Worlds Around Other Suns

An exoplanet is a planet orbiting a star other than the Sun. The first confirmed detections transformed a question that once belonged mostly to science fiction into an observational field of astronomy.

Finding a planet without seeing it

Most exoplanets are difficult to photograph directly because their host stars are vastly brighter. Instead, astronomers often detect planets indirectly. In the transit method, a planet passes in front of its star and causes a tiny, measurable dip in brightness. In the radial-velocity method, the planet's gravity makes its star move slightly back and forth, producing a detectable shift in the star's spectrum.

Hot worlds and distant worlds

Some exoplanets orbit so close to their stars that a year lasts only a few days. Others travel on much wider orbits. Astronomers have also found giant planets, rocky planets, planets with highly eccentric orbits and systems containing several worlds packed close to their host star.

The search for atmospheres

During a transit, some starlight passes through a planet's atmosphere. By analysing the wavelengths absorbed by that atmosphere, researchers can look for signatures of gases and learn about temperature, chemistry and clouds. These measurements are technically demanding, but they are beginning to turn distant planets into objects that can be studied as environments rather than points of light.

What makes a world interesting?

Habitability is more complicated than simply asking whether a planet is in its star's habitable zone. Atmospheric composition, stellar activity, planetary mass, geology and the presence of liquid solvents can all matter. The search is therefore not just for another Earth, but for a better understanding of how planetary environments arise.

Every new exoplanet adds another data point to the history of planets. Our Solar System is no longer the only example we have; it is one system among a growing population of planetary architectures.