It’s always interesting to hear about a new exoplanet being discovered on the news, but what exactly is an exoplanet?
Exoplanets are celestial bodies that orbit around a star other than our sun. We can easily detect planets within our solar system, but exoplanets tend to be so far away that methods used within the solar system no longer apply. How do we find them?
In very special cases, we can directly see exoplanets. While visible light provides limited knowledge of the universe, imaging in other wavelengths can reveal things that were previously hidden. For example, an infrared image of HL Tauri (a young star about 456 light-years away) captured by ALMA revealed noticeable dark bands in the disk of gas and dust around the star, which were most likely due to exoplanets.
Unfortunately, we can’t detect most planets in this way.
Radial Velocity Method
This method involves examining changes in the light spectrum of stars. A star in a planetary system is not stationary – it wobbles slightly due to the gravitational influence of its own planet. Due to the Doppler Effect, the light spectrum changes as the star moves towards and away from the observer.
When a planet is present, these changes are periodic and depend on the orbital speed of the planet. This method has a bias towards large planets that orbit close to their star because larger mass planets produce a more measurable effect.
The Transit method uses the intensity of light emitted from a star to find planets. When a planet travels in front of a star, it blocks out a small portion of the light, leading to a tiny dip in the intensity of light from the star. Periodic decreases in the star’s brightness tend to confirm the existence of an exoplanet. Like the previous method, this method has a bias towards large planets that orbit close to their star.
All of the techniques mentioned can’t detect exoplanets that are thousands of light-years away. Gravitational microlensing uses an effect explained by Einstein’s General Theory of Relativity to detect planets. Simply put, if a star (the lens) passes in front of another star (the source) whose light is on its way to Earth, the gravity of the nearer star will bend the light (lensing) of the source.
If the lens happens to contain an exoplanet that also crosses through the light from the source, the exoplanet’s gravity will also slightly bend the light (microlensing). Both the lens and its planet produce a spike in the light curve, so two spikes in the light curve indicates an exoplanet. This method allows us to detect even small planets that are thousands of light-years away.
How Many Have Been Detected?
To date, about 3,700 exoplanets have been discovered using the previously mentioned methods as well as a few lesser known techniques. Roughly 2,650 of those exoplanets were discovered using the Kepler Space Telescope.
Fun Fact: The closest exoplanet discovered is Proxima Centauri b, which orbits around Proxima Centauri (the nearest star to the Sun) and is 4.22 light years away!