Galactic Panspermia -   I. Ginsburg, M Lingham, & A Loeb, Inst. Theory & Computation. Nov 2018 Astrophysical Journal Letters
Panspermia is the concept that life may be able to self-propagate from one planet to another, possibly by meteorites, even Martian achondrites (rocky pieces of planetary bodies).      To survive intraplanetray travel microbes must be able to survive the shock pressure heating of our Earth's atmosphere, which should not be much above 373 K. Research has shown that microorganisms can survive such temperatures.        Some strains of bacteria are also resistant to UV radiation which would be destructive to most common Earth bound bacteria. Bacillus subtilis has survived in space for 6 years and many earth bacteria are known to surround itself with a biofilms, a collective of bacteria in a slimy, protective extracellular matrix.
Oumuamua is a 700 foot long interstellar comet discovered in our Solar System in 2017 by the Hawaiian Pan-STARRS-1 telescope. Comets are icy, dusty objects. Oumuamua is headed out of our Solar system at a speed of 70,000mph. Such a large icy snowball might have been able to protect alien microbes during intraplanetary panspermia before bombarding Earth.
While this is not an experimentally tested concept it does have supportive properties that do not allow dismissal of the idea at present.
Panspermia animation*
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