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A common theme in media is that an alien lifeform comes to Earth by clinging to a meteor that was once part of the alien's homeworld. Usually, these aliens are either bacteria, viruses, or germs, creatures that are very tough and can survive the extreme heat of a meteor entering a planet's atmosphere.

Plantlife isn't the most durable and is very susceptible to fire and heat, so it would be unlikely to survive on the exterior of a chunk of rock flying through space. If this plant instead was inside of the meteor instead of on the surface would it be able to survive being volleyed to Earth?

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    $\begingroup$ Even if it survived re-entry (or more accurately, just entry) if it is anything like plants as we know them it wouldn't have the proper ecology to support itself. It would have to amazingly self-sufficient or by sheer luck be compatible with Earth (which seems unlikely even if the only viable biochemistry in the universe is only our own since considering randomly transplanting most plants onto a random place on Earth kills it more often than not). $\endgroup$ – DKNguyen Dec 30 '20 at 0:47
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    $\begingroup$ The inside of a meteorite does not necessarily get very hot. See: astronomy.stackexchange.com/a/6500 $\endgroup$ – Wayne Conrad Dec 30 '20 at 15:48
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Eon-long stellar journeys and flaming atmospheric entries are harsh environments for any life form, not only plants; but plants have evolved a wonderful mechanism for surviving temporary harsh conditions. It is called, "a seed".

Wrapping its vital genetic information in a nutrient-rich medium and then wrapping that kernel in a hard shell, along with some genetic coding which allows that information to lay dormant until specific environmental stimuli are present, the plant's species is able to survive long frigid winters and blisteringly hot dry spells.

This mechanism, if implemented to an extreme, might allow your stellar wandering flora to survive the rigors of its millennia-long journey. And the fires of falling through the atmosphere might be just the right stimuli to let it know that it was finally time to sprout.

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    $\begingroup$ I'd also suspect that such a hyperextremophilic plant species would have no problem taking root in just about any environment it lands on. Earth would just be a bonus for a plant that perhaps could live somewhere on Mars. $\endgroup$ – elemtilas Dec 30 '20 at 1:35
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    $\begingroup$ @elemtilas environmental resilience is much more difficult, the plant has to interact with the environment to grow while the seed is built to not interact. In a nutshell, I doubt it's even possible to have a plant that can grow both on Mars and Earth, and even if it could, it would be outperformed by Earth plants. $\endgroup$ – toolforger Dec 30 '20 at 12:08
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    $\begingroup$ @toolforger -- A pox on thy realism! What you say is most likely true, let's not lose sight of the possibilities within speculative fiction! $\endgroup$ – elemtilas Dec 30 '20 at 13:57
  • $\begingroup$ @toolforger, you are assuming that plants which evolved on Earth would be optimally evolved to thrive on Earth and would therefore outperform the alien seed. Why is that a given? If the alien plant evolved somewhere darker and less nutrient-rich (like Mars) isn't it logical that it would be more efficient than our spoiled terrestrial plants? Earth might be easy street compared to its home planet. a.k.a. the superman effect $\endgroup$ – Henry Taylor Dec 30 '20 at 16:09
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    $\begingroup$ @HenryTaylor Plants that evolve in darker and less nutrient-rich environments on Earth frequently die when transplanted to brighter and more nutrient-rich ones. For instance, it may not be adapted to rapidly spread when presented with ideal conditions and so get choked out. It may even suffer "sunburn," being unable to protect itself from sunlight's damaging effects. $\endgroup$ – Mary Dec 30 '20 at 17:08
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As the previous respondent noted a seed (or better yet a spore) might survive a multi million year journey through the cold and vacuum of space followed by the heat and impact of a descent to the Earths surface if buried deep inside a meteor large enough for the its core not to heat up too much as a result of air friction and the kinetic energy of impact. Unlikely in the extreme but possible (I suppose).

This however leaves the question of how exactly the plant got into the center of the meteor in the first place and then how the meteor, which was presumably part of the planet where the spore originated got blown into space to start with.

It might be easier to just assume instead the spore (which would be minute) was just drifting in the upper levels of its home planet's atmosphere when some catastrophic event destroyed the planet. The odds of any one spore surviving and making it into space would be astronomical but if there were billions of spores in the air at the time?

After that, sunlight gradually propels the surviving spores - which are amazingly hardy BTW off in random direction into deep space. Once it drifts into Earths orbit and gets captured its low mass, the ultra thin atmosphere it initially encounters & low velocity (relative to Earth) let it drift down to the surface.

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The atmospheric entry would be the easy part: meteorites don't get hot on the inside:

"Rocky asteroids are poor conductors of heat [...].Their central regions remain cool even as the hot outer layers are ablated away." — Donald Yeomans, manager of NASA's Near Earth Object program at JPL.

The next part would be getting out, so we'd need to posit that the meteorite split on impact, or was helpfully busted by a curious human.

Surviving the space trip would be tough, but one can conceive of seeds or spores, maybe even cells, that might remain viable, especially if the species had evolved to tolerate high radiation levels and long dry cold.

Now, the hardest part: how did it get in there to begin with? On earth a seed (or fossil) could be trapped in sedimentary rock, but it's hard to imagine sedimentary rocks surviving a planet's breakup as anything more than a cloud of sand. Perhaps the clump of sedimentary rock was swept by a fast lava stream into an ocean, where it cooled quickly enough that the seed remained viable, and then later the whole assemblage of seed inside sedimentary inside igneous got ejected in a cataclysm and sent our way like a peanut M&M tossed at a seagull. In a hard sci-fi story I think I'd buy it. Can't guess what a planetary scientist would say.

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You may find an answer here:

Panspermia (from Ancient Greek πᾶν (pan) 'all', and σπέρμα (sperma) 'seed') is the hypothesis that life exists throughout the Universe, distributed by space dust, meteoroids, asteroids, comets, planetoids, and also by spacecraft carrying unintended contamination by microorganisms. Distribution may have occurred spanning galaxies, and so may not be restricted to the limited scale of solar systems.[

https://en.wikipedia.org/wiki/Panspermia

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My answer to:

https://scifi.stackexchange.com/questions/240935/novel-sentient-lifeform-enslaves-all-life-on-planet-colonises-other-planets-b/240976#240976[1]

Mentions the classic science fiction story "Seeds of the Dusk" 1938, by Raymond Z. Gallun, in which intelligent Martian plants send billions of spores into space and one lands on Earth and sprouts.

Panspermia (from Ancient Greek πᾶν (pan) 'all', and σπέρμα (sperma) 'seed') is the hypothesis that life exists throughout the Universe, distributed by space dust,1 meteoroids,2 asteroids, comets,[3] planetoids,[4] and also by spacecraft carrying unintended contamination by microorganisms.[5][6][7] Distribution may have occurred spanning galaxies, and so may not be restricted to the limited scale of solar systems.[8][9]

The first known mention of the term was in the writings of the 5th-century BC Greek philosopher Anaxagoras.[20] Panspermia began to assume a more scientific form through the proposals of Jöns Jacob Berzelius (1834),[21] Hermann E. Richter (1865),[22] Kelvin (1871),[23] Hermann von Helmholtz (1879)[24][25] and finally reaching the level of a detailed scientific hypothesis through the efforts of the Swedish chemist Svante Arrhenius (1903).[26]

Fred Hoyle (1915–2001) and Chandra Wickramasinghe (born 1939) were influential proponents of panspermia.[27][28] In 1974 they proposed the hypothesis that some dust in interstellar space was largely organic (containing carbon), which Wickramasinghe later proved to be correct.[29][30][31] Hoyle and Wickramasinghe further contended that life forms continue to enter the Earth's atmosphere, and may be responsible for epidemic outbreaks, new diseases, and the genetic novelty necessary for macroevolution.[32]

https://en.wikipedia.org/wiki/Panspermia[2]

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