Help wanted to finish my Structuring planet data

hello everyone, i was struggling to think of a way to add the border in the Planet name column, i know how to make the name and the group row headings (e.g. Jovian planets ) bold but for me its inefficient if i add a class to them and set the font weight to bold, any idea how to finish this task in an efficient way, TIA.

https://jsfiddle.net/satoshi2003/9za4m2fj/3/