"Detecting Photosynthesis on Exoplanets" Topic

"Although many of the nearby stars we will study for signs of life are older than the Sun, we do not know how long it takes life to emerge or, for that matter, how likely it is to emerge at all. As we saw yesterday, that means plugging values into Drake-like equations to estimate the possibility of detecting an alien civilization. We can't rule out the possibility that we are surrounded by planets teeming with non-sentient life, fecund worlds that have no heat-producing technologies to observe. Fortunately, we are developing the tools for detecting life of the simplest kinds, so that while a telescope of Colossus class can be used to detect technology-based heat signatures, it can also be put to work looking for simpler biomarkers.

Svetlana Berdyugina (Kiepenheuer Institut für Sonnenphysik and the University of Freiburg), now a visiting scientist at the University of Hawaii, has been leading a team on such detections and spoke about surface imaging of Earth-like planets at the recent Breakthrough Discuss conference. The emphasis was on Proxima b, but these techniques can be applied to many other systems within the 60 light year radius that Colossus should be capable of probing…"
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Amicalement
Armand

Ok, so I'm no exobiologist, but I have some questions:

Respiration and photosynthesis are two very complicated chemical pathways that evolved over a very long time on Earth to utilize chemicals from our atmosphere, and our environment and the energy from sunlight. All of it has the end point of breaking down one phosphate from adenosine triphosphate to adenosine diphosphate and making a lot of energy. In plants and animals this energy is stored by manufacturing glucose and starches.

Why would anyone expect the exact same processes to evolve on an alien planet? Ok, a living organism on a planet bathed in safe, not too ionizining sunlight will evolve a pathway to capture that sunlight and create an energy source for its metabolism. Why would you expect it to behave like terrestrial photosynthesis and swallow up CO2, break apart ATP, store starches and release O2?

I know they have no other chemical markers to go by and therefore have to look at the one process we are familiar with, but this is a good example of "absence of evidence does not mean evidence of absence". In other words using the "spectral imaging" will produce limited positive results and a bunch of false negatives. The latter witl not preclude photosynthesizing life.

I'm guessing they are taking it for given if a planet has co2 and sunlight sooner or later sometimes that uses photosynthesis will evolve. Just as if the planet has light. Eyes in one form or another will evolve.

I'm guessing they are taking it for given if a planet has co2 and sunlight sooner or later sometimes that uses photosynthesis will evolve.

Perhaps.

But on a planet with no CO2 in the atmosphere, photosynthesis could still evolve. It would be a complex pathway that would not involve CO2, not involve ATP, and not release O2 gas as a waste product. In other words it could be totally "alien"

Even on planets with CO2 as an atmospheric gas, why would the complex pathway that evolved on Earth be expected to be reproduced elsewhere under completely different conditions? What if the alien form of photosynthesis converts CO2 into H2O as a waste product (instead of O2), using an intermediary energy producing chemical that doesn't exist within living things on Earth?

Just as if the planet has light. Eyes in one form or another will evolve.

Maybe. Maybe not.

Photosensitive organs could evolve that would be so alien to us that we would not recognize them as "eyes". How about photosensitive receptors all along the outside of the creature's external surface that connects to it's brain and provides a 360 degree visual image? That may take a lot of neuronal pathways and processing power, but has a great evolutionary advantage that no predator could sneak up on it.

I think it is a mistake to assume alien life will be somewhat similar to what evolved here on Earth. I'm thinking that aliens will be really "alien".

I'm not trying to be difficult, it is a good discussion.

Evolution generally evolve the most efficient way to tackle the environment. Hence sharks/dolphins/ichthyosaours etc.
Almost all animals have their light sensitive organs on their head if it had been really advantageous to have eyes everywhere then most likely some creature would have evolved it.

Evolution is limited by physics and chemistry. The chemistry of a planet can probably be quite different but the physics diffences will not be. Slightly thicker atmosphere or more gravity doesn't change that much.

And doesn't matter what if you swim through H2O methane. You'd still need to be hydrodynamic if you want to move fast.

If you look at early multicellular life there was a lot of funky and alien looking things, but then as evolution did its thing life became more standardised and boring as more optimum shapes took hold.

Thanks for such a well reasoned reply. It's so depressing looking at what passes for dialogue on this board recently.

Evolution generally evolve the most efficient way to tackle the environment. Hence sharks/dolphins/ichthyosaours etc.

One of my favourite scientists is Kenneth Miller of Brown University. One of his favourite topics is "convergent evolution", which is what you are describing above. He is of the opinion that intelligent alien creatures will actually have a very similar look to modern humans. Biped movement is the most efficient way of moving about, whilst freeing up any other limbs for more detailed work, like evolving hands or similar analogies. This of course spurns on further brain development. The neurological network is concentrated to one area of the body with most of the important sensory apparatus, creating an analogy of a head.

So his idea of aliens fits the "Star Trek" version of aliens. I liked the movie "Arrival" much better as I thought real aliens would look, behave and act much more…..alien.

I can't disagree with what you are saying, but I will turn your convergent evolution argument around for you. While dolphins and sharks evolved in the identical environment and therefore look analogous on the outside, inside there are glaring differences. Some form of photosynthesis should occur on any planet with life and a sufficient source of safe, usable light. However, the actual chemical pathways that produce energy from the sun will also be totally different. Therefore, looking for terrestrial markers (CO2 and O2 level fluctuations) may not preclude other forms of photosynthetic life.

Almost all animals have their light sensitive organs on their head if it had been really advantageous to have eyes everywhere then most likely some creature would have evolved it.

That's not what I meant. I proposed an animal with light sensitive cells all throughout it's "skin" and having no specific organ like an eye. The entire body acts like the "eye", and can detect changes in light intensity all around. Now clearly, the lack of pinpointing of light through a specific organ, like a recessed eyeball will have other problems. For instance how will these creatures every develope microscopes or telescopes?

Fish do have something akin to your light sensitive whole body thing. The lateral line. Don't use light but pressure. Sure it has a shorter range then light detection. But water is ofte far from light friendly. But you can't get away from the pressure changes in a liquid environment.

When it comes to the photosynthesis part. I'm out of my depth. My knowable of chemistry is far to shallow to know just how many different "takes" there can be chemically to make it work.

But I statistically I would assume it would involve Hydrogen, oxygen carbon given the commonality of these elements in the universe. And that "we assume" alien life is made of the same building blocks as our self.

Of course if the alien life doesn't run on ATP like multicellular life does on earth. Then I not sure they would ever make any photosynthesis.
As photosynthesis is used to make glucose which is used to make ATP that runs all multicellular life on this planet (and much unicellular life too)

If the alien life isn't "carbon based" then all bets are off chemically. But evolution would still generally lead to environmental adaptations.
So a fast moving none carbon based creature in liquid would generally have some similarities on the outside to fish etc.

But I in my limited imagination think that either all life or the vast majority of it will be carbon based simply because of the massive availability of carbon and hydrogen next to other elements that might take their place.
I can only see none carbon based life evolving on planets with absolutely no carbon.

PS. And thank you, it's nice to have an actual science talk on the science board.

But I statistically I would assume it would involve Hydrogen, oxygen carbon given the commonality of these elements in the universe. And that "we assume" alien life is made of the same building blocks as our self.

Well, if you mean same chemicals and basic precursors, then yes.

The main structural building blocks of all living things on Earth are proteins. Even bones and teeth are protein structures seeded with various calcium salts and crystals. All the millions of different proteins in living things on Earth are made from only 20 (or 22) amino acids. However, there are hundreds of different amino acids on Earth and perhaps an infinite possibility elsewhere. Therefore, alien proteins should be expected to be very different from our own, as the amino acid precursors are alien. Therefore, any complicated chemical processes, such as photosynthesis with be totally different too.

Of course if the alien life doesn't run on ATP like multicellular life does on earth. Then I not sure they would ever make any photosynthesis.

They could have photosynthesis, but not necessarily using ATP as the energy chemical. There are lots of others.

But I in my limited imagination think that either all life or the vast majority of it will be carbon based simply because of the massive availability of carbon and hydrogen next to other elements that might take their place.
I can only see none carbon based life evolving on planets with absolutely no carbon.

I agree here. Silicon based life would have such a low metabolism rate that it would be indistinguishable to rocks.

PS. And thank you, it's nice to have an actual science talk on the science board.

Yep, it is.

Getting back to the original point:

….so that while a telescope of Colossus class can be used to detect technology-based heat signatures, it can also be put to work looking for simpler biomarkers.

Simple biomarkers for terrestrial photosynthesis, and that's my point. Other forms of chemistry will have other biomarkers. Therefore the risk is in seeing a lot of false negative results by watching O2 and CO2 fluctuations.