Extraterrestrial planets, over 100 and counting.

Imaging extrasolar planets is currently out of reach as shown in the first part, but subtle consequences of their existence can be detected. Bodies orbiting a star, induce wobbling movements on it that when measured, not only betray their existence but also their orbital period, distance to the star and minimum mass.

Measuring wobble

Not that this wobble can be seen directly, much less measured by observation through a telescope from Earth, but fortunately there is a trick that comes in handy for the job, the Doppler effect.
When a light source - the star in this case - approaches the observer, its emitted light wavelength gets "compressed", meaning colors are shifted towards the blue, while when it recedes, an opposite shift to the red occurs. You can witness this effect with sound waves, for if you stand by a railroad crossing and a train is approaching, you will hear the siren pitch change from high to low when it passes by. The underlying physical phenomena is the same, the sound wavelength shortens (higher pitch) during approach, and lengthens (lower pitch) when receding.

The starlight is split in its different color components with a special instrument called spectroscope. This is something like the prism used in school to decompose sunlight in a rainbow, only it uses a diffraction grating to achieve much better resolution and precision. Within the generated "rainbow", appear narrow dark bands corresponding to atoms and molecules present in the star's atmosphere that absorb certain very precise light wavelengths. This is how we got to know that oxygen, helium, hydrogen, calcium and many other elements are present in celestial bodies like stars and nebulae.
By the way, this is the same principle used by Edwin Hubble decades back to detect galactic motions leading to the discovery of universal expansion.

The first team to announce the discovery of an exoplanet was led by Michel Mayor and Didier Queloz from the Geneva Observatory in Switzerland. The system found by Mayor and Queloz in 1995 does not look much like the Solar System, for it consists of a 60% Jupiter size or larger body orbiting the sunlike star 51 Pegasi at a 5% the Earth - Sun distance, implying a 4.2 day orbital period and a day temperature of about 1000 oC.

To detect and measure the Doppler shift introduced by planetary tugging is tricky, and the spectrographic sensitivity available for Mayor and Queloz was good for a 13 m/s. (29 mph) or higher speed variations. This was enough for 51 Pegasi at 53 m/s. but should barely miss something like Jupiter in the Solar System, for its tugging on the Sun is just 12.5 m/s.

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