Article: The above are further subdivided into 31 planes of existence.[web 4] Rebirths in some of the higher heavens, known as the Śuddhāvāsa Worlds or Pure Abodes, can be attained only by skilled Buddhist practitioners known as anāgāmis (non-returners). Rebirths in the Ārūpyadhātu (formless realms) can be attained by only those who can meditate on the arūpajhānas, the highest object of meditation.

Question: What is the highest object of meditation?
arūpajhānas