The same Doppler Effect that changes a siren’s pitch as it passes by an observer is used by planetary scientists to find exoplanets. Exoplanets orbit other stars outside our solar system. They are new worlds that may be habitable. The technique is known as Doppler spectroscopy, also referred to as the radial-velocity method, or the “wobble method.”
How does it work?
Spectrographs are high-resolution prism-like instruments that are able to separate light waves into different colors. When mounted on ground based telescopes they can be used to measure redshifts and blueshifts from astronomical objects extremely far away, such as a star. Scientists can also measure abnormal periodic shifts, otherwise referred to as spectral wobbles. These wobbles are often caused by an unseen planet orbiting a given star.
All moving objects undergo a Doppler shift. Stars themselves are in motion, which causes a normal Doppler shift. This can make detecting the slight irregularities difficult at first. But after careful observations over long periods of time, the repeated motions due to the influence of an unseen orbiting planet become obvious.
The star will move in very small, but detectable circles or ellipses, depending on the gravitational influence of its smaller companion. These slight differences actually affect the star’s normal light spectrum and they change the wavelength of light emitted. A star moving towards us will have a shorter, more compressed wavelength, resulting in the light emitted being blueshifted. Whereas if the star is moving away, then its wavelength will be longer, more stretched out, thus the light will be redshifted.
The shifts caused by the gravitational pull from the planet happen at regular intervals. If the repetition is observed over a certain period of time, it can be determined the shift is caused by a body in orbit around the star. The next step is to determine if the body is actually a planet.
According to the Planetary Society, if the object has a mass lower than around 10 times that of Jupiter, which is approximately 3,000 times the mass of Earth, then it is most likely a planet. Larger-mass objects are most likely stars.
“If this shift is large, then it must be caused by another star pulling it, but if this shift is small, then it is likely caused by a low-mass body like an exoplanet,” says Joshua Winn, an assistant professor in MIT’s Department of Physics.
This technique works best for finding exoplanets with significant mass that are orbiting reasonably close to their parent star. This is because the two factors that affect gravity are mass and distance. The larger the mass of the planet and the closer it is to the star, the larger gravitational tug it will exert on the star, in turn creating a larger Doppler shift. And larger shifts are easier to detect.
Observing how the Doppler shift of a star changes over time in respect to its daughter planet can also reveal a planet’s orbital period (the length of its year), its lowest feasible mass, and the shape of its orbit. To date, 910 out of the over 4,000 known exoplanets (~22.75%) have been detected using the Doppler effect.
The success of Doppler spectroscopy is due to the development of extremely sensitive spectrographs such as:
- ESPRESSO, (Echelle SPectrograph for Rocky Exoplanet and Stable Spectroscopic Observations) on the Very Large Telescope,
- EXPRES (Extreme Precision Spectrometer) on the Lowell Observatory Discovery Channel telescope
- HARPS3 (High Accuracy Radial-velocity Planet Searcher the 3rd) in construction for the Isaac Newton Telescope at La Palma
The image above shows three of the four Unit Telescopes of the European Southern Observatory’s (ESO) Very Large Telescope (VLT) at the Paranal Observatory in Chile’s Atacama Desert. The striking Milky Way is clearly visible in the sky 300 nights per year, making this location perfect for research. The VLT is the most advanced optical instrument in the world, giving us the opportunity to find new worlds.