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Frameshift: Andrew Brēza had the right idea but got it backwards. It's not that the planet has a surplus of oxygen. Rather, it has a major deficit in carbon. The air isn't toxic at all, it's lacking CO2. Animals (including humans) don't use CO2, they won't care. Plants need CO2 to grow. You either need to import all the carbon your plants need, or you ...

17

We can calculate the effect on Earth's orbit by applying conservation of angular momentum. Earth's orbital angular momentum is $$\ell=mvr=mr\sqrt{GM/r}=m\sqrt{GMr}$$ with $M$ the mass of the Sun, $m$ the mass of Earth, $v$ Earth's orbital velocity at any given point and $r$ the distance from Earth to the Sun at any given point. As angular momentum is a ...

9

It sounds like a poorly conceived waste of money. Launching many space probes with 1 degree separation means launching 360 probes. If we take for good the lowest value of the range for the orbital period, 10000 years, we get that each probe would have to cover a span covering about 28 years of the orbit of the planet. Basically the luckiest probe would reach ...

8

Oxygen You're looking for a gas that will not harm animals but will impede plant growth. I suggest oxygen. Lots and lots of oxygen. In fact, the atmosphere is overwhelmingly composed of oxygen and barely has any carbon dioxide. Check out this question from Biology StackExchange for some of the science, or this old journal article for even more science.

6

There are three main issues: Cost, low probability of success, and better options. Current observations have attempted to constrain the current location of Planet Nine to within a region of sky roughly 20-40$^{\circ}$ across (Fienga et al. 2016, Holman et al. 2016). That's not a small portion of the sky (although at least it's not a completely blind search. ....

6

It would be easier with a pathogen. The chemistry of a plant selective airborne toxic is tricky. A less tricky way to accomplish your end would be a pathogen. Humans and vertebrates generally have immune systems that can adapt in an individual level to reckon with new germs and so after an initial infection, humans are immune. Maybe there could even be a ...

6

"Now, in the air of this planet there is also a gas that kill any plant they try to farm. I'm thinking about some toxic interaction with photosynthesis" Difficult, but doable with a little handwaving. You need a volatile compound called pseudoatrazine tetrafluoride, a heavy, odorless and tasteless gas that is somehow synthesized deep in the soil, ...

5

Yes, this wouldn't be a problem. The magnetic field at the outside of a planet's core very roughly scales like $$B_{\text{core}}\sim\sqrt{\frac{\rho\Omega}{\sigma}}$$ with $\rho$ the core density, $\Omega$ the rotational speed and $\sigma$ the core's electrical conductivity. The field should be a dipole, and a dipolar magnetic field falls off cubically, i.e. ...

5

Black hole's accretion disk burning rate is nowhere as constant as Sun-like star's, and its spectrum is not life-friendly (as @Demigan had already noticed). Intelligent life at or little above the tech level of modern day humanity would have to dig underground and shelter there. A more advanced civilization might be able to construct some kind of planetary ...

5

It is not possible for an object to be in the shadow of Earth permanently - although it could be in the semi-shadow called penumbra, the full shadow (umbra) does not extend that far. There is a special place called the L2 Lagrange point that allows an object to orbit around the Earth at about the same rate as the Earth orbits around the Sun - thus always ...

5

Your question can be rephrased to "What are the chances that a planet has photosynthetic life forms?", because oxygen won't last over geological times in a non life hosting planet. We can calculate it by using a modified version of Drake equation $N=R_* \cdot f_p \cdot n_e \cdot f_l \cdot f_i$ with $R_∗$ = the average rate of star formation in our ...

4

What you're trying to do is maintain a satellite in Earth's shadow while it travels around the Sun. You can't have the satellite orbiting the Sun with the same orbital period as Earth and not be in Earth's orbit. (Unless you're at L2 [or any other L-point, but only L2 is "behind" Earth], which is just barely outside Earth's umbra and unstable in ...

4

Not possible with current configuration To have an object perpetually in the earth's shadow, it must circle the earth at the same rate that the earth circles the sun, in order to keep the earth between it and the sun. To have a satellite with an orbital period of 1 year, it needs to be very far from the earth, approximately 2.1 million kilometers away (...

4

Havng a breathable atmosphere and temperatures with liquid water, etc., are requirements for a planet to be habitable for humans. Most discussions of planetary habitabilty are about planets where any of the many possible types of carbon based, liquid water using lifeforms could live. Places where humans could live unproteted, breathing the atmosphere should ...

3

The way I see it a slow collision can only happen if there is something which can dissipate momentum. Let's put some numbers on the paper... Any body approaching Earth from very far will at least impact it with a velocity of 11 km/s, which is also the escape velocity. This is greater than the velocity a satellite needs to have in order to orbit Earth. Since ...

3

PDMS Going with your suggestion of a silicon-based system, I'm going to suppose that the aliens convert a thick layer of the lunar regolith into polydimethylsiloxine, with occasional or perhaps frequent substituents on the methyl carbons. The effect of this is to give the Moon a viscoelastic "ocean" that is stable in vacuum and ultraviolet light. ...

3

I'm ignoring how the planet got there in the first place or what orbit it might have. Your life will need light and warmth to survive at all. A BH accretion disk could provide this. The time warping effects on the accretion disk apparently shifts light into higher energies causing more harmful UV radiation. Your life forms will need more resistances to ...

3

The real question should be: how likely is it that a space ship will crash on a habitable planet? Given that humans are mostly interested in habitable planets, it is save to assume that spaceships will either be very far away from any planet, or near a habitable one. So if a ship crashes on a planet, the likelyhood that it is habitable will be close to 1.

2

The answer by user 177107 to the question: https://astronomy.stackexchange.com/questions/40746/how-would-the-characteristics-of-a-habitable-planet-change-with-stars-of-differe[1] has a table with the characteristics of various types of stars, including the distance from a star where a planet would receive exactly as much radiation from that star as the Earth ...

2

If you want this to be science-based, you have some pretty big problems to overcome: Pressure The pressure near the Earth's mantle-core boundary is around 140 GPa, so your support structures need to withstand at least such pressures. However the strongest known materials have compressive strengths on the order of several hundred MPa, so you have a few orders ...

2

Black clouds of cyanide polymers. In the nitrogenous, oxygen-poor atmosphere of your world, photobiology would favor formation of cyanide. Hydrogen cyanide in nitrogen-rich atmospheres of rocky exoplanets Cyanide is unstable in the presence of oxygen, which can compete off the nitrogen to form CO. Your world is oxygen and water poor and so cyanide would ...

2

Planetary collisions cannot occur at "low speed" The minimum possible being orbital speed of roughly just under 7.8 km/s. Such a collision involving Earth sized astronomical bodies would release so much energy that both bodies would loose their structure. Debris might form a ring or even eventually a Moon, but at "relatively" low speed ...

2

How can the "new plant" grow without photosynthesis? If there is CO2, your plant basically just needs energy to extract the carbon from it. If it doesn't get it from the sun, you could imagine that it exploits geothermy, for example. The problem is if you choose to go the non-CO2 route. Because a plant is made of carbon, and to grow, it needs to ...

2

Biochemical incompatibility. Not total, of course, but it is a tap dance to get all the nutrients to you need while avoiding the stuff that will poison you. Especially since all the techniques to detoxify it are new.

2

Because there's nothing there The whole planet is a barren wasteland, where only heavy industry thrives. Because of the frequent high energy EM bombardment of the star, very few electronics survive, unless shielded heavily. This causes most of the surface life to consist of heavy manual labour. If they do want electronics, it is only in the dark mines and ...

2

The planet is a paradise. The temperature is perfect, balmy and warm, rain is regular, the biology is compatible, the mineral sources are ample and rare, and generally it's a great place to live on place. So what's the problem? The natives love the paradise as well. There's numerous extremely hardy and unpleasant organisms on the planet that love the taste ...

1

Impurities will carry the day. Like rubies and sapphires, your atmosphere is made mostly of colorless materials. If nothing else is present, the sky of Earth and exoplanet alike can have a blue tint due to Rayleigh scattering (scattering from things much smaller than a wavelength of light). Mars, on the other hand, has a red sky with a bluish tinge near ...

1

Earth did fine. Earth got hit by something big early in its planetary career. https://en.wikipedia.org/wiki/Giant-impact_hypothesis Got the Moon knocked out of it, yes she did. And here we are, living the life that life lives. I am anyway. I have a snack coming up! I conclude a big impact does not preclude life coming to exist later on. Other issues: ...

1

Mass-luminosity relationship is proportional to the 4th power, so I'd expect a mass 1.2 times that of the Sun for 2.19 times the luminosity, but that's close enough for demiurgical work. [Habitable zone] should be proportional to the light received, which decreases by inverse square, so for 2.19 times the light I'd expect something centered on 1.5 AU. ...

1

Cover it in corner reflectors Others have covered why a permanently eclipsed Moon is hard, requiring major changes to the Earth-Moon system. But the Moon could be in its own shadow if it returns all the light striking it to sender. I don't think I can post animation here, but the second image at the Wikipedia article on corner reflectors illustrates how ...

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