That very fearful demon, who was actually the son of Tvaṣṭā, covered all the planetary systems by dint of austerity. Therefore he was named Vṛtra, or one who covers everything.
Srimad Bhagavata Purana
श्रीमद्भागवतमहापुराणम्
मूल श्लोकः
येनावृता इमे लोकास्तमसा त्वाष्ट्रमूर्तिना । स वै वृत्र इति प्रोक्तः पापः परमदारुणः
yenāvṛtā ime lokāstamasā tvāṣṭramūrtinā | sa vai vṛtra iti proktaḥ pāpaḥ paramadāruṇaḥ
SB 6.9.18