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[Solifuge]

The thing is, Super-Earth is matched with another term: Mini-Neptune.

Somewhere between these two values a planet starts to resemble a gas giant more than a terrestrial planet. Where? Nobody knows. There may not even be a clear demarcation, though it seems silly to say that just ramping up the atmosphere forever would take you from something like Earth, to something like Jupiter, to eventually something like the Sun. After all, the Sun has no solid terrestrial core (probably), so can we be sure a gas giant does too?

If things in the universe are super crazy, it's not beyond the realm of possibility that terrestrial planets become the smallest gas giants with only, say, 1.5 Earth masses. With what we know that's unlikely, but anywhere between 1 and 30 Earth masses could be the limit, and it can't be fully ruled out.

The secret to Gas Giant Formation is gathering and retaining ultralight Helium and Hydrogen gases. Whether a planet will become a Gas Giant or not can be calculated based on Temperature and Mass; if a planet has enough entropy from heating, but not enough gravity from mass to counter it, Hydrogen and Helium start to move faster than the planet's Escape Velocity, bubble up to the edge of the atmosphere, and get skimmed off into deep space by gravitational interactions with other bodies.

Even Earth, if it had been cool enough, could potentially have become a Gas Planet; once you start collecting Hydrogen and Helium, mass increases, and the positive feedback loop continues until all the available gas is gathered. On the flip-side, if you took a Neptune-like world, and brought it in far enough that it's entropy exceeded it's gravity, you'd eventually have the Hydrogen and Helium drift away (lowering atmospheric pressure and gravity), the hypothetical liquid hydrogen core would boil into more atmosphere that would float away (further decreasing mass/gravity), and the temperature would start to drop off as the heat-retaining atmosphere thinned. The heavy gasses like oxygen, ammonia, or water would stay behind, and depending on the temperature you'd probably be left with a low-metal rocky iceball that resembled Pluto or Triton, a world that's mostly fluid oceans of water or azanes, or a world with a thin atmosphere with a similar composition. Though I don't think any planets like this have been positively identified yet, they call them "Cthonian Planets".

[MetalSlimeHunt]

Though I don't think any planets like this have been positively identified yet, they call them "Cthonian Planets".

There's been some hypothesizing that the illusive Cthonian Planet might have been right next door all along, as Mercury.

[Putnam]

You can't use lenses and mirrors to make something hotter than the surface of the light source itself. In other words, you can't use sunlight to make something hotter than the surface of the Sun.

I'm suddenly reminded of someone being incredulous about this here and I can't think of any reason it's false. I want to remember why anyone was incredulous.

[MetalSlimeHunt]

That's not true, mirrors slightly reverse the flow of entropy due to being unnerving. A sufficient creepy mirror can power anything.

[Criptfeind]

You can't use lenses and mirrors to make something hotter than the surface of the light source itself. In other words, you can't use sunlight to make something hotter than the surface of the Sun.

I'm suddenly reminded of someone being incredulous about this here and I can't think of any reason it's false. I want to remember why anyone was incredulous.

Because you can charge a laser with solar power and then use it to create higher heats.

[Putnam]

But that's not lenses and mirrors and it isn't using sunlight (directly)? It's specifically talking about optics and makes it very clear that it's about optics, solar power is completely ignoring the context. In other words, power storage is not mentioned at all.

Also, lasers are weird. I mean, they can be at negative temperatures, even.

[nogoodnames]

You can't use lenses and mirrors to make something hotter than the surface of the light source itself. In other words, you can't use sunlight to make something hotter than the surface of the Sun.

I'm suddenly reminded of someone being incredulous about this here and I can't think of any reason it's false. I want to remember why anyone was incredulous.

That statement is true for black-body radiation. The problem is that he then treats sunlight reflected off the Moon as black-body radiation generated by the Moon and never explains why.

IIRC, at one point in the post he says he'll come back and explain the issue, but never does. Logically it doesn't seem to make sense. Mirrors don't magically cool any light reflected off them to their surface temperature.

[Max™]

More to the point, he explains how a lens can be seen as a way of making a given light source seem to cover more area, and with enough screwing around you can get up to the point where it seems like you're surrounded by the light source, at which point you'll equilibrate at the temperature of said light source. The moon isn't heated to 6000 K by sunlight, being surrounded by fully lit lunar surface at around 390 K will end up with you peaking out at the same temperature as the fully lit lunar surface: ~390 K.

You can do screwy shit with non-focusing optics, but that's full on wizardry and out of my league.

The thing is, Super-Earth is matched with another term: Mini-Neptune.

Somewhere between these two values a planet starts to resemble a gas giant more than a terrestrial planet. Where? Nobody knows. There may not even be a clear demarcation, though it seems silly to say that just ramping up the atmosphere forever would take you from something like Earth, to something like Jupiter, to eventually something like the Sun. After all, the Sun has no solid terrestrial core (probably), so can we be sure a gas giant does too?

If things in the universe are super crazy, it's not beyond the realm of possibility that terrestrial planets become the smallest gas giants with only, say, 1.5 Earth masses. With what we know that's unlikely, but anywhere between 1 and 30 Earth masses could be the limit, and it can't be fully ruled out.

The secret to Gas Giant Formation is gathering and retaining ultralight Helium and Hydrogen gases. Whether a planet will become a Gas Giant or not can be calculated based on Temperature and Mass; if a planet has enough entropy from heating, but not enough gravity from mass to counter it, Hydrogen and Helium start to move faster than the planet's Escape Velocity, bubble up to the edge of the atmosphere, and get skimmed off into deep space by gravitational interactions with other bodies.

Even Earth, if it had been cool enough, could potentially have become a Gas Planet; once you start collecting Hydrogen and Helium, mass increases, and the positive feedback loop continues until all the available gas is gathered. On the flip-side, if you took a Neptune-like world, and brought it in far enough that it's entropy exceeded it's gravity, you'd eventually have the Hydrogen and Helium drift away (lowering atmospheric pressure and gravity), the hypothetical liquid hydrogen core would boil into more atmosphere that would float away (further decreasing mass/gravity), and the temperature would start to drop off as the heat-retaining atmosphere thinned. The heavy gasses like oxygen, ammonia, or water would stay behind, and depending on the temperature you'd probably be left with a low-metal rocky iceball that resembled Pluto or Triton, a world that's mostly fluid oceans of water or azanes, or a world with a thin atmosphere with a similar composition. Though I don't think any planets like this have been positively identified yet, they call them "Cthonian Planets".

I just checked as I recalled hearing about pebble gathering models showing a lot of promise at getting the initial cores scaled up to where they suck down the bulk of the hydrogen and helium, in part by knocking smaller planetesimals out of the sweet spot which wound up getting the troublesome issue of gas giants forming in a reasonable time cut down by hundreds or even a thousand fold?

This stands out as rather interesting, but I'm not super up to date on the field: using the different halting threshold for pebble accretion to separate gas giant and ice giant populations properly.

[Sergarr]

More to the point, he explains how a lens can be seen as a way of making a given light source seem to cover more area, and with enough screwing around you can get up to the point where it seems like you're surrounded by the light source, at which point you'll equilibrate at the temperature of said light source. The moon isn't heated to 6000 K by sunlight, being surrounded by fully lit lunar surface at around 390 K will end up with you peaking out at the same temperature as the fully lit lunar surface: ~390 K.

Nooooooo. If you're surrounded by a fully lit lunar surface, you're effectively surrounded by sunlight, since moon acts as a reflective mirror for sunlight, and thus your peak temperature will be the same as the Sun.

His argument was incredibly, totally, absolutely wrong. Even by its own standards, if you assume that lens make a light source just cover more area, it only works if you suddenly forget about Sun's existence.

[Gentlefish]

...Wasn't the question asking if you could start a fire using moonlight? The answer, regardless, is no. Focusing the lunar light wouldn't give you the temperature necessary to start a fire.

[x2yzh9]

So what if the only way to build a truly self-aware AI is to program sufficient algorithims to allow it to model and observe personality types and create individual cybernetic personas which would have to be selected and then reprogrammed into it, giving it a capacity to evolve.

[nogoodnames]

More to the point, he explains how a lens can be seen as a way of making a given light source seem to cover more area, and with enough screwing around you can get up to the point where it seems like you're surrounded by the light source, at which point you'll equilibrate at the temperature of said light source. The moon isn't heated to 6000 K by sunlight, being surrounded by fully lit lunar surface at around 390 K will end up with you peaking out at the same temperature as the fully lit lunar surface: ~390 K.

Nooooooo. If you're surrounded by a fully lit lunar surface, you're effectively surrounded by sunlight, since moon acts as a reflective mirror for sunlight, and thus your peak temperature will be the same as the Sun.

His argument was incredibly, totally, absolutely wrong. Even by its own standards, if you assume that lens make a light source just cover more area, it only works if you suddenly forget about Sun's existence.

That's assuming that the moon is a 100% reflective mirror, which it obviously isn't.

That's not the point. Okay, so the Moon probably can't be used to start fires because it has low reflectivitey and is spherical so only a tiny fraction of reflected light is making it to Earth. But the argument that it can't possibly heat anything more than its surface temperature because black-body radiation makes no sense at all.

First of all, the surface of the Moon is far from a closed system. A lot of heat is getting conducted away and radiated out on the night side. If this were not the case then its peak surface temperature would in fact be the temperature of the Sun as Sergarr said.

Okay, now imagine that someone covers the Moon in mirrors. They aren't perfect, maybe reflecting 90% of whatever light hits them, but the Moon's albedo gets a lot higher. Obviously nights get a lot brighter because of the mirrors, so more energy must be reaching the Earth. Consequently, magnifying glasses can make things hotter with moonlight than they could before. So by the article's logic, the surface of the Moon must now be getting hotter to produce these higher temperatures. Except clearly it isn't. If anything, it's getting colder because more energy is reflected away. Do you get what I'm saying?

Normally I like the What If articles, but that one is just poorly explained and flat out wrong in a number of ways.

[Reelya]

More to the point, he explains how a lens can be seen as a way of making a given light source seem to cover more area, and with enough screwing around you can get up to the point where it seems like you're surrounded by the light source, at which point you'll equilibrate at the temperature of said light source. The moon isn't heated to 6000 K by sunlight, being surrounded by fully lit lunar surface at around 390 K will end up with you peaking out at the same temperature as the fully lit lunar surface: ~390 K.

Nooooooo. If you're surrounded by a fully lit lunar surface, you're effectively surrounded by sunlight, since moon acts as a reflective mirror for sunlight, and thus your peak temperature will be the same as the Sun.

Electromagnetic energy density falls off as distance squared. Even if the lunar surface is 100% reflective, the energy density is attenuated because of the dilution of light between the sun and the moon.

The problem is that the "fully lit lunar surface" is only getting light from a small patch of the sky - the sun. Unless the moon itself was blasted by full-power sunlight from all directions, you're not going to get sun-level light from being surrounded by moonlight.

[Starver]

And if you cover the Moon with mirrors, then you don't get a full-moon's worth of 100% solar light.

If its a uniform spherical mirror, you get a single spot of reflected sunlight, visibly (if blindingly) smaller than the sun it reflects as it creates a diverging 'image'. (Although a mega-sized refocussing magnifying glass or secondary mirror could capture that image and refocus it.)

If it's multiple steerable mirrors, or shaped/positioned mirrors so as to be a better solar reflector to your location (but not just one big moon-sized solar-reflector dedicated to concentrating a Moon's circumference of solar light down onto your location), it'll be a mirror-ball, loads of little pinpricks.

The Moon takes light, may reflect a small amount 'perfectly' perhaps from suitably crystaline metals or perhaps glassy intrusions, but in all directions¹ (hence how you see all lit limbs of the moon) and similarly also takes absorbs and emits as per its particular black-body signature, at the lower energy density/temperature/whatavyer. One insufficient to be collumated and refocussed onto a spot of high-enough temperature.

Or so is my understanding.

¹ Peaking around directly back at the original light-source, due to an interesting optical effect that you can look up, if you're interested.

[Max™]

More to the point, he explains how a lens can be seen as a way of making a given light source seem to cover more area, and with enough screwing around you can get up to the point where it seems like you're surrounded by the light source, at which point you'll equilibrate at the temperature of said light source. The moon isn't heated to 6000 K by sunlight, being surrounded by fully lit lunar surface at around 390 K will end up with you peaking out at the same temperature as the fully lit lunar surface: ~390 K.

Nooooooo. If you're surrounded by a fully lit lunar surface, you're effectively surrounded by sunlight, since moon acts as a reflective mirror for sunlight, and thus your peak temperature will be the same as the Sun.

Electromagnetic energy density falls off as distance squared. Even if the lunar surface is 100% reflective, the energy density is attenuated because of the dilution of light between the sun and the moon.

The problem is that the "fully lit lunar surface" is only getting light from a small patch of the sky - the sun. Unless the moon itself was blasted by full-power sunlight from all directions, you're not going to get sun-level light from being surrounded by moonlight.

Ding, we have a winnar!

You are getting blasted by full power sunlight from 149 Gigameters away, as is everything in a sphere at that distance from the sun, at this distance at the top of the atmosphere we're looking at around 1366 W/m^2 or so.

Wanna see a neat way to check if that makes any sense?

http://www.spectralcalc.com/blackbody_calculator/blackbody.php


A black body at ~390 K will be at around 1311 W/m^2 emitted, which tells you that a body receiving that much insolation isn't going to be much warmer than that, barring (heh) pressurized atmospheres and so forth.