\begin{table}%T4
\caption{\label{table:comp2} Comparison of formation times and core masses
 for several disc densities, and for two different planetesimal sizes: 100 and 
 10~km. All cases were calculated including atmospheric gas drag.}
 %\centering
\small
\begin{tabular}{p{1.0cm} p{1.2cm} p{1cm} p{1.2cm} p{1cm}}
\hline\hline
Disc density & \multicolumn{2}{p{1.9cm}}{Planetesimal radius: 100~km} &  
\multicolumn{2}{p{1.8cm}}{Planetesimal radius: 10~km}\\
\cline{2-5}
 [MMSN] & Formation time [My] & Core mass [${M_{\oplus}}$] & Formation 
 time [My] & Core mass [${M_{\oplus}}$]\\
\hline
%\\
$\; \; 6$   & 12  &  $\; \; 60$  & 5.6  & $\; \;69$ \\
$\; \; 7$   & $\; \; 9$  & $\; \; 70$  & 4    & $\; \; 80$ \\
$\; \; 8$   & $\; \; 7$  & $\; \; 81$  & 3    & $\; \; 92$ \\
$\; \; 9$   & $\; \; 6$  & $\; \; 91$  & 2.2  & 104 \\
10  & $\; \; 5$  & 100  & 1.75 & 115 \\
\hline 
\end{tabular}
\end{table}