Gas Dwarf.

Hot-Neptunes with radii greater than 2 and less than 10 Re are relatively scarce and the most common type of planet when looking at orbital distances of less than 0,1 AU, creating what's called a Hot-Neptune Desert, planets within these radii, are rarely found between the following lines - most exoplanets with these radii cram just around the border of this zone.
In case your planet fall inside the Hot Neptune Desert, increase the orbital period / distance to an acceptable degree of at least 0,5 log10 into the desert.

Upper Limit Log10(MJ) = [1,07 * Log10(period in days)] - 2
Lower Limit Log10(MJ) =  - Log10(period in days) + 0,1

Class G - Gas Giant

A planet which is composed of mostly of Hydrogen and Helium, having some amount of volatiles like water, ammonia and methane.

A Gas Giant can be as light as ≳10 Me or as heavy as 4.134 Me (beyond that they're classified as Brown Dwarfs), Gas Giants have their atmospheres made up of ≳90% Hydrogen/Helium, nas less than 10% volatiles, a small amount of their mass comes from a solid rocky/metallic core, Gas Giants have strong magnetic fields and make perfect magnetic shields for habitable moons around them.

Type 1 - Lesser Gas Giant

A planet which is mostly made of Hydrogen alone (about ≳95%), ≲5% Helium, and less than 1% trace volatiles.

Lesser Gas Giants are very light like Saturn, having densities from anywhere less than 1,32 g/cm³ and around 0,7 g/cm³ (about as light as Saturn), and at first seem featureless, depending on how low is the concentration of volatiles and tholins in the atmosphere.

Bellow a relatively thin gaseous atmosphere (compared to their size), lies a mantle of liquid supercritical hydrogen, and at the center a rocky core about the size and mass of a Superterran.

Type 2 - Jovian Gas Giant

A planet that is similar in composition to Jupiter, 90% Hydrogen, 10% Helium, and less than 1% trace volatiles.

Jovian Gas Giants are rather heavy when compared to Lesser Gas Giants, their density wander above 1 g/cm³ and less 2 g/cm³, and they might have or not a solid Superterran-sized core depending on where and how it formed.

Jovian Gas Giants have a higher concentration of tholins and other chromophores (like ammonia and sulfur) than it's lighter counterpart, then, it's atmospheric bands appear to be tinted in red/orange/pink depending on the concentration of the materials in it.

Planet's with a mass higher than that of Jupiter aren't much larger than the planet, as the pressures involved kick electron degeneracy in the core, allowing atoms to pack more tightly, so instead of growing in size, they get denser until when they reach 13 Jupiter masses, at start to fuse deuterium.

Type 3 - Inflated Gas Giant

A planet that can be even lighter than a Lesser Gas Giant, due thermal expansion of it's atmosphere for being so close to it's star.
It's worth noting that planets with considerably less gravity or mass than Jupiter (around 24,8m/s²), when exposed to temperatures around 1800K or more will have their  atmospheres evaporated in the timescale of 1~2 billion years.

Gas giants in general (up to 10 Mj) seem to have similar sizes (0,85 to 1,15 Rj) at T <1000K, once the temperature

A reasonable radius estimate for any gas giant at a given temperature is given by:

RJupiter = 0,915 * ( 1,00027 ^ T )

Where T is the temperature in K.
Of course, aspects such as distance from the star, albedo and exact composition are ignored in this case, but it seems to offer quite a solid baseline in that aspect - further bellow you can sort out your gas giant by mass range and calculate it's radius within a 0,2~0,7 Rj margin for variance based on actual exoplanet catalog work - but that's also under the same limitations.

Type 4 - Gas Dwarf
A rocky planet that has a considerable amount of it's mass and volume due to high concentrations of Hydrogen and Helium in a thick atmospheric envelope, however it's atmosphere also contains high levels of volatiles such as ammonia, water, and methane - a Gas Dwarf also isn't as massive or large as a Gas Giant, being similar in size to Hot-Neptunes and Super-Earths.

Due their low density and low gravity - is rather probable that most Gas Dwarfs are in the most part featureless planets, the hotter ones would have a hyper-inflated haze-like atmosphere envelope, while the colder ones could have pale colored bands according to it's rotational period and chromophores in the atmosphere.

Their radii vary in between 1,2 ~ 3,9 Re, and minimum mass of 0,6 Me.
(Kepler 138d is the smallest known Gas Dwarf with 1,2 Re).

A Gas Dwarf with 1,0 Me and a density comparable to that of Saturn would have a radius of 2,0 Re.

Type 5 - Helium Titan

A planet that has a considerable amount of it's mass and volume due to high concentrations of Helium, it would be mostly made of supercritical helium and hydrogen, being ~4x as dense and ~4x as heavy as an usual Gas Giant.

Such a planet could compact 318 Me within only 7,75 Re, presenting a nearly featureless pale or gray appearance due lack of methane, that would otherwise give it a bluish tint.

In nature, the more likely way these planets form is via the Hydrogen evaporation of Hot-Neptunes and Hot-Jupiters, as a late stage of these planets life, a Helium Titan can form within a couple million years, but take up to 10 billion years to lose all of it's hydrogen, so it is possible that most of these planets still contain a large portion of hydrogen, thus, appearing as either pale jovians or a jovian-sized ice giant due methane scattering of light.

SUBTYPES (Sudarsky Classification)

S I

The planet is beyond the frost line, and it's atmosphere color is defined chemically rather than physically, temperatures are less than 150 K, and albedo range from 0,57 w/o tholins and 0,34 w/ tholins.

S II

The planet is close enough to it's star that it's clouds are made of water vapour and methane, giving it a more featureless and pale color, temperatures are less than 250 K, albedo is around 0,81.

S III
The planet is so close to it's star that the incoming solar radiation decompose cloud formations, giving it a more featureless and azure~cyan appearance due sheer Rayleigh scattering, temperatures are less than 800 K but more than 250 K, above 700 K the presence of sulfides and chlorides favor the formation of thin cirrus-like clouds, with an albedo as low as 0,12.

S IV

The planet is close enough to it's star that the main component of the upper atmosphere changes to carbon monoxide and other alkali metal impurities, which darkens the planet, having an albedo low as 0,03, temperatures are above 900 K. Those are commonly called Hot-Jupiters.

S V

The planet is either a super Hot-Jupiter, with temperatures above 1400 K, or cooler and less heavy than Jupiter, with silicate and iron clouds, and albedo at 0,55.

POTENTIALLY HABITABLE PLANET TYPE SUMMARY

• S2c type
• T1 and TT1, c through f types
• T2 and TT2, a and b types
• T3 and TT3, a through f types
• G4, subtype S-III

Other Multi-Class Types

Eyeball Planet

A planet within the Subterran-Superterran mass range, granted it does have an atmosphere and some amount of liquid water, and sufficiently close to it's star, usually a Red Dwarf, it will be tidally-locked, creating a desert hemisphere and a water ice hemisphere, having a thin temperate zone around it's terminator. Such planets are bombarded with radiation and charged particles from it's parent star, however, if it does have a sufficiently strong magnetic field, it could harbor complex life.

Rock/Ice Puffy Planet

A planet that has a density comparable to that of the Moon or an asteroid (less than 3,5 g/cm³), generally nearly or completely absent of metals and composed mainly of silicates and ices.

Puffy planets of this kind have a weak gravity when compared to Sub, Terran, and Super Terran counterparts of the same size, because of that, are probably geologically dead on their own. This kind of body is found in the solar system as the icy moons of Jupiter, and tidal heating could make them habitable to some extent.

Classifying Terran-Sized planets by Water Content

A planet within the S throughout TT classes could be also classified by it's water content, proportionally speaking - because it doesn't help much having as much water as Earth if the planet has 4x as much area, making for a lower water content, or if the planet has half the water on Earth, but have 1/3rd of the area - making for a relatively larger water content.

In this classification, I will be referring to the Mars water proportion to it's mass, ie how much of the planet's mass is made of water.

Color-codes for habitability assumes the planet that much liquid water in a stable temperature and pressure for life as we know it.

DRY

Dry - < 0,0000024%

Martian-dry - around 0,0000032%

MOIST

Moist I - up to 0,00032%

Moist II - up to 0,002%

WET

Oceanic I - up to 0,01%

Oceanic II - around 0,023%

Earth goes here.

Oceanic III - 0,04%

SOAKED

Water World I - 0,07%
Water World II - 0,2%

Water World III - 5~15%

At this point the planet has at least 5% of it's mass composed by water, and above 15% we can consider it a True-Water-World (a T3c or TT3c planet).

Earth-like biochemistry is difficult with 0,1% of the planet's mass worth of water.

Tectonic activity stops once the water amount is above ~1,1% of the planet's mass, so chemosynthesis would also be difficult, if not, non-existant beyond this point.

RELATIONSHIP GRAPHS

Here is a graph of planetary mass in Earths x Volatile content in percent.

Earth posess 0.02% of its mass in water, and so we get -1.7 in the vertical axis, and zero in the horizontal axis since the log of 1 is 0 (0.0,-1.7).

Mars has coordinates (-0.97, -5.49)

 

Planetary Density as a function of it's distance from the Sun

 

Minor Planet Density Zones

Dwarf Planet Mass/Size Relationship

 Gas Giant planet Radius x Temperature

Gas Giant planet Radius x Temperature - but it's my attempt to find a relationship

Metal/Silicate Ratio and respective representative densities - metallic material is assumed to be 8,05g/cm³ and silicate material to be 2,75gcm³

- M.O. Valent, 15/09/2020

- M.O. Valent, updated 27/09/2020