Is there life anywhere else in the universe ? This is one of the most profound questions scientists have ever asked . So far we have found no definite answer — but we have very good reasons to keep searching , and the discoveries so far are exciting . Searching carefully and honestly , rather than simply guessing , is exactly how science works .
✨ Scientists estimate there are roughly 300 billion stars in the Milky Way alone , and most of them have planets . In 2022 the total number of confirmed planets orbiting other stars — called exoplanets — passed 5,000, with many more candidates waiting to be confirmed .
Most exoplanets are too far away and too dim to photograph directly . So how do we find them? One clever method is the 'transit' technique : when a planet passes in front of its star , it blocks a tiny fraction of the starlight , making the star dim ever so slightly . By watching for these regular little dips in brightness , astronomers can detect planets they cannot even see.
📷 X-ray: NASA/CXC/SAO/R. DiStefano, et al.; Optical: NASA/ESA/STScI/Gendler; Illus · Public domainLife as we know it needs liquid water to exist . That is why scientists focus on the 'habitable zone' — the range of distances from a star where a planet is neither too hot nor too cold for liquid water on its surface . Earth sits perfectly in the Sun's habitable zone . Many exoplanets have been found in the habitable zones of their stars .
🃏 Flip each card to explore key ideas in the search for life.
? Exoplanet A planet orbiting a star other than our Sun
? Habitable zone The range of distances from a star where liquid water could exist
? Biosignature A chemical or feature in a planet's atmosphere that could indicate life
? SETI The Search for Extra-Terrestrial Intelligence — listening for signals from space
Tap each card to see the answer.
✨ Within our own solar system , several moons may hide liquid water beneath icy shells . Jupiter's moon Europa and Saturn's moon Enceladus both show strong signs of subsurface oceans — they are among the most exciting targets in the search for life close to home .
📷 NASA / JPL / University of Arizona · Public domainWhen scientists talk about alien life , they usually do not mean little green visitors . The most likely life to find first would be tiny — microbes , similar to the simplest living things on Earth . Finding even a single living microbe beyond Earth would be one of the greatest discoveries in all of history , because it would tell us that life is not unique to our planet .
🧩 Order the steps astronomers take to find and study an exoplanet.
Study the planet's atmosphere for biosignatures ? ⤒ ↑ ↓ ⤓
Confirm a planet is orbiting the star ? ⤒ ↑ ↓ ⤓
Notice a tiny regular dip as a planet passes in front ? ⤒ ↑ ↓ ⤓
Watch a star's brightness very carefully ? ⤒ ↑ ↓ ⤓
Astronomers use the James Webb Space Telescope to study the atmospheres of exoplanets . They look for 'biosignatures' — gases like oxygen , methane , or ozone that might suggest living things are producing them. If a planet had both oxygen and methane together , that would be very surprising , because these gases react and destroy each other quickly without a continual source — and life could be that source .
✨ Life on Earth exists in some astonishing places : boiling hot springs , frozen Antarctic ice, pitch-dark ocean vents kilometres below the surface , and highly acidic lakes . These 'extremophiles' show that life can adapt to conditions very different from our everyday experience .
🔗 Match each method or idea in the search for life to what it does.
Listens for signals from other civilisations Earth life in a very harsh place A gas that might be a sign of life Finds planets by tiny dips in starlight
Listens for signals from other civilisations Earth life in a very harsh place A gas that might be a sign of life Finds planets by tiny dips in starlight
Listens for signals from other civilisations Earth life in a very harsh place A gas that might be a sign of life Finds planets by tiny dips in starlight
Listens for signals from other civilisations Earth life in a very harsh place A gas that might be a sign of life Finds planets by tiny dips in starlight
The SETI programme (Search for Extra-Terrestrial Intelligence ) has been scanning the sky for radio or laser signals from other civilisations since 1960. So far no confirmed signal has been detected . The famous Fermi Paradox asks : if the universe is so vast and old, why have we not heard from anyone yet? Nobody knows the answer , and it remains one of science's greatest mysteries .
📷 Serge Yerin (YeS) · CC BY 4.0🗂️ Sort each place or clue into 'might support life' or 'very unlikely to support life'.
Europa (liquid water ocean beneath ice)
? Might support life Very unlikely to support life
Mercury (no atmosphere, extreme temperatures)
? Might support life Very unlikely to support life
Enceladus (ocean water jets into space)
? Might support life Very unlikely to support life
The surface of the Sun (millions of degrees)
? Might support life Very unlikely to support life
An exoplanet in its star's habitable zone
? Might support life Very unlikely to support life
✍️ Fill in the facts about the search for life.
A planet orbiting another star is called an _____ . The zone around a star where liquid water can exist is the _____ zone . A gas that might hint at living things is called a _____ .
exoplanet habitable biosignature asteroid corona
