Earth is the only planet we know of where life exists.
This sentence is from the commentary of Planet Earth.
Why it is know of in this sentence? Can it be know about？
What’s the difference between know of and know about?
English Language & Usage Stack Exchange is a question and answer site for linguists, etymologists, and serious English language enthusiasts. It only takes a minute to sign up.Sign up to join this community
In your sentence, there is no difference between know of and know about; the writer is free to choose either with no change in meaning here.
This particular use falls under sense 10b in the OED, found at the top of its section III for this verb covering meanings related to learning. (paywalled link)
III. To (come to) apprehend, be or become conversant with or aware of; to learn.
a. transitive. To be or have become conversant with (a body of facts, principles, a method of action, etc.), esp. through instruction, study, or practice; esp. to have a good command or practical understanding of (a subject, language, etc.); to have learnt by study or practical experience; to be versed or skilled in.
b. transitive. To understand or have learnt (a stated amount) about or of a particular subject.
Here are some of citations from sense 10b:
- 1887 M. E. Braddon Like & Unlike I. iii. 78 I know something about navigating a yacht.
- 1895 R. Kipling Second Jungle Bk. 148 The boy knows something of dog-driving.
- 1924 Amer. Mercury Sept. 14/1 I know nothing of pomology or of the flora of the Meuse.
- 1967 M. Stand Diana is Dead vii. 104 You seem to know a lot about crookery.
- 1996 K. Maristed Fall 4 ‘A. Heely’ knew zilch about horses except that sheer size plus dumb equals dangerous.
To add some nuance occasionally present between know of and know about, Collins Dictionary writes:
VERB [no cont]
If you say that you know of something, you mean that you have heard about it but you do not necessarily have a lot of information about it.
- We know of the incident but have no further details. [VERB + of]
- The president admitted that he did not know of any rebels having surrendered so far. [VERB of noun]
- I know of no one who would want to murder Albert. [VERB + of]
VERB [no cont]
If you know about a subject, you have studied it or taken an interest in it, and understand part or all of it.
- Hire someone with experience, someone who knows about real estate. [VERB + about]
- She didn't know anything about music but she liked to sing. [V amount + about]
Synonyms: be familiar with, experience, understand, ken [Scottish]
But I don’t think it would make any difference in your case here.