As far as I know, you cannot open adoption records for something like a genealogy project. Dire medical need, yes. Curiosity, no.
Whether or not the biological parents are dead, their descendants from subsequent marriages may not have known about the adoption.
And frankly, adoption makes a child legally a part of a family. Genealogy should be traced through adoptive parents. What are you going to do about adopted children from the 1800s and 1700s---when it was never even recorded that a child was adopted? Generally, adoptions prior to about 1900 were not handled through courts, but were just a matter of transferring the child from one parent to another, and then recording the birth of the child in the family Bible.
The odds of you getting those records opened are lower than the odds of winning the lottery. You'd have to show VERY good reason for a court to do so, and generally they will not without a valid medical reason.