The surest proof of extraterrestrial life apart from alien craft landing on Earth would be deliberate radio signals, which scientists diligently search for using radio telescopes and probes. Although bursts of electromagnetic radiation occur regularly in space, a patterned series of signals could indicate possible attempts at extraterrestrial contact. The scientific community also scans for simpler lifeforms like bacteria on meteorites or hostile locations.
The search for extraterrestrial life has been going on for as long as humanity has explored space. Although the size of the universe indicates life should exist elsewhere, humanity has yet to encounter other intelligent civilizations, a situation described by the Fermi Paradox. One theory is that the vast interstellar distances involved in signal transmission prohibit radio signals from being sent or received between civilizations while both are still in existence.
Another tool scientists use to explore space is the Kepler telescope, which searches for extraterrestrial life partially by looking for exoplanets, or planets that have similar conditions to those that exist on Earth. However, it is also possible that the solar system is one of the first of its kind to form and that in the future other planets that support extraterrestrial life will form.