If a string has a Planck length then why doesn't it have a huge Planck mass? Perhaps it does but its negative gravitational self-energy counteracts this huge positive mass/energy. Maybe a closed string can be thought of as a Planck energy electromagnetic wave trapped by its own gravitational field so that it is forced to orbit itself in a Planck scale loop. The positive rotational energy of the wave would be exactly balanced by the negative gravitational field so that the total rest mass/energy of the object would be zero. Maybe this would be a model of a graviton.