The first confirmed planet orbiting a Sun-like star was announced in 1995. It was nothing like what anyone expected: a gas giant roughly half the mass of Jupiter, orbiting its star every four days at a distance far closer than Mercury is to the Sun.
Thousands of confirmed planets later, the pattern holds. The main lesson of exoplanet astronomy is that our solar system is not the template.
The problem with looking directly
Seeing an exoplanet is hard for two reasons that compound each other.
Planets are faint, shining mostly by reflected light. And they sit extremely close to something overwhelmingly bright. From a distance of tens of light years, an Earth-like planet is roughly ten billion times dimmer than its star and separated by a fraction of an arcsecond.
Direct imaging works only in favourable cases: young, hot, massive planets still glowing from their formation, orbiting far from their stars, observed with coronagraphs that block the starlight. Most exoplanets are found without ever being seen.
The two workhorse methods
Transits
If a planet’s orbit is aligned so that it passes between us and its star, the star dims slightly. For a Jupiter-sized planet crossing a Sun-like star, the dip is about one percent. For an Earth-sized planet, it is around one hundredth of a percent.
Measuring that requires exceptional photometric stability, which is why the technique came into its own with dedicated space telescopes staring at the same field for years. The payoff is rich:
- The depth of the dip gives the planet’s radius relative to the star.
- The interval between dips gives the orbital period, and therefore the orbital distance.
- The shape of the dip constrains the orbital geometry.
- Light filtering through the planet’s atmosphere during transit carries a chemical fingerprint, allowing atmospheric composition to be measured.
The limitation is geometric. Only a small fraction of planetary systems happen to be edge-on from our vantage point, so transits find a biased sample — but a statistically correctable one.
Radial velocity
A planet does not orbit its star. Both orbit their common centre of mass, which means the star traces a small circle in response to the planet’s pull.
That motion shifts the star’s spectral lines by the Doppler effect — toward the blue as it approaches, toward the red as it recedes. Jupiter makes the Sun wobble at about twelve metres per second. Earth manages roughly nine centimetres per second.
Detecting shifts that small requires spectrographs of extraordinary stability, calibrated against laser frequency combs and housed in temperature-controlled vacuum chambers. The method gives the orbital period and a minimum mass, since it cannot distinguish a heavy planet in a tilted orbit from a lighter one seen edge-on.
Combine a transit and a radial velocity measurement and you get both radius and true mass — and therefore density, which tells you whether a planet is rock, ice, or gas.
The other techniques
Two further methods fill in parts of the picture the main two miss.
Gravitational microlensing exploits the chance alignment of a foreground star with a distant background one. The foreground star’s gravity magnifies the background light, and a planet orbiting it adds a brief extra spike. Microlensing is sensitive to planets at wide separations and to free-floating planets bound to no star at all, but events are one-off and cannot be re-observed.
Astrometry tracks the star’s positional wobble on the sky rather than its velocity along the line of sight, complementing radial velocity for wide orbits.
What the census shows
Several findings have reshaped expectations.
Planets are common. The statistics suggest most stars host at least one, making planets the rule rather than the exception.
The most abundant planets are of a type absent from our solar system: worlds between Earth and Neptune in size, often called super-Earths or sub-Neptunes. There appear to be two distinct populations separated by a gap in the radius distribution, probably reflecting whether a planet retained or lost a thick hydrogen envelope.
Hot Jupiters — giant planets in very tight orbits — exist, though they are rarer than early surveys implied, since they are the easiest planets to find and therefore heavily over-represented in raw counts. They almost certainly formed further out and migrated inward.
Small red dwarf stars, the most common type in the galaxy, frequently host compact multi-planet systems. Because these stars are dim and small, planets in their habitable zones transit more deeply and more often, making them the most accessible targets for atmospheric study.
The next question
Cataloguing is giving way to characterisation. The interesting question is no longer how many planets exist but what they are made of, whether they hold atmospheres, and what those atmospheres contain.
That is a much harder measurement, and the signals are close to the limits of current instruments. But it is the direction the field is now pointed.
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