Question: I am writing a story that involves the colonization of an exoplanet with very similar properties to Earth. This exoplanet would be orbiting a red dwarf, but due to certain factors (it's atmosphere and the presence of one or several moons) it is NOT tidally locked. Upon arrival from humans, to their surprise this planet already harbors complex life. It has a large ocean, continents, fauna and flora much like our own earth.

My question is: despite all the similarities, would it be plausible for this planet to be radioactively hostile towards humans? What I mean by this is, despite the similarities, could the fauna and flora have adapted to thrive in an environment which would be ultimately too harsh for our own bodies in the long run?

To provide some more context as to why i'm asking this: The plot i'm working on makes it so that when humans arrive, they can breathe without their suits, and at first it seems like the planet is just like earth, but soon after they realize that prolongued exposure to the radiation within the planet leads to early offset of cancer and thus humans can't really afford to live in this planet as it is without applying some form of alteration to the atmosphere (that's a wholenother subject i'll leave for another post haha), which would lead to harmful consequences towards the native life that has evolved to thrive within this specific environment.

Love this forum, it's been super helpful, to anyone taking time out of their day to answer: Thank you!

Answer: 
**Radiation belts.**

<https://astronomy.com/magazine/ask-astro/2020/02/what-is-the-source-of-jupiters-radiation>

> 
> Any charged particles in the space around Jupiter will experience the
> planet’s strong magnetic field and get accelerated to high energies.
> These negatively charged electrons and positively charged ions of
> hydrogen (i.e., protons), oxygen, and sulfur form Jupiter’s radiation
> belts... The dominant particle source, however, is the volcanic moon
> Io, which dumps a ton per second of sulfur dioxide gas into space
> where the molecules are broken up and ionized. These processes around
> the moon drive million-amp currents between Io and Jupiter’s
> ionosphere, where they excite intense aurorae on the planet.
> 
> 
> The highest fluxes of energetic particles are found closest to the
> planet, where they form a doughnut-shaped belt around the equator
> (which causes radio emission that astronomers have measured from Earth
> since the 1960s)...
> 
> 
> It is Jupiter’s combination of a strong magnetic field, Io’s
> prodigious source, and the magnetic coupling of charged particles to
> the planet’s rapid (10-hour) spin that drives the intense radiation.
> Earth’s radiation belts (named after James Van Allen, who discovered
> them with the U.S.’s first satellite in 1958) are much weaker.
> 
> 
> 

Your planet has a very strong magnetic field and one of its moons plays the role of Io, dumping particles that are accelerated to high speeds. Fast moving particles are the equivalent of electromagnetic radiation as regards biological damage.

Your people do not realize that these belts exist and dip down to and below the planet surface because the belt is not homogenous - it moves with the moons, with high and low tides. In addition a detection method tuned to short wavelength radiation like Xrays and gamma rays might completely overlook fast moving large ions of sulfur and iron.

As regards the prospect of life not affected by particulate radiation, why not? One could imagine Earth life durable to this sort of thing. A simple method: if radiation makes random changes in a genome you could have multiple copies of the genome and check them against each other frequently, correcting according to consensus for a given gene.
You asked specifically about "radioactively hostile" but in case you just meant "very unhealthy in the longer term" - radiation on the surface could be much higher than on Earth (e.g. like Mars) from cosmic sources, either due to the stellar neighborhood or to a weaker magnetic field (again, like Mars). As a total non-expert in this kind of biology I can suppose that the native life has adapted to this, similar to the Earth organisms mentioned in other answers.

Going outside 'radiation', there are any number of trace elements that can be present in soil, and therefore also as dust in the air, that are long-term toxic or carcinogenic for humans e.g. Arsenic (As), cadmium (Cd), chromium (Cr), and nickel (Ni). Asbestos is also a naturally occuring contaminant in various minerals: vermiculite and taconite are well-known examples.

Here's a completely different form of hostility: It seems pretty plausible based on recent research, that life can be constructed from a different set of amino acids than Earth life. And if it can be, the odds are it WILL be on other planets. As a result, if your colonists use native life as food, they will quickly or slowly develop amino-acid deficiencies unless they supplement with the essential Earth amino acids that are missing. I speculate that some non-Earthly aminos might even be actively toxic, but that's just a guess.

One last comment: If you don't want your story to feel like a throwback to the 1950's or 60's, assume that if humans can colonize an extrasolar planet, that their science and technology is MORE advanced than ours, and they will understand physics, chemistry, biology, and medicine better than we do. Unless they've gone through a radical loss of civilization (despite interstellar travel) they will NOT be caught by surprise by any of these environmental problems like radiation, toxic metals, or amino-acid deficits, any more than a team of Navy SEALs would get lost in the desert. Their satellites, or landers, or smart-phones, or body augments, will have told them all about it.