What are the reasons why planes can't go into space?
Basically it's because they have engines that need atmospheric oxygen for thrust. Stick a rocket motor on it, with enough power to achieve escape velocity, and provide a self contained compressed gas source for pressurization at say 10 or 15 thousand feet cabin altitude and make the pressure hull fully air tight (airplanes normally leak quite a lot), and away you go to the stars if you like.
Of course, now you have to get back down without burning up so you need maneuvering thrusters to control attitude on re-entry, and you need some kind of heat shield...
After working all that out, voila! You end up with the Space Shuttle.
There are two major problems with trying to take an aeroplane to space.
- The engines are air breathing, this makes them far more efficient than rocket engines, but means they only work over a relatively narrow range of speeds and atmospheric densities.
- Planes are designed with wings to use aerodynamic lift, this is far more efficient than directing thrust downwards, but again a given set of wings is only effective over a relatively narrow range of speeds and atmospheric densities.
There is a big gap between the highest speed/thinnest atmosphere at which aerodynamic flight with air breathing engines is practical and the lowest speed/thickest atmosphere at which orbit is practical. So-far the only successful way to get into orbit has been to brute-force through with big multi-stage rockets.