Detecting Exoplanets
Every star has planets and probably more than one... there's more planets in the galaxy than there are stars.
Detecting Exoplanets
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The “transit method” is a widely used technique for detecting exoplanets, and many exoplanets have indeed been discovered this way.
When an exoplanet passes in front of its parent star, as viewed from Earth, it causes a temporary and slight decrease in the star’s brightness. This drop in light intensity, or “transit,” is an indication of the planet’s presence.
How Do We Find Exoplanets? An Introduction to Five Key Methods
Exoplanets—planets outside our solar system—are fascinating objects that have captivated scientists and the public alike. But how do we actually find them when they are so far away? Let's explore five of the most prominent methods used to detect these distant worlds.
1. Transit Spectroscopy Method
This method involves watching a star for tiny dips in its brightness. When a planet passes in front of (or transits) its host star, it blocks a small fraction of the star's light. By measuring how much the light dims, scientists can determine the size of the planet. If the planet has an atmosphere, some of the starlight passes through it, which allows us to study the atmosphere's composition.
Transit of Venus (above), transit of Mercury (below).
2. Radial Velocity (Wobble) Method
Imagine a star and a planet locked in a gravitational dance. As the planet orbits, it tugs on the star, causing the star to wobble slightly. This wobble changes the light we see from the star. When the star moves toward us, its light shifts to the blue end of the spectrum (blueshift); when it moves away, the light shifts to the red end (redshift). By measuring these shifts using a technique called Doppler spectroscopy, we can infer the presence of an exoplanet.
Diagram detailing the Radial Velocity (aka. Doppler Shift) method. Credit: Las Cumbres Observatory
3. Gravitational Microlensing Method
This method is based on Einstein's theory of general relativity. When a massive object (like a star) passes in front of a more distant star, it bends and focuses the distant star’s light, much like a lens. If a planet orbits the foreground star, it creates a secondary blip in the light curve. This method is particularly useful for detecting planets that are far from their stars.
NASA infographic showing how an exoplanet can be detected via microlensing. Credit: NASA, ESA, and K. Sahu (STScI)
4. Direct Imaging Method
Direct imaging is exactly what it sounds like—taking pictures of exoplanets. This is challenging because stars are much brighter than planets. To capture an image, scientists often use special instruments called coronagraphs to block the star's light, making the faint light from the planet visible. Direct imaging works best for large planets far from their stars.
This gif uses multiple images from the W. M. Keck Observatory in Hawaii to show four super-Jupiters orbiting the young star HR 8799. The closest planet shown is almost as far from its star as Uranus is from the Sun, while the farthest has an even larger orbit than Pluto’s in our solar system. Roman will be able to directly image older, cooler worlds in tighter orbits.
Jason Wang (Caltech)/Christian Marois (NRC Herzberg)
5. Astrometry Method
Astrometry involves precisely measuring a star’s position in the sky. If a star has a planet, the planet's gravity will cause the star to make a small but detectable wobble in its position over time. By tracking this wobble, we can infer the presence of a planet. This method requires incredibly precise measurements and is most effective for detecting massive planets orbiting close to their stars.
This animation shows the observed path across the sky, over four years, of a star that is orbited by a planet.
Here is a link to Kepler's Orrery - just some of the exoplanets we've discovered so far:
Conclusion
Each of these methods has its strengths and is suited to finding different types of exoplanets. As technology improves, we’re likely to discover even more planets—and maybe even ones that could support life.
Additional Resources:
Alison Camacho, Aug 2024
