‘Abdu’l-Bahá stated there are other worlds than ours which are inhabited by beings capable of knowing God.
(Shoghi Effendi, The Light of Divine Guidance v II, p. 79)
Regarding the passage on p. 163 of the ‘Gleanings‘; the creatures which Bahá’u’lláh states to be found in every planet cannot be considered to be necessarily similar or different from human beings on this earth. Bahá’u’lláh does not specifically state whether such creatures are like or unlike us. He simply refers to the fact that there are creatures in every planet. It remains for science to discover one day the exact nature of these creatures.
(Shoghi Effendi, Lights of Guidance p. 478)
The earth has its inhabitants, the water and the air contain many living beings and all the elements have their nature spirits, then how is it possible to conceive that these stupendous stellar bodies are not inhabited? Verily, they are peopled, but let it be known that the dwellers accord with the elements of their respective spheres. These living beings do not have states of consciousness like unto those who live on the surface of this globe: the power of adaptation and environment moulds their bodies and states of consciousness, just as our bodies and minds are suited to our planet. For example, we have birds that live in the air, those that live on the earth and those that live in the sea... The components of the sun differ from those of this earth, for there are certain light and life-giving elements radiating from the sun. Exactly the same elements may exist in two bodies, but in varying quantities. For instance, there is fire and air in water, but the allotted measure is small in proportion. They have discovered that there is a great quantity of radium in the sun; the same element is found on the earth, but in a much smaller degree. Beings who inhabit those distant luminous bodies are attuned to the elements that have gone into their composition of their respective spheres."
(‘Abdu’l-Bahá, Divine Philosophy, p. 114-115)
The learned men, that have fixed at several thousand years the life of this earth, have failed, throughout the long period of their observation, to consider either the number or the age of the other planets. Consider, moreover, the manifold divergencies that have resulted from the theories propounded by these men. Know thou that every fixed star hath its own planets, and every planet its own creatures, whose number no man can compute.
(Bahá’u’lláh, Gleanings from the Writings of Bahá’u’lláh, p. 163)
Through His potency the Trees of Divine Revelation have yielded their fruits, every one of which hath been sent down in the form of a Prophet, bearing a Message to God’s creatures in each of the worlds whose number God, alone, in His all-encompassing knowledge, can reckon.
(Bahá’u’lláh, Advent of Divine Justice, p. 80)
Verily I say, the creation of God embraceth worlds besides this world, and creatures apart from these creatures. In each of these worlds He hath ordained things which none can search except Himself, the All-Searching, the All-Wise.
(Bahá’u’lláh, Tablets of Bahá’u’lláh p. 187-188)