'Super-Earths' in the billions

[Christopher]

Fair enough, I got the mechanism wrong, but aren't larger stars than our own more common? I thought in order to have stars like red and yellow giants with proportionally larger planets like the super-Jovians and super-Earths, it would require the system to form from the remnants (super/hypernovae clouds) of larger first-generation stars, the heavier ones that would have had the ability to fuse elements to the Uranium part of the table?

No, larger stars are less common. As a rule, the smaller a type of object is, the more common it is. Pretty intuitive, really, since you can make a bunch of small things out of the same amount of stuff as one big thing. The vast majority of stars in the galaxy are small red dwarfs, while big stars are much more rare, and also much shorter-lived. Remember, the paper that inspired this thread is about superterrestrial planets around red dwarfs, which is a big deal because red dwarfs are by far the most abundant type of star. So if Earthlike planets are common around them, there could be far more Earthlike planets in the galaxy than we used to think.

And stars don't fuse elements heavier than iron, except during supernovae. Fusing elements lighter than iron is exothermic: you get more energy out of it than you put into it. So that kind of fusion can produce energy and be self-sustaining as a stellar fuel source. But fusing elements heavier than iron is endothermic -- you get less out than you put in. So once fusion in a giant star's core turns it into iron, the fusion stops dead and all the mass of the star collapses in on itself with such force that the star blows up. And that titanic explosion generates enough energy to fuse elements heavier than iron -- as well as distributing all the other elements that were created in the core but mostly trapped within the star during its lifetime.

So the first-generation stars wouldn't have had any elements heavier than iron in them, not until they blew up. Since heavy elements are only introduced into the galaxy by supernovae, the percentage of heavy elements in the universe has increased over time. The oldest star systems are much more metal-poor (and to astrophysicists, "metal" means "anything heavier than helium"). This is one of the ways we determine the age of a star system, by measuring the metallicity in its spectrum. Younger stars and planets form out of material that's been more enriched by heavy elements spewed into space by supernovae.

This is part of why it's surprising to find so many superterrestrials around red dwarfs. They're the longest-lived category of star, so statistically speaking, a lot of them would be very old and metal-poor. We didn't expect to find big planets around them. And we can't yet make any assumptions about how rich those planets are in heavier elements. Many of them might be big but light, made mostly of rock and ice with little metal, which would give them relatively lower gravity.

 

[Asbo Zaprudder]

^In any case, you're not going to get nucleosynthesis in the core of a superterrestrial planet, and certainly not transuranic nucleosynthesis.

The idea was a new one on me, and I'm sceptical, but I wouldn't completely dismiss it out of hand. The nearest thing to a natural nuclear fission reactor was at Oklo in the Gabon. It makes me wonder if a natural fast breeder reactor could also occur in nature. ETA2 - Such a rector would need to use a fluid such as liquid sodium as a coolant, because water is good at slowing down (moderating) the fast neutrons, which are more efficient at converting U238 to Pu239 and less efficient at inducing fission than thermal neutrons. Doesn't seem likely to happen, but then natural fission reactors were a surprise as well.

But that's fission and neutron capture, not fusion. It's pre-existing heavy elements being converted into isotopes of similar mass. It's not the same thing as nucleosynthesis through fusion, not even remotely. Fusion of elements heavier than iron is endothermic -- you have to put more energy into it than you get out of it. So it can't be self-sustaining like a fission reaction. It takes a great deal of energy to fuse heavy elements, especially transuranics which don't occur in nature as far as we've ever discovered. Such things could only be created in supernovae. You can't get transuranic nucleosynthesis in the core of a superterrestrial planet any more than I could crush coal into diamonds with my bare hands.

Yeah, I get that - neutron capture is how elements heavier than iron/nickel are made. It's nucleosynthesis by the r- or s- process but not nucleosynthesis by fusion. I assumed that was maybe what the poster was getting at.

 

[Ronald Held]

Christopher, were you refering to either or both of these papers:
arXiv:1001.4851 [pdf, ps, other]
Title: Mass-radius relationships for exoplanets

arXiv:0707.2895 [pdf, ps, other]

Title: Mass-Radius Relationships for Solid Exoplanets

 

[Crazy Eddie]

Interestingly, I've heard it theorized that there may be more brown dwarfs in the galaxy than there are visible stars; only something like a third of all accretion disks in stellar nurseries would actually collect enough mass to begin fusing hydrogen, with the rest becoming (relatively) cold failed stars that float around and do nothing.

What's interesting to me is that, if brown dwarfs are common formations in the galaxy (I don't see why they wouldn't be) then those cold not-quite-stars could ALSO be orbited by an extensive system of planets, maybe even a few dark super-Earths of their own. Since those systems would emit almost no light of their own, they'd be extremely difficult to detect unless you knew exactly where to look.

 

[Asbo Zaprudder]

I've occasionally wondered if the abundance of stellar and sub-stellar bodies follows a power law with mass, and if so, over what mass range the law applies.

 

[lau03143]

A new estimate by astronomers says
Setting aside the distances involved, which are pretty much insurmountable for the forseeable future, what benefit would something with 10 times the Earth's gravity have for humans?

A cure for Gigantism.

 

[Christopher]

^Actually I think the opposite would be the case. The usual science-fiction conceit is that people raised in low gravity are extra-tall and people raised in high gravity are extra-short; but the fact is that bone growth is stimulated by stress and exertion, so I think that the more work people have to do fighting gravity, the more their bones will grow and the taller (and thicker-limbed) they'll end up being.

 

[Crazy Eddie]

No, they won't actually grow taller, but they will have much higher bone density and probably higher bone thickness.

 

[Christopher]

No, they won't actually grow taller, but they will have much higher bone density and probably higher bone thickness.

How do you know they won't grow taller? Have you been to the future?

 

[Nerys Myk]

No, they won't actually grow taller, but they will have much higher bone density and probably higher bone thickness.

How do you know they won't grow taller? Have you been to the future?

His next door neighbor is named Clark.

 

[Crazy Eddie]

No, they won't actually grow taller, but they will have much higher bone density and probably higher bone thickness.

How do you know they won't grow taller?

Mainly because astronauts who experience significant loss to bone density and thickness do not get shorter.

More to the point, height is one of those physical traits that is almost entirely controlled by genetic factors as opposed to environmental ones, although diet and exercise will also be factors. I do know that both chronic malnutrition AND excessive exercise in childhood can both contribute to stunted growth.

 

[Christopher]

How do you know they won't grow taller?

Mainly because astronauts who experience significant loss to bone density and thickness do not get shorter.

What?? I'm not talking about changes in the adult body. I'm talking about people who are born on high-gravity planets and grow up there. When I said that stress and exertion stimulate bone growth, I thought it was clear from the word "growth" that I was talking about childhood development and maturation. Evidently I should've made that more explicit.

More to the point, height is one of those physical traits that is almost entirely controlled by genetic factors as opposed to environmental ones, although diet and exercise will also be factors.

Well, yes, that's exactly what I'm talking about. Moving around in higher gravity equals more vigorous exercise.

I do know that both chronic malnutrition AND excessive exercise in childhood can both contribute to stunted growth.

Okay, I'll grant I overlooked something. I did a little more digging and I found this:

http://www.sciencedaily.com/releases/2004/10/041006082858.htm

Activities like “serious weight-lifting, however, aren’t recommended for children because overloading growing joints can stunt longitudinal bone growth,” and consequently stunt overall limb growth and height, he adds.

So there might be a slight increase in average height as gravity goes up, but past a certain point, gravity might indeed stunt growth.

Still, I think the standard sci-fi assumption that people raised in low gravity would be freakishly tall is implausible, since they would have less exertion to stimulate growth. Perhaps humans raised in Earth gravity would be near the peak of the height curve.

 

[Crazy Eddie]

Still, I think the standard sci-fi assumption that people raised in low gravity would be freakishly tall is implausible, since they would have less exertion to stimulate growth. Perhaps humans raised in Earth gravity would be near the peak of the height curve.

That's because you're thinking of "Growth" in generic terms that don't apply to what the scientists are talking about.

Bone and muscle tissue grows pretty much constantly, and the rate of growth is usually made to keep pace with the rate of tissue destruction/decay. This is similar to some mammal species (beavers, for example) whose teeth grow continuously throughout their lives; they're constantly chewing on things, so the growth keeps pace with wear and tear. If you removed that wear and tear, their teeth would grow a lot larger than normal. The same is postulated for human children, because -- like beaver's teeth -- childhood/adolescent growth is "mandatory" growth and is triggered by hormones, mostly independent of external environmental factors.

What you're talking about is "replacement" growth, which depends entirely on your metabolism and accelerates or decelerates depending on your average physical activity. Replacement growth doesn't contribute to overall length and size, or else weightlifters and athletes would continue to grow throughout their lives even into adulthood and college athletes would wind up seven or eight feet tall by the time they graduated. Only childhood/adolescent "mandatory" growth does that, and it can only occur in designated areas that can be triggered by hormonal factors. Replacement growth is usually accelerated by physical activity -- the more active you are, the faster your metabolism -- but the older you get, the less the replacement growth is able to keep up with your rate of physical activity and your bones and tissues loose density over time. The slowing of replacement growth does NOT result in older people suddenly shrinking down to the size of toddlers (they do shrink a little, but not by much).

Low-gravity children would be taller by virtue of reduced loads on the growth plates, first and foremost, which means that those tissues can grow much larger than usual and won't need to be replaced before they are fully formed, if ever. The result is that the fresh tissues are longer and thicker, but much less dense since the normal metabolic cues that would have triggered lots of replacement growth are absent. This ceases to be a factor in adulthood, though; the rate of replacement growth remains pretty stable and the relative lack of loads (theoretically) leads to a stable condition that can last through tot he end of their lives (as opposed to, say, a period of relatively high bone and muscle density followed by a lifetime of gradual decay).

 

[Christopher]

^Okay, I guess I was missing some information, then, and I need to reassess my assumptions. Still, I could've done without the lectures about how people wouldn't shrink, because I was never proposing anything that idiotic and I don't appreciate the straw men.

 

[Crazy Eddie]

It's not a strawman. You protested that the low gravity would result in less mechanical stress and would, in turn, stimulate less growth. "Less growth" means "shorter people," so the population would shrink.

The reason that doesn't happen is because replacement growth doesn't contribute to height, and a lack of replacement growth therefore doesn't result in a reduction of height (which is why old people don't shrink).

 

[Christopher]

It's not a strawman. You protested that the low gravity would result in less mechanical stress and would, in turn, stimulate less growth. "Less growth" means "shorter people," so the population would shrink.

No, it doesn't! A smaller increase does not in any way equate to a decrease. A smaller positive number does not equal a negative number -- that should be self-explanatory.

I was speaking ONLY of how I imagined the process of childhood growth to operate, assuming that how much growth would occur would be affected by stress and gravity. Obviously I was wrong about that, and I appreciate the clarification. But I never meant for a second to suggest that any kind of negative growth would take place, because that's COMPLETELY FRAKKING STUPID. I was only postulating different rates of positive growth followed by a stable height upon maturity.

I do take your point about how replacement growth is a separate process from the one I was thinking of. But you could've found a less insulting way of making that point.

 

[Crazy Eddie]

It's not a strawman. You protested that the low gravity would result in less mechanical stress and would, in turn, stimulate less growth. "Less growth" means "shorter people," so the population would shrink.

No, it doesn't! A smaller increase does not in any way equate to a decrease.

I am born on Earth. I grow to be six feet tall.

My son is born on Earth and raised on the moon. He grows to be five feet tall.

My grandson is born and raised on the moon. He grows to be four feet tall.

Granted it would realistically be the opposite progression, but in such a case, proceeding generations of colonists are significantly shorter than their ancestors.

I call that "decrease in height." What do you call it?

I do take your point about how replacement growth is a separate process from the one I was thinking of. But you could've found a less insulting way of making that point.

I'm still unsure of what it is you found insulting about it in the first place.

 

[Christopher]

In post #33, you said:

The slowing of replacement growth does NOT result in older people suddenly shrinking down to the size of toddlers (they do shrink a little, but not by much).

Obviously at that point you weren't talking about different generations of individuals growing to different heights, but about the same individual getting smaller. And that is something I never remotely meant to imply would happen, but it seemed to be what you were accusing me of proposing.

 

[Crazy Eddie]

Mainly using old people as a real-world example of a case where there is "less growth" because of less tissue replacement. The point is, of course, that they will loose tissue in bone and muscle mass but will not loose longitudinal mass because that's a different pattern of growth. So for reasons already explained above, the lack of replacement growth doesn't affect the height of old people for the same reason it doesn't affect the height of people who are born and raised in a low-gravity environment.

Does that make things clearer, or would you prefer to challenge me to ritual combat to restore your honor?

 

[Christopher]

I get what you're trying to say, but you chose a very poor way of expressing it that made it hard for me to understand what you were talking about because it involved lecturing me on something I already knew was nonsense. All your harping on the insanely obvious fact that people don't shrink got in the way of the part of your explanation that was actually useful.