I'm looking for another juvenile SF novel that was in my junior high school library (1959 - 1962). I got really lucky on my previous request that led me to "The Planet Mappers", but this one will be probably harder because my memory is so sketchy about this book. A bother and sister discover that their high school chemistry (physics?) teacher is helping a group of aliens who plan to blow up Earth. The human siblings team up with an alien bother and sister who are part of a group opposed to the plans to blow up Earth. The aliens want to blow up Earth in order to release a special gas trapped deep under the Earth's crust that they use as a power source or as fuel for their spaceships (hazy memory here). The chemistry (physics) teacher is helping the aliens with the calculations to determine the best way to blow up Earth. That is basically what I remember from 50 years ago (man, but I am showing my age).