Problem with "_visible"

I am working on a ‘guide to the solar system’ in Flash CS4, even though I use AS2 due to AS3 is confusing. If I rollover Venus for example, I want the rest of the planets to disappear using the ‘_visible’ command. All of the planets are buttons like ‘venus_btn’.

This is my code for ‘venus_btn’:

on (rollOver) {
 mercury_btn._visible = false;

This doesn’t work, do I need to set a class or a varible? That’s all the code I’ve used.