10.6 - Planetary Atmospheres
Exoplanets
- An exoplanet is a planet that orbits a star outside our Solar System.
- Thousands of exoplanets have now been discovered orbiting stars across many other planetary systems.
- A key reason to study them is to find out whether any could support life.
Detecting exoplanets
Exoplanets are too faint to see directly next to the glare of their star, so they're found indirectly from the light of the star they orbit.
The transit method
- When a planet passes directly between a star and us, it blocks a small fraction of the starlight.
- This causes a regular, repeating dip in the brightness of the star that reveals an orbiting planet.
The wobble method
- A star and its planet both orbit their common centre of mass.
- As the star moves slightly towards us and then away from us, the wavelength of its light shifts because of the Doppler effect.
- Detecting this regular shift in the star's light reveals the orbiting planet.
Reading a planet's atmosphere from starlight
- When an exoplanet transits its star, some starlight passes through the planet's atmosphere on its way to us.
- Gases in that atmosphere absorb light at particular wavelengths, leaving dark absorption lines in the spectrum of the starlight - a fingerprint of the gases present.
Signs of possible life
- Physicists look especially for oxygen and water, because both are essential for life.
- Detecting oxygen in particular points to the possibility that the planet could support life.
TermDefinition
ExoplanetA planet that orbits a star outside our Solar System.
Absorption linesDark lines in a spectrum showing where light of specific wavelengths has been absorbed by a gas.