Astronomers have been trying to discover evidence that worlds exist around stars other than our Sun since the 19th century. By the 1990s, technology finally caught up and led to the first discovery of a planet orbiting another sun-like star, Pegasi 51b. Why did it take so long, and what techniques do astronomers use to find these distant worlds?
Transit Method
One of the most famous exoplanet detection methods is the transit method. When a planet crosses in front of its host star, scientists can detect a slight dip in brightness. However, if you can imagine detecting an ant crossing a searchlight from miles away, you can see how difficult it can be to spot a planet from light years away! Furthermore, if the system’s angle is askew, transits will not be visible from Earth. For example, the next time a Venus transit across our Sun will be visible from Earth will be 2117, even though Venus will have completed nearly 150 orbits by then!
Wobble Method
Spotting the Doppler shift of a star’s spectra was used to find Pegasi 51b, the first planet detected around a Sun-like star. This technique is called the radial velocity or “wobble” method. Astronomers split up the visible light emitted by a star into a rainbow. These bands of light are normally smooth. However, if a planet is orbiting the star, the star will wobble slightly, causing the lines within the spectra to shift. By carefully measuring the shift, astronomers can determine the speed and size of the object pulling on the host star and if it is indeed a planet.
Direct Imaging
Finally, exoplanets can be revealed by directly imaging them. Space telescopes use instruments called coronagraphs to block the bright light from the host star and capture the dim light from planets. The Hubble Space Telescope has captured images of giant planets orbiting a few nearby systems, and the James Webb Space Telescope has only improved on these observations by uncovering more details and even scanning atmospheres for potential biosignatures!
The future of exoplanet discovery is only just beginning, improving our understanding of the Universe, where we are from, and if there is life elsewhere in our cosmos. You can find more information and activities on NASA’s Exoplanets page.
A planet passing in front of its parent star creates a drop in the star’s apparent brightness, called a transit. Credit: NASA’s Ames Research Center
As a planet orbits a star, the star wobbles. This causes a change in the appearance of the star’s spectrum called Doppler shift. Astronomers can use Doppler shift to calculate the object’s speed, direction, and mass. Credit: NASA, ESA, CSA, Leah Hustak (STScI)
