Apex domains cannot have CNAME records.
Question and response
Question
dig planet.net. NS
Answer
planet.net. 5 NS ns-992.awsdns-60.net.
planet.net. 5 NS ns-1643.awsdns-13.co.uk.
planet.net. 5 NS ns-434.awsdns-54.com.
planet.net. 5 NS ns-1312.awsdns-36.org.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0