The Siddhas are the inhabitants of Siddhaloka, wwhere all are born with fully developed mystic powers. There is aother planet inhabited by the Vidyadharas and Caranas, who are very expert in singing.
"Then Lord Brahma, by his ability to be hidden from vision, created the Siddhas and Vidyadharas and gave them that wonderful form of his known as the Antardhana.
Purport: Antardhana means that these living creatures can be perceived to be present, but they cannot be seen by vision."
"It is said that the demigods never touch the surface of the earth. They walk and travel in space only. Like the great sage Narada, the Kumaras do not require any machine to travel in space. There are also residents of Siddhaloka who can travel in space without machines. Since they can go from one planet to another, they are called siddhas; that is to say they have acquired all mystic and yogic powers. Such great saintly persons who have attained complete perfection in mystic yoga are not visible in this age on earth because humanity is not worthy of their presence."
"Beyond the region of ignorance [the material cosmic manifestation] lies the realm of Siddhaloka. The Siddhas reside there, absorbed in the bliss of Brahman. Demons killed by the Lord also attain that realm.
Purport: Tamas means darkness. The material world is dark, and beyond the material world is light. In other words, after passing through the entire material atmosphere, one can come to the luminous spiritual sky, whose impersonal effulgence is known as Siddhaloka. Mayavadi philosophers who aspire to merge with the body of the Supreme Personality of Godhead, as well as demoniac persons who are killed by Krsna, such as Kamsa and Sisupala, enter that Brahman effulgence. Yogis who attain oneness through meditation according to the Patanjali yoga system also reach Siddhaloka. This is a verse from the Brahmanda Purana."
Caitanya-caritamrta, Adi 5:29
Bhaktivedanta Book Trust. Excerpted from various texts and purports of HDG A.C. Bhaktivedanta Swami Srila Prabhupada.