A lot of people have asked this question on various forums: “Why are there navagrahas only in Śiva temples governed by siddhānta-āgama-s?” The answer hit me a while ago when I realized that the navagrahas worshiped by Śaiva-s are not the ones we know in this Brahmāṇḍa (which consists of this entire material universe, svarga, naraka-s, asura, rākṣasa, yakṣa, gandharva worlds, etc), which is in Pṛthivī-tattva.
From Pṛthivī-tattva (lowest of 36 tattva-s) to Puruṣa-tattva is the Aśuddhādhvan (“impure” pathway of the cosmos made of Aśuddhamāyā). Above Aśuddhamāyā is the Śuddhādhvan (the pure pathway), which consists of Śuddhavidyā, Īśvara, Sadāśiva, Śakti and Śiva-tattvas.
In Siddhānta, Paramaśiva (Who is beyond the 36 Tattvas and surpasses all) is worshiped as Śivasūrya (who exists in Śiva-tattva, the 1st of the 36 Tattvas) and in ritual, Śivasūrya has an Āvaraṇa of transcendental deities with the same names as the remaining 8 Grahas. This worship of Śivasūrya with the 8 Grahas as Āvaraṇadevatas in Ātmārthapūjā (personal worship) likely & gradually carried over to the Parārtha (temple worship) context, where Śivasūrya’s presence was deemed necessary.
It is imperative that Śaivas, particularly Ādiśaivas & Mahāśaivas with a good foundation in Śāstra, take up the task of articulating rich explanations for Siddhānta rituals in the light of Śaiva metaphysics.