While Goldilocks was only forced to choose between 3 bowls to find one that was just right, NASA scientists have to find that ‘just right’ zone among billions of planets and nearly 4,000 solar systems. In the case of planets, that ‘just right’ place, or the Goldilocks Zone, is the region in a solar system with the right temperatures for planets inside it to have liquid water on its surface. The Goldilocks Zone is one of the most important metrics that scientists use to determine where life might exist. This metric applies to exoplanets which, as you can probably guess, are planets outside of our solar system. There has been a recent surge in research on exoplanets and determining if they are habitable.
First of all, how are exoplanets found? While there are several different methods for finding exoplanets, the most commonly utilized ones are transits, radial velocity, and gravitational lensing, where transits are the most successful. A transit occurs when a planet passes between the star it orbits and the place where it is being observed from, so Earth in this case. When this occurs, the transit blocks the light from the star for a brief moment and this interference tips scientists off that there is an object circulating the star and blocking the light, often a planet. However, this data doesn’t only demonstrate the existence of an object, but it also can be used to determine the exoplanet’s other characteristics. The light that passes through the planet’s atmosphere can be used to determine the chemical composition can be done through exoplanet spectroscopy, which is especially important in determining if a planet is habitable. When light passes through a planet, the chemicals within it will absorb that light and ensure that the color of the light absorbed isn’t seen at a later point, meaning from Earth. The missing light colors can be compared with known chemicals that absorb the same colors, which means the atmosphere is rich in that element. For example, on a sodium-dense planet, the sodium would absorb light and block it out in a spectrum the same way sodium on Earth would and the same spectra produced would allow scientists to know the atmospheric content of the planet. In addition, the size can be determined based on how much light is able to get through and how long it takes to orbit the planet can be determined based on how far apart these transits occur. The larger the exoplanet, the more light it will block from the star. Thus, a larger dip in light emission signifies a larger planet is the cause. In addition, how far apart these dips in light emissions occur can help determine how long it takes for the planet to orbit the star. This will always be a periodic time for exoplanets, which is helpful for determining their existence, and with additional knowledge of its orbital period, can result in scientists learning more about the planet’s characteristics. Considering that more than 4,000 exoplanets have been found with transits, it becomes clear why it is such a widely utilized method.
The second most common method, radial velocity, has found only a quarter of that amount, roughly 1,000 exoplanets. Radial velocity involves how the planet and orbital star’s gravitational forces interact with each other. A planet’s circulation around a star is enabled by that star’s gravitational force; however, the planet’s gravitational force also has a small effect on the star. The way the planet’s force influences the star is usually referred to as the ‘wobble effect,’ where the star ‘wobbles’ a little as if it moves slightly around itself. This movement is usually very small, but it is caused by the star moving slightly toward the planet’s gravitational force as the planet circles around the star. How pronounced the wobble is depends on the characteristics of planets orbiting the star, including the mass and size. As larger objects have stronger gravitational forces, this method can be used to determine the size of planets orbiting a star as well as the number. Radial velocity is also a popular method for double-checking that a planet actually exists after it has been found with other identification methods. None of these methods are perfect and our knowledge of space is limited. Even with these methods, errors are bound to happen, where another large object in space might result in a transit, but those objects might not be planets. There are a variety of objects that could have resulted in a transit, including brown dwarfs – failed stars, groups of asteroids, or a companion star. Therefore, it is common practice to utilize many different techniques on the same object in order to confirm its identity as an exoplanet or other object. Radial velocity is one of the best ones for this because it confirms an object large enough to have a gravitational impact, which is something many other space objects, like companion stars, don’t.
The third most common exoplanet identification method is gravitational microlensing. Gravitational microlensing is more specifically a phenomenon in which the light is bent in the telescope it is observed through due to the way the light travels towards it. This effect occurs because stars bend space when it is near this and this bending results in the amplification of a star’s light, where the strongest amplification occurs when the planet is directly behind the star, in the direction of the observer. In a way, this makes it almost like the opposite of a transit! This method is particularly helpful if the object doesn’t emit much light. For this reason, it is the best method for detecting the furthest exoplanets from our solar system and allowing us to find out more about the furthest corners of our universe. Gravitational microlensing was actually also the method that discovered the first exoplanet. Since then, many more methods have been created and utilized, with the methods just explained being the most commonly used ones.
There are several requirements NASA looks for in exoplanets to determine if they are viable for life, and many more that are actually needed to sustain life. Generally, however, habitable planets are more generally defined as planets that can sustain life for an extended period of time. The most common characteristics NASA looks for in potentially habitable planets are if they are the same size as Earth as this would mean that the planets’ other characteristics could be used to determine if they are similar to Earth and can sustain life in the same fashion. Another important characteristic is the planet’s distance from the star, which is where the Goldilocks Zone comes in. It needs to be far enough for water to be liquid, not gas, but also close enough to be liquid and not solid, that zone in a planetary system is referred to as a Goldilocks Zone as it is ‘just right.’ Such a location usually stems from the planet’s distance from the star as well as the energy amount being emitted towards the planet. The atmospheric composition is also important which, as mentioned before, can be determined through light that comes to our planet from the exoplanet. For closer planets, satellites can be used to examine the atmosphere and use the atmosphere’s composition to determine if the planet would be toxic to life by having oxygen to support it or containing toxic chemicals.
Exoplanets are a fascinating part of our solar system, holding the potential to support life. Currently, the method for finding closer exoplanets is through transits, further ones are through gravitational microlensing, and exoplanets’ identities are confirmed by radial velocity. Even with the recent advancements in exoplanet identification technology, there are still many more that remain to be discovered and much to learn about exoplanets. One thing remains clear, as our technological abilities advance so too will our ability to find and identify exoplanets.
Sources:
Goldilocks Zone – Exoplanet Exploration: Planets Beyond our Solar System.
5 Ways to Find a Planet | Explore
Spectroscopy Infographic – Exoplanet Exploration: Planets Beyond our Solar System
How do you find – and confirm – a planet? 10 things about the search for exoplanets
Spectroscopy Infographic – Exoplanet Exploration: Planets Beyond our Solar System
Space-Warping Planets: The Microlensing Method | The Planetary Society
Color-Shifting Stars: The Radial-Velocity Method
What’s a transit? – Exoplanet Exploration: Planets Beyond our Solar System
