I'm sorry, but the FZ7 doesn't have exchangable lenses. You'll need a dSLR for those lenses.

Ex. Canon 400D - 450D - 40D -... Nikon D40 - D80 - ... Pentax, Sony and Olympus are other brands...

As far as the lenses themselves, you need to know your camera and then read reviews about the lenses. Set your budget and choose the best one... But be prepaired that you'll get what you pay for. Cheap often means less quality. So read, read, read...

Not every lens goes on every camera. Canon, Nikkor, Zuiko, Pentax and Sonylenses only work on the camera's they were designed for.

Tamron, Sigma and Tokina build lenses for different camera's, but even then they are interchangable. You can't put your Sigma for Canon on a buddies Nikon camera.

literature: www.photozone.de

PS: your FZ7 allready has a 400+ mm lense attached to it.

To get really nice shots of contrails and planes that high up, you'll have to look into telescopes. But that's a whole different story...  

Mictheslik has high altitude shots posted in these forums, just look a few topics back. He shot those birds with a 400 mm lense on his Canon 400D. Perhaps that's why you did search for a 400 mm?

The lens of the Panasonic FZ7 ( like my FZ5), goes up to 430 mm equivalent on 35 mm film.

When you put a 400 mm lens on a camera like the 400D, you have to multiply the 400 mm with 1,6 (the cropfactor of the camera). So the lens actually becomes a 640 mm one.

This is because lenses have a focal length indicated as seen on a full frame sensor ( as big as the old analogue film). Lots of dSLR's have a slightly smaller sensor, so the image is cropped a bit and this way you get a photo with a 400 mm lens that looks like a photo with a 640 mm lens taken on a full frame camera.

Still following me? It's complex, I know.

Your camera doesn't show focal lengths, just 12x optical zoom. But when you look at the side of the lens, you'll see equal to 28 - 430 mm on 35 mm. So, if you zoom in to the max and take a photo, it will look like the same photo on a full frame sensor with a 430 mm lens.

That's how it works.  

Don't shoot me on the amount of mm on the lens of the FZ7, I don't have the camera with me, so I could be a bit wrong... but not much...