The way the rule is currently stated is that words normally written with a capital letter are excluded. In practice, if somebody wanted a word to be permitted, I would probably agree if there was
any dictionary that gave it in all lower-case letters, or evidence that it was sometimes written that way.
Judging words by whether they are written with a capital is not an ideal approach, but it is the way Scrabble rules normally judge the issue, and hence it is the criterion that has been used in compiling most of the English word lists in existence, including YAWL, which Chihuahua uses.
Alan Beale, who compiled another massive word list, ENABLE, has also compiled a list of over 9000 capitalised words that he believes are
not proper names. He was inspired to do this by a Scrabble Players' Dcitionary that carelessly stated that "proper names" were not allowed. Beale says:
This is of course inaccurate, as not every capitalized word is a proper name (e.g., "Iraqi", "Napoleonic", "Pleistocene" or "Ritalin"), and not every proper name is capitalized (e.g., "pinyin", "cinquecento").
(This list, and Alan Beale's discussion of proper names, are included in the
Enable Supplement at
http://personal.riverusers.com/~thegrendel/software.html.)
One reason a word that is not a proper name may have an initial capital is that it was named after a person or place, and still retains the capital from the source word. This sometimes applies to breeds of dog: "labrador", "pekingese" (and even "chihuahua"!) are often written with a capital letter. However these words are also often written in lower case, and all three are in our word list.
The words you ask about, "oort", "petri" and "henry" are in a similar situation, often retaining the initial capital letters from the names of Messrs Oort, Petri and Henry. I haven't seen "Oort Cloud" written without a capital O, but "petri dish" is certainly written with a lower case P sometimes. (Doing an online search of
the British National Corpus for the phrase extracted 22 examples of its use, 5 of which wrote "petri" with a lower case P.) And "henry" for the electrical unit is often written with a small "h".
However, I think the real problem for "oort" and "petri" is that neither of them seems to be a word in its own right. Is a petri dish ever called a "petri"? Do lab technicians say, "Pass me that petri, will you"? Is there a plural, "petris"? Not as far as I'm aware. So I would say the phrase "petri dish" is part of the English language, but "petri" by itself is not. Likewise, I don't see "Oort" used other than in the phrases "Oort Cloud" and "Oort constants".
But if you can convince me I'm being too harsh on these words, I'll gladly reconsider.