What a decade of exoplanet data taught us
Thousands of confirmed worlds later, the most surprising result is how ordinary our own solar system looks to be rare.
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When astronomers began confirming planets around other stars in the 1990s, each discovery was an event. Today there are more than five thousand confirmed, and the catalogue grows by dozens each month. The excitement has shifted from finding planets to understanding them.
The most important lesson so far is humbling: our solar system may not be the template we assumed it was.
A zoo of unexpected worlds
Before the data arrived, theory predicted that giant planets would sit far from their stars, as Jupiter and Saturn do. Instead, early surveys found hot Jupiters, enormous planets orbiting closer than Mercury does to the Sun.
Later surveys revealed the most common type of planet of all, something between the size of Earth and Neptune, a category that does not exist in our own neighbourhood.
The bias in the sample
Part of the surprise is a selection effect. Most detection methods favour large planets on short orbits, because they produce stronger and more frequent signals. Careful statistical work is needed to correct for this and to estimate what the full population looks like.
Detection is not the same as abundance. The planets that are easiest to find are not necessarily the most common.
Atmospheres come into view
The newest telescopes can split the light that passes through a planet's atmosphere and read the chemical fingerprints within. Astronomers have detected water vapour, carbon dioxide and clouds of exotic minerals on worlds hundreds of light years away.
- Hot gas giants show winds faster than the speed of sound.
- Some small planets appear to be covered in deep oceans or thick hazes.
- A few rocky worlds seem to have lost their atmospheres entirely.
What about life?
No confirmed sign of biology has been found, and scientists are careful to say so. What has changed is the toolkit. For the first time, there is a realistic path to examining the atmospheres of rocky planets in the habitable zones of small stars.
The universe has turned out to be far more inventive than our theories, which is exactly why we keep looking.
The next decade
New observatories will image planets directly and survey many more stars. Each answer will probably generate more questions, and for working astronomers that is the point.
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