In 1991 we knew of no planets around other stars. Now we know of more than 6,000. Find out how you detect a world you can never see, and what the numbers say about life.
Almost none of these planets have ever been seen. They are far too faint, lost in the glare of their stars. We find them by what they do to the light, and the results have kept breaking the rules of what astronomers expected a planetary system to look like.
| Method | What you measure | What it gives you |
|---|---|---|
| Transit | The dip in starlight as a planet crosses the star (Kepler, TESS, JWST) | Radius, orbit period. Most planets have been found this way. |
| Radial velocity | The star's wobble, from Doppler shifts | Minimum mass, orbit period |
| Direct imaging | Block the star's light and photograph the planet | Rare: only works for young, large planets far from their stars |
| Microlensing | A star and planet in the foreground bend and brighten the light of a background star | Mass; can find planets with no star at all |
| Astrometry | The star's tiny sideways wobble on the sky (Gaia) | Mass and orbit, for massive planets |
Combine transit and wobble and you get both radius and mass, and so the planet's density. That tells you whether it is rock, water or gas.
In 1995, Michel Mayor and Didier Queloz found a planet around the Sun-like star 51 Pegasi. It had about half Jupiter's mass, but orbited in just 4.2 days, much closer than Mercury is to the Sun. Theories at the time said gas giants could only form far from their star, where ice is stable. A hot Jupiter should not exist. Then they turned out to be common among the planets that are easiest to find. The explanation is migration: planets can form far out and then drift inwards. The pair won the 2019 Nobel Prize in Physics.
The second surprise came from Kepler (launched 2009). The most common planets are between Earth and Neptune in size, super-Earths and mini-Neptunes. Our solar system has none. Our own arrangement, it turns out, is not typical.
The habitable zone is the range of distances from a star where a planet's surface could hold liquid water. For the Sun it runs from roughly 0.95 to 1.7 AU (one AU is the Earth–Sun distance, about 150 million km), depending on assumptions. Dimmer stars have zones closer in. Remember, a planet in the zone is only possibly habitable. It also needs an atmosphere, and the right amount of it.
The famous test case is TRAPPIST-1, a tiny red star 40 light-years away with seven Earth-sized planets. All seven orbit closer to their star than Mercury does to the Sun, with years of 1.5 to 19 days. Three of them (e, f and g) are in the habitable zone. They look like a promising place for water until you ask the next question, as JWST did in 2023: planets b and c show no sign of a thick atmosphere. They look like bare rock. Red dwarf stars also throw violent flares, and a planet so close is probably locked with one side always facing the star.
When a planet transits, a sliver of starlight passes through its atmosphere on the way to us. Each gas absorbs particular colours. The pattern of missing colours is a chemical fingerprint. This is transmission spectroscopy, and JWST does it. A biosignature is a combination of gases that is hard to explain without life: for example oxygen and methane together, since they react and destroy each other unless something keeps making them.
In 1961 the astronomer Frank Drake gathered a small meeting at the Green Bank radio observatory and wrote down seven terms:
N is the number of civilisations in the galaxy we could contact right now. The other terms: the rate of star formation (R*), the fraction of stars with planets (fp), the number of habitable planets per such star (ne), the fraction on which life starts (fl), becomes intelligent (fi) and builds technology we can detect (fc), and how long such a civilisation lasts (L).
The first three used to be pure guesses. Thanks to planet hunters, they are now measured. The last four remain unknown, because we have only one example of life, and the answer swings from less than one to millions depending on what you guess. The physicist Enrico Fermi's lunchtime question in 1950, “Where is everybody?”, sits behind it: if the galaxy is old and life is common, why have we seen no sign of anyone? Maybe one of the steps is far rarer than we think, which is sometimes called a Great Filter. The only honest answer is that we don't know.
Build a star and a planet and watch the dip. The left panel shows the star and the planet passing. The right shows the brightness you would measure.
Set the seven terms. The first three come with measurements. The last four are guesses, and the result shows how much the answer depends on them.
Five questions.
A news story claims: “Earth-like planet found in the habitable zone!” Explain, using the words atmosphere, habitable zone and biosignature, why that headline does not mean there is life, and what you would want to measure next.