"Jean-Marc Becker" wrote in message
...
The May 2004 issue of Scientific American has an article about gas
depleted hot jupiters which are named Chthonians (page 13). How to
pronunciate it ? Does "Ch" or "th" go silent ?

Ch is for greek letter Khi, pronounced K, ah th for theta, pronounced T.
Ctonian is a good approximation.

JMB

Reasonable assumption, but every dictionary happens to disagree with you.