Two questions run through the search for life beyond Earth. One asks what is out there, around other stars. The other asks what people are seeing in the sky. Start with the first, because it has numbers. As of October 2026, NASA has confirmed more than 6,000 exoplanets, which are planets that go around a star other than our Sun, out of billions believed to exist, and some are rogue planets that orbit no star at all. Scientists use telescopes, some of them out in space, to look for them. The second question starts with a word. UFO stands for unidentified flying object: something in the sky that the person who saw it could not identify at that moment. Bright planets such as Venus and Jupiter, and meteors, are sometimes reported as strange lights because they can look like they are hovering or moving. From 1947 to 1969 the Air Force investigated UFO reports under Project Blue Book. Of 12,618 sightings reported, 701 stayed unidentified.

The transit method gives the first set of measurements. When a planet crosses in front of its star, the star dims a little, and for small planets the dimming is tiny. An Earth-size planet crossing a Sun-like star dims it by only about 80 parts per million, which is 0.008 percent, while one big planet, HD 209458 b, dims its star by 1.7 percent. The method also has a geometric limit, because it 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. Two telescopes have used the transit method on a large scale. The Kepler space telescope launched on March 6, 2009, and left a legacy of more than 2,600 planet discoveries before NASA retired it in October 2018. TESS, the Transiting Exoplanet Survey Satellite, launched on April 18, 2018, and it watches patches of sky for transits. As of November 2026, TESS has finished a two-year primary mission and is now in an extended mission.

The wobble method meets the same difficulty, because the gravity of an orbiting planet tugs the star back and forth, and astronomers look for shifts in the star's spectrum. For small planets the effect is very small. Jupiter makes the Sun move by about 13 meters per second. Earth makes it move only about 9 centimeters per second, a speed that is very hard to measure. Repeated shifts from red to blue can be caused by one or more orbiting planets, but an Earth-size planet is a far smaller signal than a giant one. Both of these methods work best on large planets, so small planets are the hardest to find. A wobble shows up as a shift in the star's light, and not as a picture of the planet.

Finding a planet is not the same as finding a place where life could live. The habitable zone is the area around a star where a planet could have liquid water on its surface, neither too hot nor too cold. It moves with the star: every star has one, but where it lies depends on the star's size and brightness, and it is wider for hotter stars. Red dwarf stars have a narrow zone, and their flares could sterilize planets orbiting close in. Being in the zone does not make a planet habitable. A planet also needs the right size, temperature and composition to host liquid water. The zone only tells scientists where to look first. It is a guide and not a result.

The TRAPPIST-1 system is an 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. That wording is careful: more likely to have liquid water is not the same as having it. Habitable zones are also called Goldilocks zones, and the name fits, because conditions are not too hot and not too cold, and liquid water is essential for life as we know it. How could anyone tell whether a distant planet has life? Starlight that passes through or comes from a planet's atmosphere can be split into a spectrum. Scientists read its color bands like a bar code to see which molecules are present, and they look for gases that might suggest life. 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.

Reports of strange lights run into a limit of their own. One report on its own is not evidence of what something was. NASA announced in June 2022 a study of unidentified anomalous phenomena (UAP), the term for such reports. Most UAP sightings produce very limited data, and NASA says that without an extensive set of data it is nearly impossible to verify or explain an observation. A cause can also hide for years: 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.

Numbers can also organize a guess. The Drake equation, formulated largely by astrophysicist Frank Drake and first discussed in 1961 at a SETI conference in Green Bank, West Virginia, estimates the number of technically advanced civilizations in the Milky Way from a chain of factors. It multiplies the rate of star formation, the share of stars with planets, planets suited to life, and the chances that life, intelligence and radio technology arise, by how long such civilizations last. These numbers are poorly known, and the uncertainty grows with each factor. Its answer can swing from one civilization to about a million depending on how long advanced civilizations last, so it organizes what we do not know more than it predicts an answer.

One famous radio signal shows how hard a single observation is to use. 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. Astronomer Jerry Ehman noticed it days later, in the recorded data, so the signal had passed by the time anyone saw it.

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, and it sits beside the Drake equation, whose answer can swing from one civilization to about a million depending on how long advanced civilizations last. As of November 2026, scientists have not found life anywhere beyond Earth, and the search goes on. The paradox puts two things side by side, which are the lack of evidence of extraterrestrial civilizations and the high likelihood that they exist.

Words to know

• exoplanets: far-off planets
• spectrum: color bands
• habitable: livable
• atmosphere: a planet's air
• declassified: no longer secret