18 replies to this topic

[MeridianStarGazer]

F class stars die before a planet gets as old as Earth did when animals first appeared. But an F star has a wider habitable zone that can hold more planets. It has a stronger magnetosphere and solar wind to protect against cosmic rays and maybe super nova. At a given temperature distance, it gives off more visible light for photosynthesis. An F star might harbor methane or ammonia based life because they can get enough visible light at a cooler distance. And lots of energetic visible light could speed life along. As for all the UV, planets would need more atmosphere.

M class stars might harbor silica and sulfur based life. To get enough visible light for biochemical reactions, one needs to be close to the star. There is lots more heat there, too hot for water and carbon based life. While what snd carbon seem the most versatile, I don't know if they should be ruled out as the only solution in extreme environments. We look at extremes of our solar system, but our solar system has only its one ratio of frequencies, while other stars have other frequences.

K class stars are considered more habitable than G class. Fewer flares than other classes, and longer living so more time for life to develope. Lower luminosity is made up for by orbiting closer. Years can be shorter so you don't have to hibernate as long. Less UV light. But what if UV light is really needed? What if it is huberis to think we know better than the one system we know has worked? A K class star has less solar wind, and possibly less magnetic field, which could mean less protection from cosmic rays. It also has less visible light, which might slow the rate that life moves along.

Then there is the metalicity. Stars with more metalicity tend to have more planets, and planets with more actual metal.

Binary systems are the most common arrangement. Actually 3 or more. Complex, and can prevent tidal locking. They can disrupt outer planet formation so there are more asteroids far out. They can also gobble up asteroids and volatiles at medium distances. Planets can weave around among the stars. Scientists think there could be life there. Many desert plants look dead in the summer and bloom when it rains. Many animals hybernate in the winter. So maybe. But it you foy there and find a Mars, terraforming it with comets may be harder.

It seems the larger a star is, the more likely it is to have a partner. I think having another 100 AU away would stimulate space travel while leaving asteroids and kuiper objects to mine. A nearby star would also make the night sky brighter, though life would adapt.

3 body orbits are difficult to estimate and maybe impossible to solve. So hard for us to know what is there other than to look with a big telescope.

We have some fantastic telescopes going up in the next few years.

[russell23]

Paper relevant to this topic:

 

https://arxiv.org/abs/1401.2392

[SuiGeneris]

To get enough visible light for biochemical reactions, one needs to be close to the star. 

I know astronomers really like to say things like this, but man, ya'll need to talk to a microbiologist once in a while. Our best evidence of where life evolved on earth tells us it was either in the deep sea near a black smoker, or within the earth itself. 20% or more of earths biomass (amounting to about 60% of all earthly species) lives in the crust, and is comprised of organisms "rooted" near the base of the tree of life (e.g. its most similar to the last universal common ancestor than most other life). More species on earth are lithotrophs and chemoautotrophs (e.g. things that eat stuff like H2S, H2, and CH4) than are phototrophs (plants) or heterotrophs (life that eats other life).

 

In other words, the idea that you need light for life is wrong. Most species on earth, nor their ancestors, ever saw light. Life evolved in the dark places of the earth - and is where most of it remains today. Based on earthly life, any planet/moon with liquid water in their crust and some sort of abiotic processes that produce things like H2S and CO2 (e.g. volcanism, radioactive decay, etc) is a potential harbour of organic life.

Edited by SuiGeneris, 27 March 2025 - 07:49 AM.

[russell23]

I know astronomers really like to say things like this, but man, ya'll need to talk to a microbiologist once in a while. Our best evidence of where life evolved on earth tells us it was either in the deep sea near a black smoker, or within the earth itself. 20% or more of earths biomass (amounting to about 60% of all earthly species) lives in the crust, and is comprised of organisms "rooted" near the base of the tree of life (e.g. its most similar to the last universal common ancestor than most other life). More species on earth are lithotrophs and chemoautotrophs (e.g. things that eat stuff like H2S, H2, and CH4) than are phototrophs (plants) or heterotrophs (life that eats other life).

 

In other words, the idea that you need light for life is wrong. Most species on earth, nor their ancestors, ever saw light. Life evolved in the dark places of the earth - and is where most of it remains today. Based on earthly life, any planet/moon with liquid water in their crust and some sort of abiotic processes that produce things like H2S and CO2 (e.g. volcanism, radioactive decay, etc) is a potential harbour of organic life.

I think the real reason there is a focus on visible light is that the question ultimately leads to whether or not there are other technological species out there.  That requires a more restricted set of conditions - To be "Earth-like" in that sense the planet needs to have a mixed land-ocean surface so that life can develop on surface land - life that would most likely rely upon the central star's light.

 

If you read the literature on how life may have originated, there is a recognition that one option is that the Earth's life may have started in the deep beyond the reach of sunlight.
 

[moefuzz]

 

pm sent

[moefuzz]

and what of gaseous planets that have no water based oceans or terrafirma?

[russell23]

and what of gaseous planets that have no water based oceans or terrafirma?

What is the question about them?  They are not good candidates for evolving technological species.  That is why the serious long term planning to search for evidence of life involves bodies like Mars, Titan, and the interior oceans of Europa and Enceladus - not the gas and ice giants.

 

When the topic of life on other worlds is discussed the types of planets and moons that are relevant depends upon what question you are trying to answer.  Obviously it would be relevant and interesting to find evidence for any form of life but there are basically three general levels to consider:

 

1.  "Simple" single celled organisms --> most of Earth's history.  This type of life would have the broadest range of possible circumstances under which it might exist. 

 

2.  Ocean ecosystems -->  This has a much broader range than Earth-like worlds because you can have bodies like Europa and Enceladus with oceans in contact with a rocky mantle underneath an ice crust.  The rocky mantle could provide mineral elements and the ices that formed the crust and ocean may have provided the organics.  If the mantle is heated by the rocky mantle by magma plumes or smokers then you have an energy source to support life.

 

A second ocean ecosystem could be ocean planets that have a surface entirely covered by a deep ocean - much deeper than Earth's.   These planets could evolve potentially evolve life and complex organisms over time.  They are unlikely to develop any technology that humans would be able to communicate with.

 

3.  Habitable zone Earth-like terrestrial worlds --> This will be a much rarer type of planet.  You have to have fall into the right distance range to support surface liquid water.  You need to have the right amount of water to have a mixed ocean-land surface.  I can dig up the paper if needed, but one group of researchers estimated that only ~3% of terrestrial planets that form would have a mixed land-ocean surface.  In addition, the planet needs to be able to sustain a climate that permits the existence of surface water for billions of years.  So you need a planet massive enough that what happened to Mars doesn't happen, but not so massive that it holds onto too thick an atmosphere.  You probably need an elemental composition (sodium, magnesium, etc) so that the silicate crust is capable of sustaining a cycling between the crust and the interior. Too much sodium and the crust will be too light to allow for subduction and you would have a stagnant lid planet that would still have volcanism but potentially less suitable for long term climate stability for life.  Research into stellar spectra indicate perhaps ~30% of stars have the right elemental ratios to form terrestrial planets capable of plate tectonics (again I can find the reference).

 

So when we are talking about these sorts of questions - it is important to keep track of what question we are trying to answer.  If we are just talking about life then then the options are very wide.  If we are talking about complex ecosystems with multicellular organisms then it gets much narrower and we have to look at ocean worlds and terrestrial planets.  If we are talking about technological species that we could communicate with then we are looking at species that almost certainly would have evolved on Earth-like mixed land-ocean surface worlds in the habitable zone of a G0 to K5-class star.

 

Dave

[moefuzz]

 it is important to keep track of what question we are trying to answer.  If we are just talking about life then then the options are very wide.  

 

Dave

Exactly

[russell23]

Exactly

Ok, but that is exactly not the topic of this thread.  The question is not "How widespread are the potential locations where life may have developed in the universe?"   

 

The question is about the "best class of star" for life and further for native and immigrating life - so life from another planet or star system.   The answer to this question will have more restrictions because the type of star is really only significantly relevant for life on a planetary surface.  There are papers talking about the potential for life on the interiors of rogue planets ejected from their stellar orbit.  For interior ocean worlds such as Europa and Enceladus you don't even need a star.  

 

So the question asked is most specifically relevant to the third example in my previous post - habitable zone terrestrial planets with a mixed land-ocean surface.  Or alternatively potentially a Titan type planet or moon orbiting at a methane habitable zone distance.  The locations for these types of worlds are going to be much more restricted than the broader question of just "life".
 

Edited by russell23, 27 March 2025 - 06:37 PM.

[SuiGeneris]

I think the real reason there is a focus on visible light is that the question ultimately leads to whether or not there are other technological species out there.  That requires a more restricted set of conditions - To be "Earth-like" in that sense the planet needs to have a mixed land-ocean surface so that life can develop on surface land - life that would most likely rely upon the central star's light.
 
If you read the literature on how life may have originated, there is a recognition that one option is that the Earth's life may have started in the deep beyond the reach of sunlight.

To my interpretation, the titla of the thread encompasses the formation of life, and its later spread to other planets. But even with our narrower definition, I would still disagree with the gist of your claim, on a few grounds:
 
1) Earth-like life needs water, but it needs not be ocean water. Again, 20% of life on earth (by mass) lives in the crust, using what is largely fresh water that is dispersed through the rock. If anything, an ocean-free world would have more surface area for land-based life to evolve on.
 
2) There is no biological or energetic reason why chemosynthesis could not be the basis of life that gives rise to intelligence. Earth developed photosynthesis after, not before, chemosynthesis. Had light been relatively unavailable, or had their been a larger supply of the substrates used for chemosynthesis, photosynthesis may never have evolved. The energetics of chemosynthesis is significantly better than photosynthesis - photosynthesis captures 3-6% of the energy of the light absorbed by the photosynthetic pigments. The H2S/CO2 chemosynthetic pathway is about 40% efficient, some electrogenicity pathways may be upto 60% efficient. For comparison, OxPhos (what our bodies use to generate energy from sugars) is around 40% efficient.
 
Keep in mind that we humans (if you consider us intelligent) are heterotrophs - we generate energy by breaking down chemicals. The only difference between us and chemotrophs is where those chemicals come from. We eat other life; they eat gasses and rocks.

 

Ok, but that is exactly not the topic of this thread.  The question is not "How widespread are the potential locations where life may have developed in the universe?"   
 
The question is about the "best class of star" for life and further for native and immigrating life - so life from another planet or star system.   The answer to this question will have more restrictions because the type of star is really only significantly relevant for life on a planetary surface.  There are papers talking about the potential for life on the interiors of rogue planets ejected from their stellar orbit.  For interior ocean worlds such as Europa and Enceladus you don't even need a star.  
 
So the question asked is most specifically relevant to the third example in my previous post - habitable zone terrestrial planets with a mixed land-ocean surface.  Or alternatively potentially a Titan type planet or moon orbiting at a methane habitable zone distance.  The locations for these types of worlds are going to be much more restricted than the broader question of just "life".

And again, regardless of the context you try to spin around it, I disagree. Whether you're talking about where life in general could arise, where technological life could arise, or where technological societies could immigrate to, you cannot assume earth represents the norm, nor the conditions that led to us a necessity. Even looking at how earthly life evolved points to evolutionary pathways independent of photosynthesis. Life on earth developed photosynthesis because the conditions (eventually) were favourable for it. The exact same high-energy molecules developed by photosynthesis are produced by chemotrophs and lithotrophs; access to light is not needed for the sorts of energetics that enable intelligent creatures such as ourselves.
 
For technological intelligence to evolve, you probably need some sort of surface access and a oxidizing atmosphere. That gives you a space to build stuff, and chemistry compatible with "primitive" technology like fire. Outside of that, assumptions like you need large bodies of surface water, or light for photosynthesis, etc, are all applying earthly expectations to situations where they need not exist.

 

In terms of where a technological intelligence may migrate too - if you've mastered interplanetary or interstellar travel, you're probably able to make a pretty broad range of planets your home.
 

So when we are talking about these sorts of questions - it is important to keep track of what question we are trying to answer.  If we are just talking about life then then the options are very wide.  If we are talking about complex ecosystems with multicellular organisms then it gets much narrower and we have to look at ocean worlds and terrestrial planets.  If we are talking about technological species that we could communicate with then we are looking at species that almost certainly would have evolved on Earth-like mixed land-ocean surface worlds in the habitable zone of a G0 to K5-class star.
 
Dave

Again, please talk to your local microbiologist before making assumptions like these. The oldest unambiguous evidence we have of life on earth are complex, interacting microbial communities (stromatolites). The idea that "simple" life cannot form complex assemblies outside of a narrow range of conditions is simply wrong - we see these sorts of communities in every environment on earth, even in the "primitive" environments found within the earths crust, and they are present throughout the fossil record, right back to the very beginning. The amount of evolutionary change required to go from a free-living single-celled organism, to obligate colonial organism, to a truly multicellular organism, is astoundingly small. "Small" being that we can drive it in the lab over the course of a few months, with changes to less than 1% of the genes in the organism. In other words, neither complexity, nor multicellularity, is hard to do.

 

As I stated above, there is no biochemical or energetic need for oceans or sunlight for complex life. Earthly life developed both before we had photosynthesis; there is no reason to expect other planets would be incapable of following a similar evolutionary path.

Edited by SuiGeneris, 28 March 2025 - 08:30 AM.

[russell23]

To my interpretation, the titla of the thread encompasses the formation of life, and its later spread to other planets. But even with our narrower definition, I would still disagree with the gist of your claim, on a few grounds:
 
1) Earth-like life needs water, but it needs not be ocean water. Again, 20% of life on earth (by mass) lives in the crust, using what is largely fresh water that is dispersed through the rock. If anything, an ocean-free world would have more surface area for land-based life to evolve on.
 
2) There is no biological or energetic reason why chemosynthesis could not be the basis of life that gives rise to intelligence. Earth developed photosynthesis after, not before, chemosynthesis. Had light been relatively unavailable, or had their been a larger supply of the substrates used for chemosynthesis, photosynthesis may never have evolved. The energetics of chemosynthesis is significantly better than photosynthesis - photosynthesis captures 3-6% of the energy of the light absorbed by the photosynthetic pigments. The H2S/CO2 chemosynthetic pathway is about 40% efficient, some electrogenicity pathways may be upto 60% efficient. For comparison, OxPhos (what our bodies use to generate energy from sugars) is around 40% efficient.
 
Keep in mind that we humans (if you consider us intelligent) are heterotrophs - we generate energy by breaking down chemicals. The only difference between us and chemotrophs is where those chemicals come from. We eat other life; they eat gasses and rocks.

 

I don't disagree with anything you say here - except that you are describing scenarios that are not "Earth-like".  Understand I am aware of the far reaching possibilities for microbial life to potentially originate in many non-Earth-like situations.  In some of those circumstances that microbial life could evolve more complex creatures and ecosystems.

 

Let me list a few examples of places that could potentially have some form of life in the Solar System:

 

~1.  Mars - microscope sub-surface life.  I suppose if there are underground caverns you could have more complex creatures as well.  The fact that Mars had oceans for its first billion or so years would likely increase the chances of such circumstances.

 

~2.  Europa & Enceladus.  Among the icy moons of the Solar System these two are particularly interesting because they have global oceans beneath their ice crusts.  Those oceans are in direct contact with their rock mantles. That is significant because the rock mantle may provide minerals for life chemistry and certainly the ices that were collected with the moons will have additional raw materials.  So you have liquid water, minerals, energy source from tidal heating which probably generates smokers etc.  Clearly any life in these oceans would not involve sunlight - but instead chemosynthesis. 

 

The reason the other icy moons (except Titan) are generally less interesting is because they have larger water fractions which leads to multiple ice/ocean layers and the oceans in those cases are sandwiched between ice layers.  So the raw materials available are more limited and probably the energy supply too.  

 

~3. Titan.  In some respects Titan is the most Earth-like body in the Solar System.  The surface has methane/ethane lakes, rains, seasonal changes etc.  The hydrocarbon based surface hydrological cycle could potentially provide for a different kind of life chemistry than found on Earth.   The interior ocean of Titan may provide another option too.

 

~4.  Venus upper atmosphere.   The evidence of phosphine in the atmosphere of Venus at a level equivalent to ~ 1atm of pressure is still being debated, but you can't rule out that some sort of organisms have evolved that can sustain themselves in those conditions at a depth of ~ 1atm of pressure. 

 

Others?  Perhaps but these would seem to be the most promising locations in the Solar System to search for life.

Here is my frame of reference for this thread.  The title of the thread is best class of star for life.  I've listed examples above that do not require energy from the star for life.  So I understand the broad ranging possibilities for life that you are talking about.  But whether the OP asks it or not, the framing of the question around "best class of star" is really only relevant to surface life.  The type of star doesn't really matter for life deep inside an icy moon, or for life that evolves inside terrestrial planets that absorbed water during formation. 

 

When I say "Earth-like" I'm not suggesting that all life in the universe must exist in Earth-like conditions.  What I'm getting at is identifying the essential characteristics of an Earth-like planet because Earth is highly dependent upon the Sun for maintaining its complex surface/ocean ecosystems.  And if we talk about what is the best type of star for life then there are some characteristics of Earth are going to also be general requirements for complex surface ecosystems on other planets orbiting other stars. 

 

What makes a planet Earth-like?  This could obviously be debated, but in simple terms it is a terrestrial planet with a mixed land-ocean surface that interacts with an atmosphere and has a day-night cycle.  It also relies significantly upon starlight to provide energy for the surface ecosystems and maintenance of an atmosphere and "stable" climate over billions of years.   Earth-like planets will be the planets most dependent upon the type of star.   Specifically, the best type of stars for Earth-like planets are approximately G0 to K5 spectral classes.

[russell23]

And again, regardless of the context you try to spin around it, I disagree. Whether you're talking about where life in general could arise, where technological life could arise, or where technological societies could immigrate to, you cannot assume earth represents the norm, nor the conditions that led to us a necessity. Even looking at how earthly life evolved points to evolutionary pathways independent of photosynthesis. Life on earth developed photosynthesis because the conditions (eventually) were favourable for it. The exact same high-energy molecules developed by photosynthesis are produced by chemotrophs and lithotrophs; access to light is not needed for the sorts of energetics that enable intelligent creatures such as ourselves.

For technological intelligence to evolve, you probably need some sort of surface access and a oxidizing atmosphere. That gives you a space to build stuff, and chemistry compatible with "primitive" technology like fire. Outside of that, assumptions like you need large bodies of surface water, or light for photosynthesis, etc, are all applying earthly expectations to situations where they need not exist.

 

In terms of where a technological intelligence may migrate too - if you've mastered interplanetary or interstellar travel, you're probably able to make a pretty broad range of planets your home.

 

Let's think about technology for a moment.  In order to become technological you're going to need the ability to study chemistry and physics, record what you have learned, energy sources to mine mineral resources. Fire is an early means to energy creation that leads to other energy sources as scientific knowledge develops.   How would you master interplanetary travel without chemistry, physics, materials science etc?  What speculations can we come up with for a species that has science and technology without also having evolved on a planet with the broad Earth-like characteristics?  

 

Again - there may be numerous different types of worlds that are capable of having life evolve and sustained.  I'm not saying otherwise.  But most of those don't require a specific type of star for their life.

[russell23]

Again, please talk to your local microbiologist before making assumptions like these. The oldest unambiguous evidence we have of life on earth are complex, interacting microbial communities (stromatolites). The idea that "simple" life cannot form complex assemblies outside of a narrow range of conditions is simply wrong - we see these sorts of communities in every environment on earth, even in the "primitive" environments found within the earths crust, and they are present throughout the fossil record, right back to the very beginning. The amount of evolutionary change required to go from a free-living single-celled organism, to obligate colonial organism, to a truly multicellular organism, is astoundingly small. "Small" being that we can drive it in the lab over the course of a few months, with changes to less than 1% of the genes in the organism. In other words, neither complexity, nor multicellularity, is hard to do.

 

As I stated above, there is no biochemical or energetic need for oceans or sunlight for complex life. Earthly life developed both before we had photosynthesis; there is no reason to expect other planets would be incapable of following a similar evolutionary path.

 

Again, the thread is about best type of star for life.  That implies surface ecosystems or you don't really need to ask the question.  Also it appears there is some misunderstanding of meaning here.  Any living organisms have incredibly complex biochemistry. What I specifically said was "complex ecosystems".  Sure stromatolites were complex.  I learned about them in my paleontology classes in college.  Would you say stromatolites are as complex an ecosystem as the modern Earth's ecosystems? 

 

But even that question doesn't really matter.  The point is that Earth has sustained for billions of years an atmosphere with a hydrologic cycle that has all three phases of water.  It has maintained conditions that have allowed life on the planet to evolve.  Stromatolites were an early more complex life form.  Then there was the mysterious Ediacaran fauna ~700 million years ago.  With the Cambrian explosion 540 million years ago multi-cellular life finally really took off into a rich and evolving diversity. Eventually that life found its way from the oceans to the land.  That took ~ 3 billion years of evolution and life modifying the atmospheric composition.  And yet the complex surface ecosystems of today are still dependent upon the simplest microorganisms for nutrient fixing in the soil, helping break down food in our guts etc. 

 

Today's surface ecosystems are a whole different level of complexity than the time of the stromatolites.  And that takes the right kind of star, the right kind of planet with the right kind of atmosphere and climate - which is ultimately supported by the right amount of surface water.  The Earth's oceans have played a significant role in stabilizing the Earth's climate enough that it has provided life time to evolve into it's current state.

[SuiGeneris]

I'm breaking your reply into pieces to keep my thoughts coherent...hope this makes sense.
 

I don't disagree with anything you say here - except that you are describing scenarios that are not "Earth-like".

I literally based every point of what I wrote in my last reply based on what has happened here on earth. Life (likely) evolved in the crust, and continues to live there today. Complex biomes and organisms appear in every environment we've looked at, and appear early in earths evolutionary history. Earthly chemosynthesis is more energetically favourable than is earthly photosynthesis. Those are all based on what happens here on Earth. Its not hypothetical in any form - its literally what is happening beneath our feet, on our oceans, and yes, even across the surface, this very second.
 
My literal complaint throughout this thread is that people are making claims which we know are false based on what has happened here on earth.
 
I'm not sure how I could be any more "earth like" than literally describing how life on earth works.
 

Here is my frame of reference for this thread.  The title of the thread is best class of star for life.  I've listed examples above that do not require energy from the star for life.  So I understand the broad ranging possibilities for life that you are talking about.  But whether the OP asks it or not, the framing of the question around "best class of star" is really only relevant to surface life.  The type of star doesn't really matter for life deep inside an icy moon, or for life that evolves inside terrestrial planets that absorbed water during formation.

But is the title only relevant to surface life? I'd argue its not - stellar class appears to play some role in determining what kinds of planets, and in what numbers, are formed. If life can evolve on Jovian-like moons, than perhaps the best class of star for life is a class of star that tends towards larger number of gas giants. If chemosynthesis is common across extra-terrestrial life, than star-planet configurations which promote geological activity (e.g. tidal flexing) may be the best class of star for life. Even if life is 1:10,000 as likely to evolve around an M- or K-type star than a G-type star, M/K-types may still be the "best class" as their huge numbers would mean that life is more commonly found around them. Likewise, a relatively quiescent star is also good for life - wherever it lives - as having solar flares strip off the atmosphere and irradiate the surface would be lethal to earth-like life; even that which lives in the crust.
 

When I say "Earth-like" I'm not suggesting that all life in the universe must exist in Earth-like conditions.  What I'm getting at is identifying the essential characteristics of an Earth-like planet because Earth is highly dependent upon the Sun for maintaining its complex surface/ocean ecosystems.  And if we talk about what is the best type of star for life then there are some characteristics of Earth are going to also be general requirements for complex surface ecosystems on other planets orbiting other stars.

And again, in terms of raw numbers of species, most of earth's life lives independently of the surface and sun. The vast majority of earths biological diversity - including earthly life's evolutionary origins - exists entirely away from the light of the sun and does not rely directly or indirectly on photosynthesis for survival. And while relatively rare today, there are very complex surface biomes that are independent of light and oceans here on earth (Antarctic biomes being the largest examples, but similar can be found elsewhere). Neither sun nor oceans are necessity for complex surface life.

 

What makes a planet Earth-like?  This could obviously be debated, but in simple terms it is a terrestrial planet with a mixed land-ocean surface that interacts with an atmosphere and has a day-night cycle.  It also relies significantly upon starlight to provide energy for the surface ecosystems and maintenance of an atmosphere and "stable" climate over billions of years.   Earth-like planets will be the planets most dependent upon the type of star.   Specifically, the best type of stars for Earth-like planets are approximately G0 to K5 spectral classes.

But why does a planet need to be earth-like to be friendly to life? Genetic and metabolic mapping suggests that the LUCA on earth was a thermophilic chemotroph - e.g. earthly life evolved in an environment hostile to most life present today. That early earth was very different from the earth today - much hotter, much thinner crust, completely different atmosphere, higher UV and X ray emissions from the sun, etc. And yet life - including surface life - did just fine.

[SuiGeneris]

Again, the thread is about best type of star for life.  That implies surface ecosystems or you don't really need to ask the question.  Also it appears there is some misunderstanding of meaning here.  Any living organisms have incredibly complex biochemistry. What I specifically said was "complex ecosystems".  Sure stromatolites were complex.  I learned about them in my paleontology classes in college.  Would you say stromatolites are as complex an ecosystem as the modern Earth's ecosystems?

 
Stromatolites still exist, and still are part of earths modern ecosystems. Depending on how you measure complexity, stromatolites can be considered very complex. They have a complex and highly structured three-dimensional organization with different organisms living in different layers. Like plants, they have dedicated structures for transporting waste and nutrients, and some appear to have developed active transport mechanisms - making those structures more "advanced" than the passive transport found in plants. Modern stromatolites have a degree of species diversity roughly equivalent to that of a temperate forest. And that's just the stromatolite itself, which in turn is part of a larger ecosystem. Even in the early earth, the first stromatolites were part of a larger ecosystem containing other free-living and colonial organisms, and potentially the first protists.

 

The first large organisms on land were fungi - not photosynthetic plants - and did not depend on the sun for energy.

 

BTW, most of the genetic and biochemical diversity of earthly life is found in microbes. The "big life" you think is so central to ecosystems provides an additional degree of complexity that is pretty much a rounding error compared to that created by microbes

 

 

But even that question doesn't really matter.  The point is that Earth has sustained for billions of years an atmosphere with a hydrologic cycle that has all three phases of water.  It has maintained conditions that have allowed life on the planet to evolve.  Stromatolites were an early more complex life form.  Then there was the mysterious Ediacaran fauna ~700 million years ago.  With the Cambrian explosion 540 million years ago multi-cellular life finally really took off into a rich and evolving diversity. Eventually that life found its way from the oceans to the land.  That took ~ 3 billion years of evolution and life modifying the atmospheric composition.  And yet the complex surface ecosystems of today are still dependent upon the simplest microorganisms for nutrient fixing in the soil, helping break down food in our guts etc. 
 
Today's surface ecosystems are a whole different level of complexity than the time of the stromatolites.  And that takes the right kind of star, the right kind of planet with the right kind of atmosphere and climate - which is ultimately supported by the right amount of surface water.  The Earth's oceans have played a significant role in stabilizing the Earth's climate enough that it has provided life time to evolve into it's current state.

And again, you're ignoring the lessons taught to us by earthly life about what is possible and what drives evolution. Surface life on earth evolved the way it did because of the physical conditions present on the surface. It evolve to deal with oceans of water...because there are oceans. It evolved photosynthesis...because there was enough light to compete with more efficient chemosynthetic pathways. And there are places on the earths surface  (and interior, and beneath the ocean) that deviate far from those "norms"...and complex life is found in those biomes as well. Surface, sun, and ocean not required. Life evolves to fit the niches available.

 

If you want to claim that complex life needs an earth-like planet to evolve on, you need some sort of active and plausible process that would prevent life from evolving past a simple state on non-terrestrial planets. Because if life on earth has shown us anything, it is that life will evolve to fill any available niche, and develop complex ecosystems in the process. 

Edited by SuiGeneris, 28 March 2025 - 11:50 AM.

[russell23]

 
Stromatolites still exist, and still are part of earths modern ecosystems. Depending on how you measure complexity, stromatolites can be considered very complex. They have a complex and highly structured three-dimensional organization with different organisms living in different layers. Like plants, they have dedicated structures for transporting waste and nutrients, and some appear to have developed active transport mechanisms - making those structures more "advanced" than the passive transport found in plants. Modern stromatolites have a degree of species diversity roughly equivalent to that of a temperate forest. And that's just the stromatolite itself, which in turn is part of a larger ecosystem. Even in the early earth, the first stromatolites were part of a larger ecosystem containing other free-living and colonial organisms, and potentially the first protists.

 

The first large organisms on land were fungi - not photosynthetic plants - and did not depend on the sun for energy.

 

BTW, most of the genetic and biochemical diversity of earthly life is found in microbes. The "big life" you think is so central to ecosystems provides an additional degree of complexity that is pretty much a rounding error compared to that created by microbes

 

 

And again, you're ignoring the lessons taught to us by earthly life about what is possible and what drives evolution. Surface life on earth evolved the way it did because of the physical conditions present on the surface. It evolve to deal with oceans of water...because there are oceans. It evolved photosynthesis...because there was enough light to compete with more efficient chemosynthetic pathways. And there are places on the earths surface  (and interior, and beneath the ocean) that deviate far from those "norms"...and complex life is found in those biomes as well. Surface, sun, and ocean not required. Life evolves to fit the niches available.

 

 

I don't have a problem with anything you are saying above.  The problem is you are misinterpreting what I am saying - or I am not being clear enough about what I am trying to say.  Everything you've said in the above quote I am aware of - well except the part about stromatolites still being around today.  That's pretty cool - the ultimate living fossils!  But I get it about the microorganisms.  You're not telling me anything I don't already have an understanding of.

 

I also fully understand that life on Earth shows the adaptability of life to exotic conditions very different from the surface. It is incredible the way life adapts.  If you go back and look at what I have said, you will see I have not denied that.

 

My point is that, the question of this thread is what type of star is best for supporting life on a planet.  By its nature that question means that we are talking about the surface of a planet, because interior ecosystems are not going to rely on the Sun.  It also means we are talking about life that uses the starlight as the primary energy source - photosynthesis.  

 

If it is non-photosynthetic life then it isn't relying on the Sun and we are back to the star doesn't matter for the life chemistry.  But even for non-photosynthetic life the star will still matter for maintaining the climate of the planet if the planet has surface life (land or ocean).

 

You seem to be getting hung up on what "complexity" means.  A case can be made that microorganisms are complex.  On a biochemical basis all life is complex.  What I mean by "complex surface ecosystems" is in an ecological structural sense.  You have a planet with surface life that is more than just microorganisms and fungi.  You also have creatures we classify as plants, animals, fungi, ... everything.  And, I consider the oceans part of the "surface" too.  But having a mixed land-ocean surface is part of the Earth-like condition. 

 

The atmosphere, biology, geochemistry and surface structure and internal activity of the Earth are all interacting to create what this planet is.  And because of this integration of all aspects of the Earth, the complex surface ecosystem structures the Earth has are very sensitive to a long term stability of the climate so that all three states of water can exist at the surface.  This is what I am trying to get at.  If you change the underlying physical nature of the star, the planet, and the planetary architecture of the stellar system, it is possible to end up with a world that really only can support microorganisms or ecosystems with much less structural diversity and complexity.   Or a world that cannot support life on the surface at all.

 

Which is what the title of this thread suggests it is about. 

 

If you want to claim that complex life needs an earth-like planet to evolve on, you need some sort of active and plausible process that would prevent life from evolving past a simple state on non-terrestrial planets. Because if life on earth has shown us anything, it is that life will evolve to fill any available niche, and develop complex ecosystems in the process.

 

That is not a claim I made.  What I said is that to be an Earth-like planet, the planet needs to have a mixed land-ocean surface.  If there is no ground, it is not Earth-like. If there is no ocean, it is not Earth-like. 

 

I did not say that you could not have complex ecosystems on other types of worlds.  In fact, I pointed to Europa and Enceladus as examples of ocean worlds that could have complex life.  But they are ocean worlds - not Earth-like worlds.  Although, for all the interest in ocean worlds like Titan, Enceladus, and Europa, recent research suggests a rich biosphere may not be possible on these ocean worlds:

 

https://www.liebertp...9/ast.2023.0055

 

 

It would not be a process that prevents life on other types of worlds, it would be a lack of resources and conditions.  And you're also bypassing the question of having the chemical conditions necessary to allow abiogenisis.  We don't know how life originated, but not all places are equally likely to have life originate and, if it does originate, survive long enough to evolve complex surface ecosystem structure (Mars).  It took Earth about 3 billion years to get from the first microorganisms to diverse multi-cellular ecosystems. It took another few hundred million years for that life to figure out how to cover the land.

 

There are a lot of physical factors that can limit the complexity and diversity of life. Life on Earth has adapted to every condition because it has had billions of years to do so and great conditions - but even life has limits of conditions.  At some point, the temperature becomes too hot for organic molecules to maintain structure. The surface of Venus (~460 C) is very hot for example. Most organic molecules decompose by 300 deg C.  PAH's decompose by around 600 C so I guess if you had a biochemistry that could be based upon PAH's maybe you could have some kind of exotic life there.  But there is not evidence for complex ecosystems on the surface of Venus. 

[SuiGeneris]

My point is that, the question of this thread is what type of star is best for supporting life on a planet.  By its nature that question means that we are talking about the surface of a planet, because interior ecosystems are not going to rely on the Sun.  It also means we are talking about life that uses the starlight as the primary energy source - photosynthesis.

I disagree that this is what this means at all; as I pointed out above, there are a lot of ways this could be interpreted. I'd still argue that the question "what type of star is best" is answered by "the type of star that is most likely to generate planets (or moons) compatible with the evolution of life". Whether life is on the surface or elsewhere is an irrelevancy in that calculation.
 
But even if we take your "surface life" as the only meaning for "best star", that still does not necessitate photosynthesis. Photosynthesis is remarkably inefficient - more inefficient (in terms of fraction of energy converted into work) than any other metabolic process we know of on earth, and up to 1/40th as inefficient as the more efficient chemosynthetic pathways discovered so far. On earth, the geology of the planet is such that opportunities for chemosynthesis are limited. But you don't need a lot more abiotic methane, or H2S, or H2, or free iron, etc, than earth currently generates to render photosynthesis moot from an energetics perspective.
 
Again, life evolves to suit the local conditions. A resource (like light) may not be used if it is energetically unfavorable compared to what the competition is using. 
 

If it is non-photosynthetic life then it isn't relying on the Sun and we are back to the star doesn't matter for the life chemistry.  But even for non-photosynthetic life the star will still matter for maintaining the climate of the planet if the planet has surface life (land or ocean).

 
But the star does matter for the environment it creates on the planet, whether the life on that planet uses photosynthesis or not. A star that pumps out large amounts of X-rays, or strips off a planets atmosphere, or which gets too hot, or which is in a multi-star system that ejects planets, etc, would sterilize a planet regardless of where the life lives.
 

You seem to be getting hung up on what "complexity" means.  A case can be made that microorganisms are complex.  On a biochemical basis all life is complex.  What I mean by "complex surface ecosystems" is in an ecological structural sense.  You have a planet with surface life that is more than just microorganisms and fungi.  You also have creatures we classify as plants, animals, fungi, ... everything.  And, I consider the oceans part of the "surface" too.  But having a mixed land-ocean surface is part of the Earth-like condition.

How we measure the complexity of a biome is pretty tightly defined in biology, so I'm not "hung up"; I'm using the primary ecological definitions (specifically alpha and beta diversity). You're using your own definition that runs opposite of how diversity is conventionally measured.
 
There is nothing "special" about animals, or plants, or fungi, etc, in generating diversity. You don't have more ecological diversity because of the presence of your favored phyla. 
 

The atmosphere, biology, geochemistry and surface structure and internal activity of the Earth are all interacting to create what this planet is.  And because of this integration of all aspects of the Earth, the complex surface ecosystem structures the Earth has are very sensitive to a long term stability of the climate so that all three states of water can exist at the surface.  This is what I am trying to get at.  If you change the underlying physical nature of the star, the planet, and the planetary architecture of the stellar system, it is possible to end up with a world that really only can support microorganisms or ecosystems with much less structural diversity and complexity.   Or a world that cannot support life on the surface at all.
 
Which is what the title of this thread suggests it is about.

Again, you're making assumptions that are not even supported by life here on earth - which by definition, is life that evolved on an earth-like planet. Firstly, while all three stages of water exist on earth, the vast majority of life only ever encounters liquid water. For most life, gaseous or solid water is a death sentence. So why would a planet need all three to support life? Heck, the earth has gone through phases where there was little to now solid water, and those were some of the periods of the most florid growth and evolution of life we know of.
 
As for "structural diversity and complexity", again, you've chosen your own ruler to measure this. By the standards used to measure biome complexity, the presence of large animals and large (vascular) plants is typically a rounding error on the degree of complexity we measure. In terms of where "complex organisms" evolve (using your definition - e.g. large, multicellular life), again, this wasn't on land or on the ocean surface. The oldest evidence we have for animal life are sponges from the sea floor; next oldest are fossilized worm tracks - from within the sea floor. So these "complex organisms" did not need land, nor the ocean surface, nor light, to evolve. They were bottom-dwellers and filter-feeders. The only group of large-organisms that did not evolve in the ocean are vascular plants; most large organism diversity remains in the oceans today.
 
 

It would not be a process that prevents life on other types of worlds, it would be a lack of resources and conditions.  And you're also bypassing the question of having the chemical conditions necessary to allow abiogenisis.  We don't know how life originated, but not all places are equally likely to have life originate and, if it does originate, survive long enough to evolve complex surface ecosystem structure (Mars).  It took Earth about 3 billion years to get from the first microorganisms to diverse multi-cellular ecosystems. It took another few hundred million years for that life to figure out how to cover the land.

I've not "bypassed" anything; to have the materials to support life, a planet a prior must have the materials to support abiogenesis. Its not like abiogenesis used different chemistry than the life it turned into. Even panspermia doesn't get around that issue. And again, using earth as our model, abiogenesis appears to have occurred within the earths crust or around sites such as black smokers. The high UV output of the early sun pretty much abrogates abiogenesis on the surface.

 

Also, it did not take earth 3 billion years to evolve multicellularity; the first time it happened was between 3 and 3.5 BYA - e.g. 500 million to 1 billion years after life first appeared. It's been present ever since, and independently evolved at least 25 additional times since then. Heck, you can even generate it in a few months in the lab. It's not that big of an issue.

 

As for land, the oldest evidence we have for life on land is from 3.2 billion years ago (and indirect evidence for 3.7 billion years ago) - about the same time multicellularity evolved.

 

Neither multicellularity, nor conquering land, was all that difficult on earth. Why would it be elsewhere?

[Lard Greystoke]

If you were to go by evidence, I would say G2.

[MeridianStarGazer]

If you were to go by evidence, I would say G2.

If I were to bet my lineage, that is where I would send them. And where I would send most radio signals. Most likely the nearest star with advanced life is just far enough we can't send a detectable signal to them.

K class stars live longer and have less UV. But G class stars have a bigger heliosphere and stronger magnetosphere to protect against cosmic rays. Likely also more solar cell or photosynthesis potential at any given temperature, if that matters.

Edited by MeridianStarGazer, 10 April 2025 - 03:34 PM.