Life Beyond Earth: How are exoplanets detected?
The quest to find planets beyond our solar system, known as exoplanets, has captivated astronomers and scientists for decades. Since the first confirmed detection in the 1990s, our understanding of the universe has expanded remarkably. The search for these distant worlds has fueled curiosity about extraterrestrial life and challenged our views on planetary formation and the uniqueness of our solar system.
Detecting these exoplanets is no small feat. Despite the vast distances, astronomers have developed ingenious methods like transit photometry, radial velocity and other techniques. But how do these techniques work? What have we learned from exploring these ‘exoplanets’?
What are exoplanets?
As the name suggests, planets beyond our solar system are known as exoplanets. These exoplanets consist of elements similar to those found in our solar system’s planets, but the proportions of these elements can vary significantly. Some exoplanets might be rich in water or ice, while others are abundant in iron or carbon. We’ve discovered lava worlds with molten surfaces, planets with the density of Styrofoam and dense planetary cores still orbiting their stars.
We know from NASA’s Kepler Space Telescope that there are more planets than stars in the galaxy!
Exoplanets vary greatly in size, ranging from gas giants larger than Jupiter to small, rocky planets similar in size to our Earth. They might be hot enough to boil solid metal, or cold enough to be locked in a freezer! Some orbit their stars so closely that a year lasts only a few days, while others orbit around two stars simultaneously. There are also sunless rogue exoplanets, wandering through the galaxy in perpetual darkness.
How do we find exoplanets?
Exoplanet detection is the process of identifying and studying planets that orbit stars beyond our solar system. This field of study has rapidly advanced since the discovery of the first exoplanet in 1995.
Radial Velocity Method:
When a planet orbits a star, it causes the star to move in a small, circular or elliptical orbit around the center of mass of the system. This movement induces a slight shift in the star’s spectral lines due to the Doppler effect which is a phenomenon where the frequency or wavelength of a wave changes based on the relative motion between the wave source and the observer. If the star is moving towards us, its spectral lines appear slightly blueshifted (wavelength decreases), and if it is moving away, they appear slightly redshifted (wavelength increases). The first exoplanet had been found using this method, and 1089 planets have been discovered ever since. This process involves:
- Spectral Analysis: Astronomers use high-resolution spectrographs which are captured through spectrometers to analyze the star’s light spectrum, measuring the precise wavelengths of its spectral lines.
- Detection of Doppler Shift: The star’s spectral lines shift back and forth due to the gravitational pull of an orbiting planet, which astronomers detect as variations in the star’s radial velocity.
- Planet Characterization: The size and period of the planet’s orbit can be inferred from the radial velocity variations. This method is particularly sensitive to massive planets orbiting close to their host stars.
Transit photometry:
When an exoplanet transits in front of its host star as seen from Earth, it blocks a small fraction of the light emitted from the star. This causes a periodic dip in the star’s brightness, which can be detected by sensitive telescopes. 4182 planets have been found using this method. This process involves:
- Photometric Observations: Astronomers use photometric instruments to measure the brightness of stars over time with very high precision, often down to parts per million.
- Detection of Transits: As the exoplanet passes in front of its host star, the star’s brightness decreases slightly. This periodic dimming indicates the presence of an exoplanet.
- Characterization: The depth and duration of the transit provide information about the planet’s size and orbital period. Multiple transits allow astronomers to determine the planet’s orbital inclination and atmospheric composition through transmission spectroscopy.
Some other methods of detecting these exoplanets are as follows:
- Direct imaging is a technique used to detect exoplanets by capturing actual images of them, typically larger planets, by blocking out the light of their host stars. It relies on advanced telescopic technology to block out the bright light of a star, allowing the much fainter light from any orbiting planets to be detected. This is often done using a device called a coronagraph, which masks the star’s light. 81 planets have been detected using this method
- Gravitational Microlensing is a unique technique used to detect exoplanets and other celestial objects by observing the bending of light due to gravitational fields. This occurs when the gravitational field of a star or planet bends and magnifies the light of a more distant star as seen from Earth. This effect has been predicted by Einstein’s theory of General Relativity. 217 planets have been discovered by this method.
- Astrometry is a technique used to detect exoplanets by measuring the tiny wobbles or shifts in the position of a star caused by the gravitational influence of an orbiting planet. When a planet orbits a star, the gravitational pull of the planet causes the star to move slightly in a small circular or elliptical orbit around the centre of mass of the system. This movement can be observed as a periodic shift in the star’s position relative to distant background stars. Only 3 planets have been detected using this method.
What’s next for exoplanet detection?
With the development of the James Webb Space Telescope, the pursuit of exoplanets has been expedited. Our search for extraterrestrial life remains a mystery, but we use multiple technologies that suggest the presence of other life in the vast universe. How do we find that out? You will find that in the next article!
