The Stars Our Destination

[Agemegos]

Originally Posted by AlexanderHowl

Actually, any A-type through M-type main sequence star may have habitable planets,

"Actually"? How do you know that?

I reckon that it is very optimistic to suppose that an A5 to F0 or K6 to M0 star might have a planet habitable to humanity. A4 or hotter and M1 or cooler don't seem plausible to me.

The main problem with A stars is that they only remain on the main sequence for about two billion years or less, whereas on Earth it took over two billion years for the oxygen atmosphere to form. Maybe the evolution of photsynthesis and the accumulation of oxygen goes a little faster on planets where the light is a bit brighter, and under suns where a more favourable bolometric correction means a highr ratio of illumination to heating. I've calculated the effect: optimistically the late A types are what you'd call "perhaps possible", not "actually", and early As are oit of the question.

M type stars and late Ks have several problems. First is that during their pre-main-sequence development they star out a lot brighter in the T Tauri phase than they end up as main-sequence stars: they spend a very long time on the Hayashi track and get a lot dimmer during it, which means that their protoplanetary disks form at a much higher temperature than they end up at. So their planets form out of planetismals that have condensed at high temperature: we expect them to be very poor in volatiles. Then you have the fact that they flare violently and eject a lot of stellar wind in their first few billion years, which would tend to strip away the atmospheres of planets in their Goldilocks zones. Then you have the problem that their low effective surface temperatures durin the main sequence mean that their light is rich in IR and poor in visible, and is more effective at heating than at driving photochemical reactions. That means that any planets they have that are the right temperature for liquid water are rather dimly illuminated, with slow photosynthesis: the formation of an oxygen atmosphere is slower than it was on Earth. I calculate that the planet of an M5 star would take 15–20 billion years to reach its oxygen catastrophe, and that is longer than the current age of any stars in the Universe. Late K and all M stars are "maybe possible", not "actually", and anything cooler than M2 seems to me to be "clearly out of the question".

[AlexanderHowl]

What we know about exoplanets and planetary formation seems to change every year, if not every month, so I think that having habitable planets around stars that are M7 through A5 (which are over 99% of main sequence stars) is reasonable until observational evidence contradicts that statement. We have found planets around stars that were A0 during their main sequence, so I am not sure that we can actually say what may be out there.

Anyway, I think we have different ideas of what is habitable. Garden type planets (other than what the Earth and any that we terraform), will likely not be habitable because the difference of amino acid composition of the life will cause the development of proteins that humans will find toxic (imagine a world where the animal dander is as poisonous to humans as cobra venom and you get the idea). Habitable planets will be the Ice and Ocean worlds that humans can terraform into Garden worlds, so planets around young stars will be just as acceptable as planets around old stars.

[Agemegos]

Originally Posted by AlexanderHowl

What we know about exoplanets and planetary formation seems to change every year, if not every month

I know it well, having followed the field since Dole's Habitable Planets for Man was current. The fact that a field is changing does not imply that it is not making progress, or that the our current state of knowledge is worse than total ignorance and unbounded optimism.

so I think that having habitable planets around stars that are M7 through A5 (which are over 99% of main sequence stars) is reasonable until observational evidence contradicts that statement.

I disagree, and so does every qualified researcher in the field. That doesn't prove that you are wrong, but it does offer a way for everyone else to escape from the problem that you described in the OP. Assume that physicists and astronomers know more about physics and astronomy than AlexanderHowl does, and the problem is reduced by six orders of magnitude.

We have found planets around stars that were A0 during their main sequence

Yeah, but no sign that they were ever inhabitable.

Anyway, I think we have different ideas of what is habitable. Garden type planets (other than what the Earth and any that we terraform), will likely not be habitable because the difference of amino acid composition of the life will cause the development of proteins that humans will find toxic (imagine a world where the animal dander is as poisonous to humans as cobra venom and you get the idea).

Cobra venom and things like that are extremely toxic not because they are from an incompatible biochemistry, but because they are precisely evolved to act as enzymes upon very specific parts of Earthly biochemistry. Alien life is far more likely to be indigestible or allergenic than super toxic. Meanwhile, I have actually eaten cake that was made with levo-glucose, and I'm fine.

Habitable planets will be the Ice and Ocean worlds that humans can terraform into Garden worlds, so planets around young stars will be just as acceptable as planets around old stars.

I've done a few back-of-envelope calculations of the thermochemical energy requirements of producing an oxygen atmosphere, comparing them to the insolation of a habitable planet. Being extremely optimistic I calculated that it would take at least two to five millennia to produce a breathable atmosphere on a lifeless planet, and that the time span required is more plausibly tens of thousands of years.

[AlexanderHowl]

Concerning terraforming, it does not take much more than 1,000 years if you are willing to use technology to accelerate the process if you are already starting with a nitrogen rich atmosphere. The Earth possesses around 1 quadrillion metric tons of oxygen, which only requires the thermal decomposition of around eleven hundred thousand cubic kilometers water (simply enough if you have fusion). A TL10 civilization could use ~300 TW of fusion generation to convert 1100 cubic kilometers of water into oxygen every year through thermal decomposition. The hydrogen produced as a byproduct of the thermal decomposition could be combined with small quantities of the atmospheric nitrogen to form ammonia for fertilizers to promote plant growth (and/or combined with carbon to form methane if greenhouse gases were required). The fusion byproducts (He-3 if you are using D-D fusion) have commercial value as well, so you might actually end up making a profit on the terraforming business.

[Fred Brackin]

Originally Posted by Phantasm

In my own setting, M2 is the coolest I'm going for habitable worlds, though any M could have planets. I haven't put an upper limit on the F type stars having habitable planets yet, though I'm angling for F5 at present.

Crossing Space p. 103 with the Wikipedia entry i referenced before you could have oxygen on everything but maybe an F0 but even your F5 might be short on land-based vertebrates.

Your F stars are going to need to be late in thei rmain sequence time though.

[Fred Brackin]

Originally Posted by AlexanderHowl

I think that having habitable planets around stars that are M7 through A5 (which are over 99% of main sequence stars) is reasonable until observational evidence contradicts that statement.

An A5 has only 1.3 billion years on the main sequence. That would allow for nothing more than _maybe_ the appearance of the first anerobic lifeforms right before the star went red giant on a really Earth-like time scale.

If you subtract out the half a billion years the planets would be molten you're needing to have oxygen-producing life develop at triple speed. I wouldn't include that in my default "reasonable" assumptions.

[Agemegos]

Originally Posted by Fred Brackin

An A5 has only 1.3 billion years on the main sequence. That would allow for nothing more than _maybe_ the appearance of the first anerobic lifeforms right before the star went red giant on a really Earth-like time scale.

If you subtract out the half a billion years the planets would be molten you're needing to have oxygen-producing life develop at triple speed. I wouldn't include that in my default "reasonable" assumptions.

Bear in mind that AlexanderHowl's "reasonable" assumption is that a planet with no oxygen is more habitable than one with.

[Agemegos]

Originally Posted by AlexanderHowl

Concerning terraforming, it does not take much more than 1,000 years if you are willing to use technology to accelerate the process if you are already starting with a nitrogen rich atmosphere.

"Does"? That would be "actually", I suppose.

The Earth possesses around 1 quadrillion metric tons of oxygen

Besides some oxidised sediments such as the banded iron formations.

A TL10 civilization could use ~300 TW of fusion generation to convert 1100 cubic kilometers of water into oxygen every year through thermal decomposition.

Paying costs plus compound interest for a thousand years? What are the cost and running cost to 300 TW of generating capacity? 300 TW is about half the total power output of a Kardashev I civilisation. Any GM or setting designer who doesn't want to deal with a setting that has too many planets has only to suppose that they haven't gotten around to that yet.

Have you calculated the effect of 300 TW of heat on the surface temperature of a planet the size of Earth? Hint: Earth receives about 700 TW of solar power, of which it absorbs about 455 TW. Waste heat from your project will raise the temperature by 39K for a thousand years even if generation is 100% efficient.


In any case, the problem that you indicated in your original post is a consequence of your making extravagant assumptions, it's not anything that anyone has to worry about except you.

[David Johnston2]

Originally Posted by Fred Brackin

An A5 has only 1.3 billion years on the main sequence. That would allow for nothing more than _maybe_ the appearance of the first anerobic lifeforms right before the star went red giant on a really Earth-like time scale.

a lo
Ocean worlds have a lot of potential for colonization though even if there's no chance that they'll turn into shirt sleeve environments without a few thousand years of hard work.

[AlexanderHowl]

Originally Posted by Agemegos

"Does"? That would be "actually", I suppose.



Besides some oxidised sediments such as the banded iron formations.



Paying costs plus compound interest for a thousand years? What are the cost and running cost to 300 TW of generating capacity? 300 TW is about half the total power output of a Kardashev I civilisation. Any GM or setting designer who doesn't want to deal with a setting that has too many planets has only to suppose that they haven't gotten around to that yet.

Have you calculated the effect of 300 TW of heat on the surface temperature of a planet the size of Earth? Hint: Earth receives about 700 TW of solar power, of which it absorbs about 455 TW. Waste heat from your project will raise the temperature by 39K for a thousand years even if generation is 100% efficient.


In any case, the problem that you indicated in your original post is a consequence of your making extravagant assumptions, it's not anything that anyone has to worry about except you.

The Earth receives 170 PW of solar energy, so I have no idea where you are getting your numbers from, as 300 TW is less than 0.2% of 170 PW. If you look at thermal decomposition, the process operates at around 90% efficiency because it just heats water to around 2,000 K, and fusion reactors tend to specialize in heating water. In addtion, a K1 civilization needs to consume a minimum of 170 PW of energy, meaning that you could simultaneously terraform 500 planets with the energy required for a K1 civilization.

If you are using 300 TW of D-D fusion, you are producing around 6 kilograms of He-3 per second (one gram per 50 GJ). Even at $100 per gram (the 2015 price was over $20,000 per gram), that would generate over $18 trillion per year in exportable He-3. Since He-3 would likely be used for energy generation everywhere, you would have a great market.