I live in northern Mississippi, where I think it is usually fine to have already planted pea plants. I did so and they have recently started coming up and are still very small (less than an inch tall). Unfortunately, we are supposed to have extremely cold (for Mississippi) weather coming up, with it expected to get as low as 9 °F. How can I protect my peas?
I live in Wisconsin, where we also plant when the temps can get below freezing, and I think that you're right to be concerned - 9 F is awfully cold for your plants—according to this sustainable farming site, it's just below the lowest temperature spring peas can stand without being killed.
Assuming that you have a typical garden-sized row or rows of peas, you have a couple of options for protecting the plants:
Plastic is pretty light-weight and will give you a few degrees' protection; you'll need to anchor it to keep it from blowing off the plants if a wind arises. You can anchor it with stones or bricks, or by making a trench around the plot about 2-3 inches deep and wide, and then burying the edge of the plastic in the trench. You'll need to remove it when/if the sun comes out, in order to avoid cooking your plants.
Fabric such as old sheets, blankets or bedspreads gives you more protection from cold temps but can be too heavy, crushing young plants under its weight; on the other hand, you won't have to worry about cooking the plants if the sun comes out. If the plants are up and twining, though, then the support that they're twining round could take the brunt of the weight (if properly staked). If you have relatively small seedlings and use fabric, I recommend putting a brick between each seedling/pair of seedlings to keep as much weight as possible off of the plants. This can be tricky!
Whichever method you choose will be at least a short-term need. Best of luck to you!