Do we know definitively that there are no other planets in this system?
The presence of a large, Jupiter-sized planet in a system is thought to be helpful for deflecting asteroid impacts. Obviously we are talking 'to scale' given TRAPPIST-1 itself is around the size of Jupiter!
The more we learn about this system is going to be fascinating though - the supposed inward migration of these planets may even help us understand more about how our own system formed.