It is still beyond current technology to look for individual living things. When astronomers search for extraterrestrial life, therefore, the targets are ecosystems—environments on other worlds that could harbor life. As far as we know it, life requires three basic ingredients: liquid water, a consistent source of gentle warmth, and a small set of basic chemical elements such as carbon, nitrogen, sulfur, and phosphorus. (Liquid water provides hydrogen and oxygen, as well.) If all three of these requirements are found together anywhere on Earth, life is always present; extrapolating to the universe, environments with these requirements may well also harbor life as we know it.