ok, i think i understood that
<thead>
<tr>
<th></th>
<th>Name</th>
<th>Mass(10<sup>24</sup> kg</th>
<th>Diameter(km)</th>
<th>Density(km/m<sup>3</sup></th>
<th>Gravity(m/s<sup>2</sup></th>
</tr>
</thead>
<tbody>
<tr>
<th rowspan="4">Terrestrial Planets</th>
<th>Mercury</th>
<td>0.330</td>
<td>4.879</td>
<td>5427</td>
<td>3.7</td>
<tr>
<th>Venus</th>
<td>4.87</td>
<td>12,104</td>
<td>5423</td>
<td>8.9</td>
</tr>
<tr>
<th>Earth</th>
<td>5.97</td>
<td>12,756</td>
<td>5514</td>
<td>9.8</td>
</tr>
<tr>
<th>Mars</th>
<td>0.624</td>
<td>6,792</td>
<td>3933</td>
<td>3.7</td>
</tr>
<tr>
<th rowspan="4" colspan="">Jovian Planets</th>
<th>Jupiter</th>
<td>1898</td>
<td>142,984</td>
<td>1326</td>
<td>23.1</td>
</tr>