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The question "Are we alone?" is older than the modern UFO phenomenon. Long before flying saucers appeared in newspapers, philosophers wondered whether other worlds might contain life. The discovery of planets orbiting other stars transformed that philosophical question into a scientific one. For centuries, humanity knew of only the planets in our own solar system. Today, astronomers have confirmed thousands of planets orbiting other stars, with many more candidates still being studied. This has changed our understanding of the universe. Planets are not rare exceptions. They appear to be common. That does not mean that life is common. But it makes the possibility much harder to dismiss. Earth occupies one tiny part of an enormous universe. Our galaxy alone contains hundreds of billions of stars. Many of those stars have planets. Some planets orbit at distances where temperatures could potentially allow liquid water to exist on their surfaces. Water is not proof of life. But it is one of the ingredients scientists consider important when assessing habitability. Mars provides a useful example. Mars was once warmer and wetter than it is today. Evidence shows that liquid water existed on its surface in the distant past. Rivers and ancient lakebeds left geological traces. Scientists therefore ask whether microscopic life could once have existed there. The question is not whether little green people lived on Mars. It is whether biology ever developed there. That is a much more scientifically meaningful question. The moons of the outer solar system are also fascinating. Europa, one of Jupiter's moons, appears to contain a vast ocean beneath its icy surface. Enceladus, a moon of Saturn, has also revealed evidence of water beneath its surface and has produced plumes containing material from its interior. These environments are extremely different from Earth. Yet they demonstrate that liquid water and potentially interesting chemistry may exist beyond our planet. If life can survive in extreme environments on Earth, scientists must consider whether life could potentially exist in similarly unusual environments elsewhere. Life on Earth is remarkably adaptable. Microorganisms have been found in places once considered impossible for life. Some survive in intense heat. Others live in highly acidic environments. Some exist deep beneath the Earth's surface. Others survive in conditions with very little sunlight. This does not prove that life exists elsewhere. It does show that the range of environments capable of supporting life may be larger than once assumed. The search therefore extends beyond planets that look like Earth. Scientists are interested in chemical processes. They study atmospheres. They look for combinations of gases that might be difficult to explain without biological activity. One of the most exciting developments in astronomy has been the ability to study the atmospheres of distant exoplanets. Powerful telescopes can analyse the light passing through those atmospheres. In some cases, scientists can identify chemical signatures. The interpretation is difficult. A gas that appears biological may also be produced through non-biological processes. This is why extraordinary discoveries require extraordinary verification. A possible biosignature is not the same thing as discovering life. Scientists must consider alternative explanations. They must compare observations. They must attempt to reproduce the results. They must allow independent researchers to examine the evidence. This careful process may seem slow. But it is what protects science from false discoveries. The search for intelligent life is even more difficult. Microbial life could be widespread without producing technology. A planet could contain simple organisms for billions of years without developing a technological civilisation. Earth itself existed for billions of years before humans developed radio technology. The universe may therefore contain countless worlds with life but