It's certainly possible in principle to correct a TTS by using some code language, but Duo has not chosen to give us access to that. I don't know why, but it's probably both more costly and more technically complex (they'd have to create some interface we could work with). It's the same for all languages. Our present TTS is much better than the first one we had, and several other languages have also had theirs exchanged for better ones.
The voice is not quite perfect on this sentence, as of September 25th, 2017, so I've taken the liberty of re-recording it.
Like Arnauti noted above, the stress changes the meaning here. If you stress plánet, it means "the (air)plane". But if you stress planét, it means "planet".
Please find a correct recording on http://duolingo.vydea.io/22728587ccc54513bf2919a65b16a36e.mp3
For more info on re-recordings, please check the info thread: https://www.duolingo.com/comment/23723515
Thanks for listening. Ha en bra dag! :)