Topic: Are larger rocky planets better for life?

[Nemesis]

Link to full article

Our planet is changing before our eyes, and as a result, many species are living on the edge. Yet Earth has been on the edge of habitability from the beginning. New work by astronomers at the Harvard-Smithsonian Center for Astrophysics shows that if Earth had been slightly smaller and less massive, it would not have plate tectonics - the forces that move continents and build mountains. And without plate tectonics, life might never have gained a foothold on our world.

"Plate tectonics are essential to life as we know it," said Diana Valencia of Harvard University. "Our calculations show that bigger is better when it comes to the habitability of rocky planets."

The team found that super-Earths would be more geologically active than our planet, experiencing more vigorous plate tectonics due to thinner plates under more stress. Earth itself was found to be a borderline case, not surprisingly since the slightly smaller planet Venus is tectonically inactive.

"It might not be a coincidence that Earth is the largest rocky planet in our solar system, and also the only one with life," said Valencia.

[Vipre]

Interesting, but he seems focused only on human habitability. Marine life exists in vastly different environments from super hot to bitterly cold, at bone crushing pressures and even in methane atmospheres.

"It might not be a coincidence that Earth is the largest rocky planet in our solar system, and also the only one with life," said Valencia

Sounds pretty convinced. Surprising considering NASA isn't even convinced of that yet.

[Lloyd007]

Well in a survey of 48 life bearing rocky worlds all of them had plate tectonics.... all rocky worlds without plate tectonics were barren... 

Maybe there's no life on Venus because the atmosphere is so thick you can cut it with a knife and it's so hot it can melt metal...

Maybe there's no life on Mars because it's a cold barren desert planet with no liquid anything...

Just a thought...

[Nemesis]

I think that part of the idea is that to encourage the evolution of life you need an environment that changes over fairly long periods of time.  With plate tectonics and oceans combined you get species forced to adapt to changing conditions.  Without the changing environment evolution may well stagnate.

Some think that the tides of our large moon helped species evolve so as to colonize the land.

[Vipre]

I think that part of the idea is that to encourage the evolution of life you need an environment that changes over fairly long periods of time.  With plate tectonics and oceans combined you get species forced to adapt to changing conditions.  Without the changing environment evolution may well stagnate.

Some think that the tides of our large moon helped species evolve so as to colonize the land.

Not a scientist obviously but isn't one piece of driving evolutionary force the predator prey relationship? Take Eurypterids...

Eurypterids are believed to be the extinct aquatic ancestors of scorpions and possibly all arachnids.

Some geologists believe that giant arthropods evolved due to higher levels of oxygen in the atmosphere in the past. Others, that they evolved in an 'arms race' alongside their likely prey, the early armoured fish

Humans have "evolved" in just that fashion albeit technologically for the same reason.

The article strikes me as implying that unless the planet is just like Earth then life on it is impossible when in fact we don't know what possible forms life might come in to begin with. For all we know there may be life on Jupiter, but not life we'd recognise when we first see it.

It focuses on rocky planets, but what of a rocky oceanworld with no tectonics, just because it doesn't have plates that slide around means the ocean will be incapable of supporting life? It says tectonics help keep our temperature where it needs to be, but what if you ditched tectonics and just moved the planet closer to the sun? The increased heat from the sun offsets the lack of recycled carbon dioxide. It seems focused clearly on "primate" life rather than "life" in general.


Eurypterid quote

[Nemesis]

One piece is indeed the predator prey relationship.  There are of course others. Environmental conditions especially.  Prey adapt colours that conceal them where that is useful.  Methods of cooling or heating are also adapted as needed.  Also there are purely reproductive adaptations such as "mating displays".

Things that cause changes in the environment will upset the status quo and drive evolution in new directions.  Plate tectonics cause ongoing changes over time and will keep forcing new adaptations. The patterns in the land by raising and lowering it.  Changes in airflow by the same.  Location, equatorial, temperate or arctic.  Changes in what land masses are near each other.  Changes in ocean currents that raise or lower the local temperatures, especially if they keep recurring. 

Changes in the land masses can also act to isolate populations from one another.  Over time they will adapt differently to pressures and become different species.  Conside equines such a the horse, zebra and donkey.

These changes can force new adaptations, they can also bring new species into the mix when two landmasses collide or approach.  Australia is a good example. Placental mammals in limited types arrived with the Aborigines and the local marsupials had a big die back (which may have been at least partly from ongoing climate changes).  More changes have happened with the introduction of other species in modern times.

Assuredly from the appearance of the surface of Mars, Venus, Mercury and the Moon there are forces that change them.  But they seem to be catastrophic rather than gradual so they would tend to wipe out life rather than drive its evolution.

I'm not a scientist either.  I tend to read a lot on things that are of interest, science, technology and history.  Which gives me a lot to think about and I sometimes get the pleasure of finding real scientists discovering what I've speculated about.

[Lloyd007]

It might well be that this guy is 100% right and plate tectonics are a necessary ingredient in the creation of life but to try and say it's more than a coin flip based on the three  large rocky worlds in our solar system is imo not possible. If Venus or Mars were barren for reasons other than the fact that one is an oven and the other has no liquid water and is frozen He might have a point but right now his "Yes it is" is as plausible as someone else's "No it's not"

[Nemesis]

I agree that is why I posted the subject as a question.  In case the reasons behind the articles thought patterns were not known to you (and other readers) I explained them as far as they are known to me from prior reading on the subject. 

Deductions based on limited data are always questionable. 

So far we have 5 rocky planets and have detailed information on only one of those 5, have visited very briefly a second (the moon) and have probes that have done limited searching of the surface of a 3rd and orbital probes of the other 2.  That is not enough to tell us if any of those other 4 ever had life.  At this point we don't know if life originated on one of the others and then spread to Earth or not.  We don't know if they ever had a thriving ecosystem.  Its been deduced that Mercury and the Moon didn't retain an atmosphere for a long enough time and that Venus was always too hot.  Until they have been thoroughly explored that is only educated guesswork.

[Beeblebrox]

I know this answer doesn't touch on plate tectonics but it does address the size factor in the planetary generation of life.  There is a certain minimum size that must be achieved in order for a planet to retain an atmosphere--whatever the atmosphere may happen to be composed of.  Below that threshold the atmosphere tends to evaporate into space at a slower or quicker rate depending on how much the planet misses the size threshold by.