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Space nets are a theoretical space organism designed by Tim B, to quote him:

Imagine a life-form shaped like a giant net. It gathers space dust into itself to grow, using light from stars both to power its growth and for propulsion as a massive light-sail.

The strands of the net are far enough apart that gravity falls off faster than it gathers and the strength of the strands is more than enough to keep its shape.

These creatures could grow to theoretically unlimited size, just constrained by raw materials and solar energy. You could well see them sweeping into star systems and raiding the rings around planets, asteroids and even small moons for raw material — cleaning them out then moving on growing all the time.

They could reproduce by firing off spores — or more likely just by splitting in two once their size became too large for them to sustain with available resources.

Is this kind of organism even possible? If so, how would it evolve?

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Will this organism survive in space? With some adjustments, yes.

It gathers space dust into itself to grow

Digestive organs are generally not flat. Even single-celled organisms use vacuoles, stomach-like organelles, to store and break down food. Assuming this creature is built so that it absorbs dust into a cell-like exterior membrane, so it doesn't need a multitude of mouths, it cannot have a central digestive organ - it will have to produce its own "vacuoles" to break down food wherever the food appears. Fortunately, this occurs in nature! Bacteria do this, just on a much smaller scale.

However, this method of digestion severely limits the size of the particles the organism can eat. Anything larger than the width of the membrane in which vacuoles are created will tear the creature because vacuoles won't be able to hold food. If the membrane can expand slightly, expect food as large as a foot in diameter. Even with an expandable membrane, large objects will pass through the organism completely.

Using light from stars both to power its growth and for propulsion as a massive light-sail.

A common misconception about photosynthesis is that plants directly use light energy. This is false - light aids the reaction to produce glucose, the stuff plants actually use, from water and carbon dioxide, with oxygen as a byproduct. In order for a sail-like organism to photosynthesize as we know it, it will need to take in specific gasses that may not be present in every system. This is problematic.

Unless this organism has a completely different form of photosynthesis, in which light is directly used as energy, it will not be able to rely on starlight for fuel. However, movement using light is reasonable - solar sails are a sound method of propulsion.

We'll come back to radiation later.

The strands of the net are far enough apart that gravity falls off faster than it gathers and the strength of the strands is more than enough to keep its shape.

The idea of gravity falling off of objects aside, strands don't work well in this situation. While they may help reduce drag, a fully intact, plane-like membrane allows the transfer of energy and matter; strands that are not touching take much longer to circulate nutrients. Additionally, strands will not digest as much matter, which these creatures do not come in contact with often. This creature cannot afford to let that food slip through.

Furthermore, gravity wouldn't be a huge problem in space. If this organism travels to rings, it will most likely fall into a weak, breakable orbit, while dust clouds and asteroid belts do not pose a threat at all. It will still keep its shape regardless, or it will contract muscles or rearrange vacuoles to adjust itself.

These creatures could grow to theoretically unlimited size, just constrained by raw materials and solar energy.

These constraints are very real limits. Tim's creatures will find huge amounts of matter in remote areas, grow extremely large over thousands of years to store the matter and maintain a flat shape, and, when all matter is gone, they will slowly consume themselves on the way to the next location. Raw materials are a very real, very limiting constraint. Still, this is possible.

You could well see them sweeping into star systems and raiding the rings around planets, asteroids and even small moons for raw material — cleaning them out then moving on growing all the time.

Rings? Sure, orbits work well.
Asteroids? If we're talking dust clouds around them, or small particles, sure!
Small moons? Absolutely not, for several reasons:

  • If these moons don't contain what the creatures need, they will have wasted indispensable energy on a voyage to a useless rock. Asteroids and rings, in contrast, have more various compositions between particles and objects.
  • It has taken 17 million years to create the 1.15 mile deep Grand Canyon. That's a rate of about 14.8 million years per mile. If a small moon is 10 miles across, and this creature eats at half the average 4 mph speed of the Colorado River over time (2 mph is still fast for something with this method of digestion) it will take about 300 million years just to eat a hole through the moon - let alone break apart the entire thing!

They could reproduce by firing off spores — or more likely just by splitting in two once their size became too large for them to sustain with available resources.

Sure, why not. Splitting will work if these organisms are just giant eukaryotes - prokaryotic organisms won't work, as explained below. Spores would potentially work as well, but may be slightly more complex to evolve.

light from stars

... is actually one of your biggest problems! There is more radiation in space than just visible light, and it will both mess up this organism's DNA and fry its body. Since an organism with a permeable membrane cannot have impermeable radiation shielding, let's consider an alternative:

  • Shield the nucleus, or wherever the organism keeps reproductive material (DNA) and keep it in the center of the sail.
  • Have an extremely high reproductive rate (spores come in handy here!) so that irradiated individuals are replaced by newborns relatively quickly
  • Generate new membrane quickly so particles knocked out by radiation are replaced
  • Channel all heat out as quickly as possible, maybe by converting radiation into a longer wavelength and re-emitting it. Not entirely sure how an organism would accomplish this but it's possible
  • Have a reflective exterior to take in as little radiation as possible

Could these organisms evolve? Unlikely.

The evolution of a cell-like organism unaccustomed to gravity can be explained in cosmic dust clouds. After life occurs, or simple cells are introduced, the abundance of dust and gases will help shield developing organisms from radiation (think ozone). If cells have enough to eat, and evolve, eventually they may take the shape of a solar sail, and scale up as they consume matter.

However, abiogenesis (life occurring from inorganic materials) in a dust cloud, or panspermia (life hitching a ride on something) surviving space radiation, is unlikely. Consider organisms that have been genetically engineered as an alternative; it doesn't have to occur naturally to work well.

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"Is this kind of organism even possible?" No, because it couldn't evolve.

Ninety percent of life on planet Earth is microscopic, protozoa, single celled organisms. There would need to be an extensive ecology, a complete biosphere, before complex, multicellular organisms could evolve. Even the evolution of space-based microbes would be mind-bogglingly improbable. It might take trillions of years to happen. This is just getting to first base in the evolutionary stakes. It's going to take even longer for multicellular organisms like space nets emerge.

The proposal for this creature involves that gathers space dust. But without explaining what or how this space net creature is able to do this. Without specifying what the space dust consists of, there is no way of knowing what the organism will need for it to grow, reproduce and move. The creature needs to be made of something in the first place. Depending on what it is made of will determine how much energy is required for it to assimilate material, digest and constitute that matter into its 'flesh'.

Space-based lifeforms will only have access to minimal amounts of energy. There is literally microscopic amounts of matter available for them to consume.

These creatures could grow to theoretically unlimited size, just constrained by raw materials and solar energy.

Provided by unlimited size, you mean unlimited microscopic size, because that is what "just constrained by raw materials and solar energy" means when only microscopic amounts of raw materials and solar energy are available.

You could well see them sweeping into star systems and raiding the rings around planets, asteroids and even small moons for raw material — cleaning them out then moving on growing all the time.

Sorry this is romantic dream. The velocities these space nets will move at would be tiny. Not just in interplanetary terms, but by terms of planetary motion. This doesn't mean at the rate that planets move, but the rates that things move on the surfaces of planets.

The current description of the design of this creature doesn't possess any plausible biological mechanisms to justify its putative existence as an even hypothetical space-based lifeform.

I agree with Zxyrra. Space nets won't evolve, but they might be products of space-faring species that decided to create their genetically engineered space-based lifeform. However, they would have to design the biological structures and functions necessary to make a space-net organism viable.

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    $\begingroup$ I agree with you that it could hardly evolve. But it could be bio-engineered, at least its ancestor. It might have been devised as something to clean up debris in orbit around a very enthousiastic but not very far-sighted civilization that eventually found it's orbit riddled with space junk... Who knows? $\endgroup$ – Burki Nov 4 '16 at 11:08

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