54 votes
Accepted

How early was the Earth liveable for today's humans?

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 ...
  • 97.8k
52 votes
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Fantasy map that has minimal land at the equator. What does this mean for rainforests and the world as a whole?

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'...
  • 1,008
45 votes

Where would space habitats get their oxygen from?

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 ...
  • 33.3k
37 votes
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A planet illuminated by a black hole?

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 ...
  • 97.8k
36 votes

Keeping an "hot eyeball planet" wet

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 ...
  • 249k
35 votes
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Extremely small planet to live on

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 ...
34 votes
Accepted

Is a Jupiter-sized planet plausible in a habitable zone?

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....
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33 votes

Earth loses its sun as is sent hurtling through space in darkness. How do humans survive for the next 1,000 years?

If there is little or no warning, there is no chance of survival. However, with some preparation, it is feasible to survive deep underground - a few km would do. The interior of the Earth is quite ...
32 votes

What would cause plants to survive on the ground while restricting animals to the air?

Big Smelly Mangrove Animals cannot come to the ground because there is no ground. Only an endless swamp with nothing to stand on. The air is thick with CO2 from decomposition. It will not kill you ...
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28 votes
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What would the consequences be of a high number of solar systems being within close proximity to one another?

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 ...
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27 votes

A Habitable Zone Within a Habitable Zone--Would that Make any Difference?

No habitable zones at all. Consider a habitable zone. It receives enough radiation from its star (or star pair) such that water does not freeze or evaporate. From OP "one habitable zone is deep ...
  • 287k
26 votes
Accepted

Could a world with a extremely bright night sky exist?

If you don't want the light to come from a single source, presumably you need roughly uniform illumination across the night sky. One way to achieve that is with a reflection nebula, a cloud of dust ...
  • 97.8k
25 votes
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Habitable moon of a gas giant: working out the sizes and distances

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....
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24 votes

Is a solar system around a black hole possible?

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 ...
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23 votes
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Can a gas giant have its own habitable zone?

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 ...
  • 45.7k
23 votes

Are Geologically Dead Planets Habitable?

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 ...
  • 40.2k
23 votes

What would cause plants to survive on the ground while restricting animals to the air?

There is one type of animal that lives on the ground. source Your plants are myrmecophytes or the alien equivalent. The plant - insect alliance is so successful that all existing plants are allied ...
  • 287k
22 votes

Keeping an "hot eyeball planet" wet

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 (...
  • 9,571
21 votes
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Close the door on your way out - Life lit by a blue dwarf star

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 ...
  • 1,064
21 votes
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How long after a global nuclear disaster would the world become habitable by humans again?

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 ...
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21 votes
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Where would space habitats get their oxygen from?

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 ...
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20 votes

A planet illuminated by a black hole?

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 ...
  • 5,631
20 votes
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Why would Earth be long-term unsuitable for an advanced alien species that's already colonized it?

"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, ...
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18 votes
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How could you keep a rogue planet warm(ish)?

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. ...
  • 38.1k
18 votes
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What is the theoretical maximum number of habitable planets in one solar system?

This is based off the information found in the link provided by Juraj. The answer is 2,862,106 earths in the goldilocks zone How to get 2,862,106 earths in habitable orbits: Rules, they have to all ...
  • 34.4k
18 votes

Why would Earth be long-term unsuitable for an advanced alien species that's already colonized it?

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 ...
  • 287k
17 votes

How early was the Earth liveable for today's humans?

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 ...
  • 46.3k
17 votes

Extremely small planet to live on

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 ...
  • 249k
17 votes

''Habitable'' planet close to a star

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 ...
17 votes
Accepted

Could bacteria survive on a star?

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 ...
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