No discovery yet
Let us begin with the plain status: as of now, no confirmed evidence of life beyond Earth exists. There have been periodic announcements and claims, and some have received a lot of attention, but none has held up as confirmed. The search is nonetheless a serious scientific field, often called astrobiology, which asks where life might exist, what it might look like and how we could tell.
Starting from the only example we have
The only life we know is on Earth, so researchers start from it. Life here needs liquid water, a source of energy, and certain chemical elements, especially carbon, hydrogen, nitrogen, oxygen, phosphorus and sulfur. Life has also been found in extreme conditions on Earth, such as deep ocean vents, hot springs, ice and rock far below the surface. That suggests the range of habitable environments may be wider than once assumed.
This focus on Earth-like life is a practical starting point rather than a claim that all life must be like ours. Scientists keep an open mind about other possibilities but have no way to search for something they cannot define, so they use known biology as a guide.
Habitable zones
For planets around other stars, a common concept is the habitable zone: the range of distances from a star where a planet with a suitable atmosphere could have liquid water on its surface. Too close, and water boils away. Too far, and it freezes. The zone’s location depends on the star’s brightness, so it is closer in around dim stars and farther out around bright ones.
“Habitable zone” does not mean “inhabited” or even “habitable”. A planet in the zone can still be unsuitable if it has no atmosphere, is bombarded by radiation or has a very different composition from Earth. In our own solar system, Venus and Mars are near the edges of the zone but have very different conditions from Earth, which is a reminder that distance alone does not decide.
Finding the planets
Astronomers have confirmed thousands of planets around other stars. Methods used in exoplanet detection include watching for tiny, periodic dips in a star’s brightness when a planet passes in front of it, and measuring small wobbles in a star’s motion caused by an orbiting planet’s gravity. Direct imaging is also possible in some cases, particularly for large planets far from their stars.
These surveys have shown that planets are common and that they come in many types, including some with sizes and orbits comparable to Earth’s. Estimating how many are actually Earth-like is hard, and the numbers depend on definitions and assumptions.
Looking for biosignatures
The next step is to study the atmosphere of a planet to look for biosignatures, chemical clues that might point to life. When a planet passes in front of its star, some starlight filters through its atmosphere, and gases leave a pattern in the spectrum. Researchers might look for combinations of gases, such as oxygen together with methane, that are hard to maintain without a continuing source.
But this is difficult and uncertain. Non-biological processes can produce some of the same gases, giving false positives, and the signals are tiny. For this reason, scientists emphasize that any claimed detection would need extensive checking and a careful ruling out of alternatives. A single gas is not enough.
Searching our own solar system
Closer to home, missions explore places where life, probably microbial, might exist or have existed. Mars is of interest for its ancient history of liquid water, and robotic missions look for signs of past habitability and chemicals that might be preserved in rocks. Some of the icy moons of the outer planets are thought to have oceans of liquid water beneath their ice shells, making them targets for future study. Careful procedures are used to avoid contaminating other worlds with Earth microbes and to avoid mistaking contamination for a finding.
Listening for technology
A different approach is to look for signs of technology. The Search for Extraterrestrial Intelligence, known as SETI, has used radio telescopes to listen for narrow-band signals, which are not known to be produced by natural processes, and some projects also look for brief pulses of light. So far, no signal has been confirmed as being of artificial extraterrestrial origin. Some unusual signals have been traced to human technology or natural causes.
A lack of detection does not say much yet, since only a tiny fraction of possible targets, frequencies and times have been searched. The Drake equation, a framework proposed in 1961, breaks the question of the number of communicating civilizations into factors such as the rate of star formation and the fraction of planets where life arises. Several of its terms are very uncertain, so the equation is better seen as an organizing tool than a way to calculate an answer.
The Fermi question
The physicist Enrico Fermi is credited with asking why, if intelligent life is common, we see no evidence of it. This question, the Fermi paradox, has many proposed answers, ranging from life being rare to distances being too great to the possibility that we are simply not looking in the right way. None is established.
A balanced view
- Nothing has been found, and no reliable evidence of life beyond Earth exists yet.
- Many of the ingredients thought to be needed for life appear to be widespread.
- The detection of microbes would be a major scientific event, and the search methods keep improving.
- Claims should be judged cautiously, with attention to whether results have been independently checked.
The question is one of the most appealing in science because either answer is significant. Finding life elsewhere would change our view of biology, and finding none in many careful searches would suggest that Earth is more unusual than we thought.