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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?

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  • The prepositions: Of & About, are not necessary in the example sentence. better phrasing would be "Earth is the only Known planet where life exists."
    – RedOculus
    Dec 7 '19 at 17:17
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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.

  1. 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:

  1. 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]
  2. 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.

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  • I think the relevant definition isn't the one you've cited above. It's definition 2a under the final "phrasal verbs" section - To be aware or cognizant of (a person or thing as existing, an event as having occurred, etc.). Where to know about X usually implies knowing much more about X than simply the fact of it existing. Dec 7 '19 at 17:13
  • Advances in astronomy over the past couple of decades mean we now know about thousands of "exoplanets" that we weren't aware of before (as well as knowing much more than we used to about other planets within the solar system). Given that, I think if OP used about rather than of (admittedly, an unlikely choice of preposition), this might more strongly imply that we know there is no life on any other planets (we know about many planets, and know that only Earth supports life). As opposed to only knowing for sure that there's life here (but it might be elsewhere too). Dec 7 '19 at 17:23

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