Number of habitable worlds within170 light years?

[malloyd]

Originally Posted by Agemegos

What, because of the lack of oxygen and toxic concentration of CO₂ in the atmosphere?

Little thing like that!

That's a lot of the hard part of answering the question too. There are very likely many thousands of worlds in that radius with reasonable temperatures, gravities, radiation fluxes, and volatiles inventories, but if your requirement for habitable is requires breathable air, the answer could easily be zero.
Breathable air only exists because of rather complicated biological processes, and we really have no good constraints even on how many worlds have life, let alone life that happens to have exactly this or a sufficiently similar biochemistry to dump something as weird as free oxygen into the air.

[cupbearer]

Originally Posted by lugaid

At a guess, travel at 1pc per week would take almost exactly 52 weeks to travel from the center to the rim. 170ly / 3.26ly/pc = ~52.15pc.

You guessed exactly right! Lol

[cupbearer]

Originally Posted by malloyd

That's a lot of the hard part of answering the question too. There are very likely many thousands of worlds in that radius with reasonable temperatures, gravities, radiation fluxes, and volatiles inventories, but if your requirement for habitable is requires breathable air, the answer could easily be zero.
Breathable air only exists because of rather complicated biological processes, and we really have no good constraints even on how many worlds have life, let alone life that happens to have exactly this or a sufficiently similar biochemistry to dump something as weird as free oxygen into the air.

Yeah, but i gotta settle on something, so asking the GURPites what they think because I know people will have opinions !

[cupbearer]

Originally Posted by SydneyFreedberg

Fascinating! Why 170 ly? Can we scale up these numbers to larger regions? What's the percentage of stars in the galaxy that would have habitable planets, using these same assumptions? (I'm presuming the core might have a lower percentage of habitable systems...).

The answer is what the comment right below your post says. Im playing with the idea of a setting you can travel many months across and started with an idea about base speed

[David Johnston2]

More than a thousand if you set the bar low and just settle for liquid water. Actual breathable air without many centuries of terraforming? I suspect the number would be more like a hundred.

[ericthered]

So life shows up on earth pretty quickly, which gives hope for finding life most times you get what space calls an "Ocean world". I don't think that the oxygen energy storage loop is particularly unlikely: oxygen is the third or fourth most common element in the universe, and its energy storage properties are amazing. It did take 2 billion years (roughly) to show up on earth though, and that could be slow, or it could fast. Sample sizes of one are tricky. The roughly breathable atmosphere of earth seems to show up at the same time as multicellular life, or conversely that multicellular life shows up once all of the oxygen sinks on earth are used up and the oxygen level spikes.



So there is a fair argument that "water planets" are slow pressure cookers that eventually give multicellular life. That is in fact what we would assume, if not for Fermi and his blasted paradox. Which makes everyone go back and furiously try to make the numbers lower.

[Anthony]

Originally Posted by ericthered

So life shows up on earth pretty quickly, which gives hope for finding life most times you get what space calls an "Ocean world". I don't think that the oxygen energy storage loop is particularly unlikely: oxygen is the third or fourth most common element in the universe, and its energy storage properties are amazing. It did take 2 billion years (roughly) to show up on earth though, and that could be slow, or it could fast.

Oxygen showed up on Earth as a way for cyanobacteria to kill the competition, not as an energy store, and after it showed up it took another 2 billion years for there to actually be enough of it for higher life forms to breathe (there are a lot of minerals that will happily pull oxygen right out of the atmosphere, so until they all get oxidized you don't wind up with much atmospheric oxygen). That timing is probably less variable than the timing on generating oxygen producing microbes.

[Pursuivant]

Originally Posted by ericthered

So there is a fair argument that "water planets" are slow pressure cookers that eventually give multicellular life. That is in fact what we would assume, if not for Fermi and his blasted paradox. Which makes everyone go back and furiously try to make the numbers lower.

IMO, something that Fermi didn't consider was the unique nature of Earth's formation - a massive interplanetary collision which not only provided a huge "nurse moon" but also gave the planet an extreme axial tilt.

While the Moon doesn't do that much to protect the Earth from asteroids, it might play a big role in helping to maintain the Earth's magnetic field, by keeping the earth's core molten, which is critical in preventing deadly cosmic rays from reaching the planet.

From a biological point of view, tides and seasons aren't quite as important, but their constant action might serve to literally stir the pot by moving water (hence nutrients) around. Vulcanism also plays a role by constantly bringing denser elements to the planet's surface.

If planets with molten rock/metal cores and extensive vulcanism are rare, that means that planets with habitable surfaces are rare (due to constant bombardment by cosmic rays), limiting life, and sapient life, to just planetary oceans.

Decades of wishful thinking to the contrary, while ocean-dwelling life might develop sapience it's unlikely to develop extensive tool use. (Consider that there species of cetaceans and octopi which are very smart, and sometimes demonstrate tool-using behaviors, but haven't shown much inclination towards tool making.)

If the surface of most planets is hostile to terrestrial life, aquatic species would have no inclination to spend much time on land, much less evolve to live there.

Add three more layers to Drake's Equation and Fermi's Paradox might not be so paradoxical:

"x chance that a planet has the right geophysical makeup to potentially block cosmic rays."

"x chance of a lucky planetary collision which doesn't destroy the planet, but which does provide an Earth-like moon to "keep the pot boiling."" (Alternately, the presence of other gravitational forces which generate vulcanism.)

"x chance that terrestrial life evolves."

[malloyd]

Originally Posted by cupbearer

Yeah, but i gotta settle on something, so asking the GURPites what they think because I know people will have opinions !

The thing is with no particular evidence leaning in any particular direction, you can pick whatever you want for the game.

Want a space opera like universe with a thousand shirtsleeve planets - life is common and the oxygen cycle is universal.

Want a setting where there are thousands of planets where you could set up habitation domes with some effort but the two known oxygen worlds are prizes of immense value worth fighting over, life is rare but the oxygen cycle is normal.

Want a setting where there are thousands of worlds with alien biospheres but none of the have breathable air and the dozen different alien species in campaign flee for their lives if the Terran's airsuits spring a leak and start spewing horrible war grade chemical weapons into the atmosphere. You can do that too.

[SydneyFreedberg]

What Malloyd said. Also, my personal guess for the Fermi Paradox solution is that, even if all life-bearing worlds evolve intelligence, they're very unlikely to evolve it at the same time. Modern humans have been around for, what, 100,000 years, at most, and transmitting radio waves for 100; even if you round that way up to a 1 million year lifespan for our species (before we go extinct/transcend/whatever), that's 1/500th of the time since the Cambrian Explosion (ca. 500 mya). So figure a planet with macroscopic (i.e. big enough to see) multi-cellular life has a 1/500 or 0.2% chance of having intelligent life on it right now.