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We know what the weapon is.

That "Breakthrough Starshot" would have a 1 megaton yield if it hit a planet in another solar system. Funny thing is that meteors explode with that kind of yield in our atmosphere all the time and often only astronomers notice.

Bigger projectiles and more of them would be deadly traveling at 0.1c.

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I think though really life sucks inside the "frost line"; the Earth is actually a dry place that is rich in Silicon and Magnesium and poor in Carbon. Most of the "places with liquid water" are some combination of underground, tidally heated, geothermal, etc. Be it Europa or Pluto, the generic "large" outer solar system body has liquid water inside because water is 50% or so of the mass of the body.