What you should refer to as a source are publications that have authority. They can be scholarly works, they can be periodicals anything that is stable enough so that you can go back to it for verification. Yes, Wikipedia has its "Cite this page" but it starts with the following notification:
IMPORTANT NOTE: Most educators and professionals do not consider it appropriate to use tertiary sources such as encyclopedias as a sole source for any information—citing an encyclopedia as an important reference in footnotes or bibliographies may result in censure or a failing grade. Wikipedia articles should be used for background information, as a reference for correct terminology and search terms, and as a starting point for further research.
As with any community-built reference, there is a possibility for error in Wikipedia's content—please check your facts against multiple sources and read our disclaimers for more information.Wikipedia is the most often used source in Wikidata. It "deserves" some choice words and maybe more but let's leave it at that.
What is more interesting is that resources like VIAF and the JPL Small-Body Database make it in this listing. They have authority and never mind if it is only there so that we can verify a statement or that it is the origin of the information, it has relevance.
By referring to these sources, they gain relevance. When these sources are US governmental resources like the JPL, the use in Wikidata may even strengthen their position for future funding.
By using sources responsibly we contribute back. It is very much like what we expect when people quote Wikipedia; it is called attribution. When we use sources responsibly, we may even find errors in external data. They will welcome such contributions and we will all share in the sum of all knowledge.