Can a planet hit by a meteor shatter into two pieces? Alternately, if not a meteor, an advanced interstellar weapon?

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I like TimB's answer discussing the Giant Impact Hypothesis. I see that in a comment on his answer, you wrote

Thank you! I actually had this Moon origin theory in mind. But I'm actually wondering about an inhabited planet, in the immediate run. Is there any possibility of survivors, say, after the shattering? For a while, at least? Any idea?

The answer is most certainly no. I'd like to go in-depth as to why that won't be happening.

In an answer to one of my questions, Serban Tanasa discussed the conditions of impacts. He cited Stewart et al. (2015) (which cites an earlier work by the three, Stewart et al. (2014), giving the formula for the energy released in an impact, $Q_{S}$ , as $\begin{matrix} (1) & Q_{S} = Q_{R} (1 + M_{p} / M_{t}) (1 - b) \end{matrix}$ $Q_{R}$ is calculated as $\begin{matrix} (2) & Q_{R} = \frac{0.5 μ V^{2}}{M_{p} + M_{t}} \end{matrix}$ In this latter paper, they mention that for grazing impacts, $b > \frac{R_{t}}{R_{t} + R_{p}}$ , so we'll assume here that $b < \frac{R_{t}}{R_{t} + R_{p}}$ .

You mentioned a meteor. A meteor will do next to no damage to a planet. In this case, $R_{p} ≪ R_{t}$ , so we're left with $b < 1$ . To do some real damage - enough to do what you want - we need $R_{t} \approx R_{p}$ . So $b < \frac{1}{2}$ . We can, though, say that the projectile isn't too massive. We'll toe the line and have $b = \frac{1}{2}$ .

Assuming the densities are the same, $R_{p} \approx R_{t} \to M_{p} \approx M_{t}$ . We now have $Q_{S} = Q_{R} (1 + 1) (1 - \frac{1}{2}) \to Q_{S} = Q_{R}$ This means that $\begin{matrix} (3) & Q_{S} = \frac{1}{4} μ V^{2} \end{matrix}$ $μ$ , the reduced mass, is $μ = \frac{M_{p} M_{t}}{M_{p} + M_{t}}$ Assuming both bodies are Earth-like, this means that $μ = 2.985 \times 10^{24}$ It doesn't matter what $V$ is. Looking at one of the graphs, I see that whatever $V$ is, $Q_{S}$ is going to be off the charts: