From a scientific perspective, this is a scientific question, and can be approached through scientific sources. (What one considers 'credible' about extraterrestrial life may vary from person to person.)
From the perspective of the study of the Qur'an and hadith, some ayat of the Qur'an have been interpreted (past and present) to suggest that there is extraterrestrial life. Some hadith also speak of life on other "earths". In particular, there are a number of Shi'i hadith on this theme, which one can find in sources like Bihar al-Anwar, vol. 55. One can also interpret the Qur'an and hadith to speak of life before/after this universe.
The following works in English may be useful:
* Islamic Theology and Extraterrestrial Life (book)
*Shīʿī Imāmī Thought on Existence, Life, and Extraterrestrials (article)
* A Broader Perspective on “Humans”: Analysis of Insān in Twelver Shīʿī Philosophy and Implications for Astrotheology (article)
Prophet Easa (As) will come back to the earth from the sky after the reappearance of Imam Al-Mahdi (AS) and will one his followers. Imam Al-Mahdi (AS) will establish on the earth the government of peace justice and peace which will stay as long as the governments of injustice stayed.
After your firm decision to be an obedient servant of Allah in every matter in your life, you need to know what to do and what to avoid. You need to read books on Islamic practical laws which are available online like www.Sistani.org
If you have any question, you can ask trusted scholars who can guide you.
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