  We consider the semilinear elliptic equation $$ -\Delta u = |x|^\alpha u^p
\quad \hbox{in } \mathbb{R}^N, $$ where $N\ge 3$, $\alpha>-2$ and $p>1$. We
show that there are no positive solutions provided that the exponent $p$
additionally verifies $$ 1<p<\frac{N+2\alpha+2}{N-2}. $$ This solves an open
problem posed in previous literature, where only the radially symmetric case
was fully understood. We also characterize all positive solutions when
$p=\frac{N+2\alpha+2}{N-2}$ and $-2<\alpha<0$.
