GJ 3991 B is probably over 6 billion years old , making it the oldest among these objects as well .
Among these , GJ 3991 B is not only the coldest , but the only one in a short - period orbit with another star .
coldest     is probably over 6 billion years old , making GJ 3991 B the oldest among these objects as well .
Among these , GJ 3991 B is not only the coldest , but the only one in a short - period orbit with another star .