For most of human history, the question of whether other stars had planets was pure speculation. As recently as 1990, the answer was genuinely unknown. Today the confirmed count is above 5,000 and climbing, with thousands more candidates waiting.
What changed was not better telescopes for looking at planets. Planets are still, in almost every case, far too faint and far too close to their star to see. What changed was the realisation that you can detect something you cannot see by watching what it does to something you can.
The core problem
Imagine a searchlight in Los Angeles and a moth flying near the bulb. Now try to photograph the moth from New York.
That is roughly the challenge. A star like our Sun is around a billion times brighter than a planet like Earth reflecting its light. And from any realistic distance, the two appear separated by a hair's width. Direct imaging is possible in a small number of favourable cases, but it cannot be the main method.
So astronomers use indirect ones.
Method 1: the transit
This is the workhorse, responsible for the large majority of discoveries.
If a planet's orbit happens to be edge-on from our point of view, the planet passes in front of its star once per orbit and blocks a tiny fraction of the light. For a Jupiter-sized planet crossing a Sun-like star, the dip is about 1 percent. For an Earth-sized planet, about 0.008 percent — eighty parts per million.
Measuring that requires photometry stable to a few parts per million, which is why the technique really took off from space, above the shimmer of the atmosphere. NASA's Kepler mission, launched in 2009, stared at a single patch of sky containing about 150,000 stars and watched for these dips. It found over 2,700 confirmed planets. Its successor TESS now surveys almost the whole sky for the same signature around brighter, nearer stars.
The transit tells you two things directly. The depth of the dip gives the planet's radius relative to the star. The interval between dips gives the orbital period, and from that the orbital distance.
The catch: the alignment has to be right. For a planet in an Earth-like orbit, the geometric chance of a transit being visible from a given direction is under half a percent. So for every transiting planet we see, hundreds are missed.
Method 2: the wobble
A star does not sit still while a planet orbits it. The two bodies orbit their common centre of mass, so the star traces a small circle of its own.
We cannot see that motion, but we can measure it in the star's light. When the star moves toward us, its spectral lines shift slightly blue; when it moves away, slightly red. This is the radial velocity or Doppler method.
Jupiter makes the Sun move at about 12 metres per second. Earth moves it about 9 centimetres per second — walking pace for a giant ball of plasma, measured from light-years away. Modern spectrographs such as ESPRESSO in Chile can reach a precision of a few centimetres per second, which is one of the quiet marvels of modern instrument-building.
Radial velocity gives the planet's mass, though technically a minimum mass, since an orbit tilted relative to our line of sight produces a smaller apparent wobble.
This is why the two main methods are so powerful together. Transit gives radius; radial velocity gives mass. Combine them and you get density — and density tells you whether you are looking at a ball of rock, a water world, or a gas giant.
The first confirmed planet around a Sun-like star, 51 Pegasi b, was found this way in 1995 by Michel Mayor and Didier Queloz, who shared a Nobel Prize for it in 2019.
Method 3: gravitational lensing
Mass bends light. When a foreground star drifts across our line of sight to a more distant star, its gravity focuses the background light and the distant star briefly brightens. If the foreground star has a planet, the planet adds its own small, sharp spike to that brightening curve.
Microlensing is uniquely good at finding planets far from their stars, and even free-floating planets bound to no star at all. Its drawback is brutal: the alignment never repeats. Each detection is a one-off event that can never be observed again.
Method 4: direct imaging
Sometimes it does work. If a planet is very large, very young — and therefore still glowing with the heat of its own formation — and orbits far from its star, a telescope with a coronagraph to mask the starlight can capture it as a genuine dot of light.
The HR 8799 system, with four giant planets imaged over years of orbital motion, is the showcase example. Direct imaging is the only method that lets us take a spectrum of the planet's own light rather than inferring things through a star's, which makes it the long-term goal even though it currently works for only a small and unrepresentative sample.
What we have found
The first surprise was how strange the early discoveries were. 51 Pegasi b is a gas giant orbiting its star every 4.2 days, closer than Mercury is to the Sun. These "hot Jupiters" were not supposed to exist — giant planets were meant to form far out where it is cold enough for ice. Their existence forced a rewrite of planet formation theory, introducing the idea that planets migrate inward after forming.
The second surprise was the most common type of planet in the galaxy: something between Earth and Neptune in size, often called a super-Earth or sub-Neptune. Our Solar System has nothing in that range at all. The most typical planet in the Milky Way is a kind of world we have never visited.
Other findings worth knowing:
- Planets are ordinary. Statistically, most stars have at least one.
- Red dwarfs, the most common stars, frequently host compact systems of small planets. TRAPPIST-1, 40 light-years away, has seven Earth-sized planets, several in the temperate zone.
- Proxima Centauri, the nearest star to the Sun, has a roughly Earth-mass planet in its habitable zone.
- Circumbinary planets orbiting two stars at once are real.
Reading atmospheres
The current frontier is not finding planets but characterising them.
When a planet transits, a sliver of starlight passes through its atmosphere on the way to us. Molecules there absorb specific wavelengths, imprinting a fingerprint on the spectrum. Comparing the spectrum during transit to the spectrum outside it isolates the atmosphere's signature.
The James Webb Space Telescope was built partly for this. It has detected carbon dioxide, water, methane and sulphur dioxide in exoplanet atmospheres, the last being evidence of photochemistry driven by starlight.
The long-term prize is a combination of gases that chemistry alone struggles to explain — oxygen alongside methane, for instance, since they react with each other and should not coexist without something continuously producing them. That would be a biosignature: not proof of life, but a result demanding an explanation.
We are not there yet. Rocky planets have thin atmospheres and faint signals, and every claimed detection so far has come with heavy caveats. But the gap between "we do not know if other planets exist" and "we are arguing about the chemistry of specific atmospheres on named worlds" was closed in roughly thirty years. That is a remarkably short time for a question that old.
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