Run Mac OS 10.6 from external HDD on new MacBook Pro

Recently I bought a new MacBook Pro because I needed it for work. I also have some software that only runs on Mac OS 10.6 and earlier, and there are no newer versions available. Therefore I installed Mac OS 10.6 on an external HDD, so I could run this from the external drive on my new MacBook, at least I thought. But when I start up my MacBook from this drive, it gets stuck in the startup screen (the one with just the apple logo, but it is the one from OS 10.6). Is there something I can do to make it work, or is it impossible to run this version of OS on new MacBooks, even from an external harddisk? When I start up my old MacBook Pro (mid 2010) from the external drive, it works.