Past participle form of "exit"?
When we have a word ending in a single vowel and then the consonant 't', the consonant is only doubled before suffixes if that syllable is stressed. So when there is no stress we observe just a single 't'. In the following examples the stressed syllables are premarked with an apostrophe:
- 'rocketed
- e'licited
- 'billeted
- 'ratcheted
- 'exited
However if the last syllable is stressed then we will see a doubling of the consonant:
- ga'rotted
- 'vetted
- re'potted
- a'betted
- e'mitted
This is just a rule of thumb as there are special rules for certain prefixes, and compound words and loan words from other languages will not necessarily follow the rule.
Edit: Please also see Janus' interesting comment below about loan words with silent 't's below!