In 1961, at a SETI conference in Green Bank, West Virginia, astrophysicist Frank Drake and others discussed a short chain of factors. The Drake equation, formulated largely by Drake, estimates the number of technically advanced civilizations in the Milky Way. Drake first wrote it as an agenda for discussion at the 1961 Green Bank conference. The seven terms are the rate of star formation, the fraction of stars with planets, and the number of planets per star that could support life. After those come the fractions of those planets that develop life, intelligent life and detectable technology. The last term is how long such civilizations send out detectable signals. The equation multiplies these factors together, and the uncertainty grows with each factor. The question it was written for is still open. As of November 2026, scientists have not found life anywhere beyond Earth, and the search goes on.

Scientists have found thousands of planets around other stars, and as of October 2026, NASA has confirmed more than 6,000 exoplanets, out of billions believed to exist. Astronomers also use the habitable zone, the area around a star where a planet could have liquid water on its surface, to decide where to look first. Being in the zone does not make a planet habitable, since it also needs the right size, temperature and composition to host liquid water. The TRAPPIST-1 system is one example. As of November 2026, it is described as lying about 40 light-years away, with seven Earth-sized rocky planets around a red dwarf star. The three in the habitable zone are more likely to have liquid water. The habitable zone moves with the star: every star has one, but where it lies depends on the star's size and brightness. Red dwarf stars have a narrow zone, and their flares could sterilize planets orbiting close in.

The two main ways of finding planets each have limits. In the transit method, a planet passing in front of its star dims the star. An Earth-size planet crossing a Sun-like star dims it by only about 80 parts per million, which is 0.008 percent. The transit method only works when a planet's orbit lines up almost perfectly with our view, so many planets are never seen this way. Larger planets make a dip that is more noticeable and easier to detect. In the wobble method, a planet tugs its star back and forth, and repeated shifts in a star's spectrum from red to blue can be caused by one or more orbiting planets. Jupiter makes the Sun move by about 13 meters per second, while Earth makes it move only about 9 centimeters per second, a speed that is very hard to measure.

The later terms are where the equation becomes uncertain. In 1961 the conference estimated between 1000 and 100,000,000 civilizations in the Milky Way. Many of its terms are largely or entirely conjecture, and the fraction of life-bearing planets that develop intelligence remains particularly controversial. Depending on how long advanced civilizations last, the answer can swing from one civilization to about a million. These numbers are poorly known, and the uncertainty grows with each factor, so the equation organizes our ignorance and does not make a prediction. The Fermi paradox is the seeming inconsistency between the lack of evidence of extraterrestrial civilizations and the high likelihood that they exist. It grew from a 1950 lunch conversation at Los Alamos in which physicist Enrico Fermi asked a question about where everybody was. It is an open question with no agreed answer.

Scientists have offered many possible answers to the Fermi paradox, and none is confirmed. The Great Filter, an idea introduced by Robin Hanson in 1996, says something natural makes advanced civilizations unlikely to arise. The Rare Earth hypothesis says complex life depends on so many special conditions that it is exceedingly unusual. One radio signal remains a puzzle. The Wow! signal was a strong, narrow radio signal picked up on August 15, 1977 by Ohio State University's Big Ear telescope, which was searching for extraterrestrial intelligence. It lasted the full 72 seconds the telescope could watch that spot, at 1420 MHz, and astronomer Jerry Ehman noticed it days later in the recorded data. It has never been heard again, and no explanation has been confirmed. A one-time signal cannot be checked, which is why it stays a puzzle and not evidence of anyone's message. A 2017 idea that comets caused it was dismissed by astronomers, including members of the original Big Ear team.

The other half of the story is about reports from the sky. UFO stands for unidentified flying object: something in the sky that the person who saw it could not identify at that moment. NASA announced in June 2022 that it would commission an independent study team to examine unidentified anomalous phenomena (UAP) from a scientific perspective. The team had about eight to twelve members from academic, independent and commercial institutions. It published its final report on September 14, 2023, a series of recommendations for how the agency could help move understanding of UAP forward. NASA's study found no evidence that UAPs are extraterrestrial, and says there are too few high-quality observations to draw scientific conclusions. NASA uses the term UAP to be consistent with the National Defense Authorization Act. One report on its own is not evidence of what something was.

The story of reports from the sky began in 1947. Businessman Kenneth Arnold said he saw nine fast objects near Mount Rainier that moved "like saucers skipping on water"; a newspaper wrongly described them as saucer-shaped, which gave us the term flying saucer. In June 1947 rancher W.W. Brazel found unusual debris of tinfoil, rubber strips and sticks near Roswell, New Mexico. The Roswell Army Air Field press release said a "flying disc" had been recovered. Almost immediately the military said the "saucer" was a weather balloon carrying a radar target made of foiled paper on a balsa wood frame, though Brazel did not believe the debris was from a weather balloon. In 1994 the Air Force reported that the debris came from Project Mogul, a secret Army Air Forces balloon-borne research project meant to detect Soviet nuclear tests.

Washington, D.C., in July 1952 is one case. A series of unidentified flying object reports were made from July 12 to 29, and the best known were on the weekends of July 18-19 and July 26-27. Airport controller Edward Nugent spotted seven objects on his radar, and one object vanished in all three radar centers at the same time. The reports drew the largest Pentagon press conference since World War II, on July 29, 1952. An analysis by the Civil Aeronautics Administration found a temperature inversion, a warm moist layer of air over a cool dry one, in almost every instance, and General Samford said the unknown radar contacts were not caused by solid material objects. Not everyone was convinced by that explanation. Both the radar reports and the visual reports came from near National Airport.

The University of Colorado UFO Project, led by physicist Edward Condon and funded by the Air Force from 1966 to 1968, delivered its report in November 1968, and the Air Force released it in January 1969. Condon wrote that nothing from the study of UFOs in 21 years had added to scientific knowledge and that further extensive study probably could not be justified on those grounds. The report was written by 37 scientists who studied 59 reports in detail, and with fewer reports it led to the end of Project Blue Book on Dec. 17, 1969. Of 12,618 sightings reported to the project, 701 had stayed unidentified. Each Blue Book report had been classed as identified with an astronomical, atmospheric or human-made cause, or unidentified. Blue Book's own conclusions were that no reported UFO was a threat to national security.

Two more parts of the story come from before and after the Condon Report. In 1953, a panel led by physicist H.P. Robertson had concluded that 90 percent of reports could be easily attributed to astronomical or weather phenomena or to ordinary objects such as aircraft, balloons and birds. Parts of its report stayed classified until 1979, which fueled suspicions of a cover-up. In 2014 the CIA declassified a history of its U-2 spy plane program showing that U-2 and later OXCART flights accounted for more than half of all UFO reports in the late 1950s and most of the 1960s, a cause the public could not have known about at the time. Ordinary aircraft are a common cause of UFO reports: seen from odd angles or in poor light they can look unusual, and secret high-flying planes in the 1950s and 1960s caused many reports.

The Pentagon's All-domain Anomaly Resolution Office, called AARO, says many reports are listed as unidentified simply because sensors could not collect enough information to name what was seen. Most UAP reports produce very limited data, and NASA says that without an extensive set of data it is nearly impossible to verify or explain an observation. AARO says the most useful reports include recorded, time-stamped and geolocated data for both observer and object, so they can be compared with air traffic, balloon and satellite tracking databases. AARO lists common causes of UAP reports: windborne debris like plastic bags and mylar balloons, birds, balloons, aircraft, drones, rocket launches, and sunlight glinting off satellites. As of November 2026, asked whether it has found any evidence of extraterrestrial technology, AARO answers "No," adding that examination of UAP reports is ongoing.

Claims about life on other worlds meet a similar standard. A potential biosignature is a substance or structure that might have a biological origin but needs more data or further study before scientists can reach a conclusion about life. NASA reported in 2025 that a Perseverance rock sample called Sapphire Canyon contains potential biosignatures: spots with two iron-rich minerals. As of November 2026, the finding stands as a question for further study, not a confirmed discovery, because the same minerals can also form without life. Starlight that passes through or comes from a planet's atmosphere can be split into a spectrum, and scientists read its color bands like a bar code to see which molecules are present. As of November 2026, the Habitable Worlds Observatory is planned to be the first telescope designed specifically to search for signs of life on planets orbiting other stars.

Words to know

• equation: math sentence
• paradox: puzzling contradiction
• conjecture: educated guess
• biosignature: possible life sign
• inversion: layered air