Wikipedia:Reference desk/Archives/Science/2017 November 8

= November 8 =

Earth-temperature brown dwarf
How massive would a brown dwarf have to be to have Earth surface temperature? Also what would his radius be then? Would this object be dense enough for a spacecraft to land or float on it (maybe with the aid of a balloon)? I'm thinking of something like WISE 0855-0714 but warmer. 93.136.44.140 (talk) 02:04, 8 November 2017 (UTC)
 * Temperature is a bit independent of size, and the older it is, the cooler it will be. Also at what point do you measure the temperature? If it is all gas the temperature may appear to radiate in the infrared at 300 K, but deep down it would be hotter. See Y-type star and Sub-brown dwarf for what we have. Graeme Bartlett (talk) 03:56, 8 November 2017 (UTC)
 * Well the core creates some heat (esp. if you need to fuse deuterium to get >300 K) so the temperature should over time level off asymptotically towards some limit, so let's say the object is free-floating, pretty old and at/near that temperature, and it's ~300 K. I suppose I'd want that temperature to be at the cloud layer at 1 bar pressure, but I'll settle for blackbody temperature if we don't know enough about sub-brown dwarfs to estimate in that detail. Can we infer mass & radius from this or will they depend heavily on metallicity and such? I'm wondering about this because I was intrigued by the fact that all brown dwarfs down to planets of Jupiter's mass have roughly the same radius due to matter degeneration. 93.136.44.140 (talk) 04:38, 8 November 2017 (UTC)
 * Easy: gas giants don't have surfaces!. μηδείς (talk) 16:48, 8 November 2017 (UTC)
 * Let's call the surface the place where atmospheric pressure is 1 bar, as in Jupiter. 93.139.45.186 (talk) 17:35, 8 November 2017 (UTC) (original poster)
 * As the first reply says they get cooler with time as they don't do any fusion, or at least only the larger ones do and that stops after a while. The heat they generate is from gravity as they formed and it dissipates with time. There's not a great deal of difference between them and a gas giant like Jupiter except mass - they don't do anything too much different. They are not stars. To float you need something that is less dense than what it is floating in, the atmosphere can contain lots more chemicals than the sun but is still mostly hydrogen and helium so unless you can have a strong ball containing a lower pressure than outside like a submarine but much much lighter that would be pretty much impossible except with a large hydrogen balloon. Dmcq (talk) 17:59, 8 November 2017 (UTC)
 * Ok, I see I had some wrong assumptions. How about this: suppose there's a planet 5 billion years old and of Jupiter's composition, which evolved like Jupiter up to, say, 1 billion years after its "birth", when it was ejected by deus ex machina from its star system and ended up floating in interstellar space. Aliens descend into its atmosphere and measure that at the altitude where pressure is 1 bar, the temperature is 300 K. Is this enough data to estimate the planet's mass and radius, or do brown dwarfs vary too much in characteristics, or do we simply not know enough about this yet? 93.139.45.186 (talk) 19:51, 8 November 2017 (UTC)
 * Aliens in orbit around this body could estimate the mass directly from the gravitational force on their ship, and the radius by direct observation of occultations. The temperature and pressure would be irrelevant to such observations. μηδείς (talk) 22:43, 8 November 2017 (UTC)
 * Yeah sure, but can we? Is there a model that describes the relationship between temperature, radius, etc.? I'm interested in how to calculate that stuff. 93.139.45.186 (talk) 23:00, 8 November 2017 (UTC)
 * The speed of a body in orbit about a primary is independent of the orbiting body's mass, except that when we can calculate a barycenter of a system we can calculate the relative masses. If your brown dwarf has satellites of known distance and orbital velocity, then its mass could be determined.  But if all we have are distant images, then we are making guesses based on libido and presumed composition.  But a comparison of Saturn, large, cold and less dense than water with Jupiter, not too much larger, but hotter and denser, shows that there are many presumptions made when we don't have the full body of information needed, and are going on guesses due to brightness of the size and albedo.  You can check the history of observations of Ceres and Pluto to see how difficult accurate measurements are without the needed observations.  Of course your aliens would have those observation once they were in orbit, as they would know their own mass and orbital velocity and the actual albedo and specifically the radius of your brown dwarf. μηδείς (talk) 02:32, 9 November 2017 (UTC)
 * Ah yes, that makes perfect sense. I completely forgot how different Saturn is from Jupiter. I was hoping that one could make some nice relation like M-sigma or Stefan-Boltzmann law, given that there is a real and not very high limit on planet radius at low temperatures, but yes there would be too much variety in initial conditions to estimate anything. Thanks for the help folks. 93.139.45.186 (talk) 05:26, 9 November 2017 (UTC)


 * To be clear, let's remember that the atmosphere of Saturn and the atmosphere of Jupiter each contain layers of water vapor clouds. Jupiter's is at a depth of only 3 atm (300,000 kPa) -- see .  Saturn's is about ten times denser but still plausibly human-breathable, if supplemented with a trace of oxygen and if the humans are modified to survive industrial grade stink bomb and strong alkaline pH.  On Saturn the main issue is keeping afloat, though the winds suggest the presence of extractable energy that can be used by sufficiently clever gliding organisms.  With Jupiter the main issue is that 3g gravity, rather than normal Earthlike gravity on Saturn.  The problem for colonizing a brown dwarf is that I would assume that the heat generated from within scales at least linearly with the mass, probably more than that, and of course so does the gravity.  On the other hand it may not have a sun, which makes it ever so slightly cooler.  It seems like a tall order, though this talk is no substitute for a measurement (after all, the internal circulation of heat is also a factor, and I won't even guess at that!) Wnt (talk) 15:04, 11 November 2017 (UTC)
 * Your point being that if humans weren't humans, but levitating hydrothermal vent bacteria, they could live in levitating hydrothermal vent conditions? μηδείς (talk) 21:46, 11 November 2017 (UTC)
 * Well, flamingoes survive severely alkaline conditions somehow, so I don't think the evolution has to go quite that far afield. Hopefully the levitation part can be provided by organisms/ecosystems native to Saturn already... Wnt (talk) 02:33, 12 November 2017 (UTC)
 * Of course, but these would be levitating bacteria, not humans who were levitating bacteria. Raymond Luxury Yacht (Throat-warbler Mangrove) 21:18, 12 November 2017 (UTC)

Name of fallacy: mixing up unrelated variables?
How would you call it when you lump together not quite analogous variables (for example, unemployed astrologers and unemployed astronomers) and calculate the average (or whatever)?--B8-tome (talk) 13:46, 8 November 2017 (UTC)


 * To mix apples and oranges? By the way you might like James–Stein estimator where they mix up the speed of light, tea consumption in Taiwan, and hog weight in Montana all together ;-) Dmcq (talk) 14:39, 8 November 2017 (UTC)


 * The corresponding article, Apples and oranges, has some further information under the 'See also' section. --Hofhof (talk) 20:36, 8 November 2017 (UTC)


 * Except there's no prohibition against combining apples and oranges or categorizing them together, only comparing them as if they were interchangeable. Comparing unemployed astrologers to unemployed astronomers could be a useful metric to figure out how gullible a society is. However, adding them together because you think they're similar fields - as in the OP's scenario - would be an error. I have a feeling there is a term from statistics that describes this situation, but I can't seem to put my finger on it. Broadly, it's an error in classification. Matt Deres (talk) 13:46, 9 November 2017 (UTC)


 * It isn't a fallacy, but in statistics this sort of situation is known as heteroscedasticity. Looie496 (talk) 01:12, 9 November 2017 (UTC)
 * Yes, and along the lines that Matt points out, it's not even always problematic. Certainly this is not a formal fallacy. Formal logical fallacies are about using logic faultily, not about making bad decisions. If anything, this may (sometimes) be classed as a informal fallacy, because the problem is with the assumptions, not the logic of any statement or argument. SemanticMantis (talk) 16:53, 9 November 2017 (UTC)

Industrial production of methane
What makes production of methane in an industrial scale so difficult? Can't you just use any biomass mixed with the proper bacteria to obtain it? Wouldn't the CO2 balance of burning this methane be neutral? --Hofhof (talk) 20:38, 8 November 2017 (UTC)
 * But methane is produced on an industrial scale in landfills by bacteria from any biomass. So, it is not very difficult. See landfill gas. Ruslik_ Zero 20:45, 8 November 2017 (UTC)
 * That's not industrial production, that's just production. It only counts as an industry if you catch it and use it. Our article on that is at Landfill_gas_utilization. A perhaps more relevant article on general biomass production of methane is at Biogas. SemanticMantis (talk) 22:07, 8 November 2017 (UTC)


 * The question of carbon neutrality depends on the time scale in question. If for example, all the biomass was produced by plants and animals that were grown in a given year, then yes, the burning of methane produced by them would only release CO2 in to the atmosphere that had already been in the atmosphere in the beginning of the year. However, you also have to account for all the energy inputs to the hypothetical biogas plant (does is use any electricity, do people drive there to work, etc). Not to mention the carbon footprint of the biomass in question (E.g. corn has a massive carbon footprint, due to all electricity (and thus usually fossil fuels) needed to make the fertilizer for it (Haber process). All these considerations are part of why Life-cycle_assessment of energy and food production is very difficult to do properly. SemanticMantis (talk) 22:07, 8 November 2017 (UTC)


 * One issue with methane production is in producing only methane. Contaminants with sulphur in them are both smelly and also corrosive when burned. Andy Dingley (talk) 22:11, 8 November 2017 (UTC)