53

I love this one. I really do. Quick Answer: I'd put the date at roughly 2.3 billion years ago, give or take. This is the date of the Great Oxygenation Event. It's when organisms (bacteria) began putting oxygen into the atmosphere in large quantities as a waste product of photosynthesis. The atmosphere before that has a lot of carbon dioxide, which wouldn't ...


52

Earth's rainforests are definitely not the lungs of world. Actually, they consume all (or most of) the oxygen they produce. The phytoplankton (seaweed and microscopical organisms) are the truly world's lung - if we may say there's such a thing - they are responsible for more than 50% of all the oxygen thrown into the atmosphere. So, answering your question....


44

According to this lovely image from NASA (article here), the source of onboard oxygen in current spacecraft is mainly water electrolysis. The hydrogen so produced is processed with carbon dioxide to reclaim some of the water and produce either solid carbon waste, or acetylene for propulsion. This isn't a 100% closed cycle, so you'll have to add more water ...


37

What keeps my planet's water from irreversibly concentrating over time on the frigid wastes while the rest of the planet dries up? When ice piles up, it will exercise pressure. The closer to the terminator, the less the ice. As a consequence, pressure gradient will tend to push the ice sheet toward the terminator, where it will melt, returning water to the ...


37

This scenario is quite problematic for two main reasons: evaporation and peak wavelength. The black hole's lifetime is too short We can make a rough estimate of the properties of the Hawking radiation coming from the black hole. First, let's start with the luminosity. Since $L\propto M^{-2}$, where $L$ is luminosity and $M$ is the mass of the black hole, it ...


35

Our known quantities are: Radius of the body: 50 metres Density of the body: same as Earth's, 5515 kilograms per cubic metre This is enough to calculate the acceleration due to gravity on the surface of the body. We multiply the density $\rho$ and the volume $V$ to get the mass, multiply it by Newton's universal gravitational constant $G$, and divide by ...


33

We have already found exo-planets matching this criteria. For example HD_100777_b has a mass just slightly higher than Jupiter and orbits its star at the same distance from the sun that our earth does. (The star is a similar size to our sun but I didn't check the brightness so I don't know for sure if it's in the habitable zone). You can explore the known ...


26

Your environment is quite similar to that in a globular cluster. At its densest, a globular cluster may see peak stellar number densities of $\sim1000$ stars per cubic parsec, which implies a mean separation of about 20,000 AU. This leads us to conclude that many, if not most, planets will be stripped away through encounters with other stars, leading to a ...


25

Let's work out some factors. Luminosity You gave the radius of the inner edge of the habitable zone as 1.976 AU and the outer edge as 2.808 AU. From this, we can calculate the luminosity of the star. There's an explanation of how to do this on Planetary Biology. The formulae are $$r_i=\sqrt{\frac{L_{\text{star}}}{1.1}}$$ $$r_o=\sqrt{\frac{L_{\text{star}}}{...


24

Yes. You can take binary or trinary star systems and swap one of the stars for a black hole and nothing changes in the orbital dynamics. Depending on the layout of the solar system planets can orbit the stars, the black hole, or some mixture of the above. Some of those planets could be in the habitable zone (liquid water). And some of those planets could ...


23

I wouldn't expect long term habitability. A planet's magnetic field provides much of its shielding against stellar and cosmic radiation -- but the magnetic field is created by a "dynamo" action of circulatory motion in the liquid iron core of a planet. Earth's moon once had a magnetic field, but lost it when its core solidified in the billions of years ...


21

In both cases the shear volume of necessary oxygen (and other elements used in human breathable air) would make it very difficult or impossible to build such a habitat, no? No, actually; I don't think so. The accepted atmospheric composition of Earth is 78% nitrogen, 21% oxygen, 0.9% argon, and 0.05% everything else, including carbon dioxide (about 0....


21

Librations. That is, the tidally locked planet is not in a perfectly circular orbit, and so the portion of the planet that is sun-facing is not constant. This is because the rate of rotation is (extremely nearly) constant, but the rate of revolution around the sun changes due to the non-circular nature. For the Earth's Moon, this is only a few degrees. If ...


20

Yes, it is plausible as the timescales of which a red dwarf is a blue dwarf are quite big, to the point where if your icy planet is distant enough it will thaw and potentially develop life. What matters is the placement of your planet and its size, as if your icy planet is too small it won't retain a atmosphere and if it's too far the temperature increase ...


20

From Hawking radiation? No. The Hawking radiation emitted is inversely proportional to the black hole's size. To make the black hole glow with enough light to be as bright as a star from Hawking radiation alone, it would need to be very small. The problem with very small black holes is they also have very short lifetimes due to the Hawking radiation ...


20

"It's the smell!" -- Smith I think the easiest answer is that there is something in Earth's biology that disagrees with them... something that can be "managed" with continuous medical intervention, but would be really annoying for them to try to settle permanently without completely terraforming the planet. Imagine, for example, that 99% of the aliens ...


19

It's possible, but heat generated by the Kelvin-Hemlholz mechanism may be too variable to complex life to develop solely as a result of this source of heat. This paper suggests that the temperature of Jupiter, when it first finished an initial phase of contraction, was quite high, at around 25000K. At this temperature, it would have a small habitable zone ...


18

The aliens are humans. The aliens did do the calculations and they did find a habitable planet, 100,000 years ago. They are depleting their homeworld - here. On arriving on Earth and settling the colonists, the mothership immediately started working on finding the next world. These calculations take a very long time. Almost none of the humans on ...


17

Probably about 5-10 years minimum Fallout would not be a major long-term problem, the timescale on which radiation due to fallout would present a serious danger would be less than 5 years. See this article which says: Radioactive material which takes longer than 24 hours to return to earth is called delayed or global fallout. Some of the delayed fallout ...


17

First matter first: to have a body in a spherical shape, you need to exceed a certain radius, dictated by the material. Most likely with 50 meters you will have a potato shaped object. Moreover, to have a decent gravity you need more mass. Just as a reference, Ceres has a radius of 473 km, a mass of 0.00015 Earth masses and a surface gravity of 0.029 G. ...


17

You're basically talking about Venus. Or, more accurately, Venus if it had started out with a lot less water and CO2. Less water and CO2 to start with mean you never get the runaway greenhouse effect Venus has, leaving you a planet that's a lot like Earth, just dryer and hotter. Any rainfall you DID get would be the higher latitudes and that's where you ...


17

Some simple molecules can exist in stars - but not the right ones. Contrary to popular belief, many different types of molecules can exist in stellar atmospheres, especially cool stars like red dwarfs. Some are even helpful spectral diagnostics; TiO bands are very common in stars cooler than 4000 K, which actually encompasses most of the stars in the galaxy,...


16

Humans require an oxygen atmosphere to breathe, and require multicellular life to eat. They also require temperatures roughly similar to those found today. It has been shown through geologic methods that the oxygenation of the atmosphere occurred during the Precambrian era, reaching levels possibly high enough to support human life around 1.9 billion years ...


16

Problem 1: The supernova The first concern I have is one that Zeiss Ikon's answer discusses. To form a black hole, you need some sort of energetic event, likely a supernova. However, a supernova releases three extremely problematic sources of energy: High-energy photons, like gamma rays, that have the potential to strip away the atmosphere of any pre-...


15

Age: The time a star spends on the main sequence is roughly inversely proportional to the luminosity, as given by the formula $$T \approx \ 10^{10} \text{years} \cdot \left[ \frac{M}{M_{\bigodot}} \right] \cdot \left[ \frac{L_{\bigodot}}{L} \right] =10^{10} \text{ years} \times \left[\frac{M}{M_{\odot}} \right]^{-2.5}$$ where $M$ and $L$ are the mass and ...


15

Rogue planets can be kept warm. The key points can be found in the Wikipedia entry on rogue planets. Interstellar planets generate little heat nor are they heated by a star. In 1998, David J. Stevenson theorized that some planet-sized objects adrift in the vast expanses of cold interstellar space could possibly sustain a thick atmosphere that would ...


15

The closest feature we have on our planet that gets close to appear like this is the Giant's Causeway. To obtain this you need to have magma intruding a rock, then cooling down to form the pillars, and the surrounding rocks being eroded away. But you want it made on scales like El Capitan: With the right combination of factor (gravity, volcanic activity) ...


15

You can't, unfortunately. Without surface water you won't have a sustainable hydrologic cycle to keep life flourishing in the variety and density of a tropical jungle. Water will disappear into the crust beyond the reach of any root system (the Earth's crust is estimated to contain as much water as all the oceans, if not significantly more) in greater ...


14

Size and Gravity Here's a handy equation if you want to find the radius of a planet with the same surface gravity as Earth (or near it), but with a different density. If you want to change the size but keep the same gravity, you need to mess with the density. $$ r = {{g}\over{{{4\pi}\over{3}} * G * \rho}} $$ Where $ r $ is the radius in meters, g is Earth ...


14

Nitrogen (N₂): 60.4% — 78.08% = −18.48% N₂ (↓22%) Is used in the atmosphere to reduce the percentage of oxygen in the air (if O₂ is too much our atmosphere may burn out). It haven't any very important effect in life. N₂ is a inert gas, it can't burn and don't make any special reaction in the air. However bacteria breath N₂ and make amino acids > then ...


Only top voted, non community-wiki answers of a minimum length are eligible