By Jim Shih
Jim Shih has a MSc degree in Astronomy from the University of Amsterdam. After completing his degree, he worked as a guest researcher at the Anton Pannekoek Institute for Astronomy, also at the University of Amsterdam. He is interested in studying exoplanets and learning about the unthinkable properties and diverse compositions they hold with models and observations – they are just so mind-blowing.
In his free time, he likes to sing and photograph things he finds beautiful (photography account IG@jimmyshootttt). He is also a huge fan of monster movies, especially Godzilla.
We humans have always possessed an enduring curiosity and a sense of wonder about other worlds. What could they look like? How different could they be from the one we are living in? We have in fact always been thrilled by our ability to imagine and speculate about the variety and differences that might exist among planets, as seen in science fiction – from the metallic city of Cybertron in Transformers to the hostile desert land of Arrakis in Dune, and the endless ocean surface on Miller’s planet in Interstellar to the worlds harbouring different forms of life in Star Trek. Yet, even if the stories we came up with are far from scientifically rigorous, they are still so powerful in inspiring the readers and, perhaps most importantly, in motivating us to look up and search for real exoplanets. That is why, when it finally happened, the discovery of exoplanets was undoubtedly one of the most fruitful revelations in humanity’s quest to understand the universe.
What are planets?
Before jumping into discussing what exoplanets are, let us first take a step back and begin with a more familiar term – planet. The word “planet” we use today originated from the Greek word πλανήται [planḗtai], which by the literal meaning translates to ‘wanderers’. In ancient Greece, this term was initially used to describe the two celestial objects (i.e., Sun and Moon) and the five bright dots (i.e., Mercury, Venus, Mars, Jupiter, and Saturn). The reason being that when seeing them from Earth, they appeared to be “wandering” in the sky as their position changes with respect to the fixed background stars over the span of time.
Since then, however, over the course of centuries with the progression of astronomical discoveries and the advancements in instrumentation, the modern definition of a planet has gradually evolved into one that refers to the giant rocky or gaseous bodies revolving around the Sun in the solar system. In 2006, the International Astronomical Union (IAU), an international scientific organisation for the science of astronomy, passed a resolution during the annual General Assembly that defines a planet as a celestial body that:
- is in orbit around the Sun
- has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape
- has cleared the neighbourhood around its orbit
Following the adoption of this resolution, our solar system now has eight such bodies that satisfy these three requirements, namely – Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune (See Fig. 1).
Author’s note: The adoption of resolution also resulted in the famous “demotion” of Pluto’s planethood. Prior to the resolution, Pluto had been commonly known as the ninth planets of the solar system after it was first discovered in 1930. However, since Pluto has not “cleared its neighbouring region of other objects”, it does not satisfy the 3rd requirement of IAU’s definition of a planet and was therefore downgraded to a dwarf-planet. That being said, many astronomers today still refuse to acknowledge IAU’s review on Pluto’s new status (unofficially), due to personal belief.
What makes a planet an “Exo”-planet?
How is an exoplanet different from a planet? Believe it or not, this question is not a tricky one at all. The prefix “exo” simply refers to planets that orbit a star that is not our Sun. With this definition, therefore, any planet found outside of our solar system can be called an exoplanet.
It might come as a surprise to many that the study of exoplanets is a relatively new field in astronomy. In fact, before the 1990s, without definitive evidence of the existence of exoplanets, humanity had only been able to speculate about them. It was not until 1995 that the detection of the first exoplanet orbiting a sun-like star was finally revealed by a Swiss research team led by Didier Queloz and Michel Mayor (Nature paper, closed access). This milestone marks the beginning of a new subfield of astronomical research – exoplanet research. Since then, the number of known exoplanets has exploded. Today (as of August 2024), more than 5700 exoplanets have already been catalogued, and this number is only expected to grow at an accelerating rate amid the ongoing and upcoming exoplanet search missions, such as the Transiting Exoplanet Survey Satellite (TESS), PLAnetary Transits and Oscillations of stars (PLATO), CHaracterising ExOPlanet Satellite (CHEOPS), and others.
How do we detect exoplanets?
The current understanding of star-planet systems tells us that we should expect every star to have at least one planet orbiting around it. Therefore, given our best estimate that the Milky Way is made up of approximately 100 billion stars, one can only imagine the sheer number of exoplanets yet to be detected. That being said, despite their seemingly large number, planet-hunting is not an easy task at all. In fact, detecting exoplanets is a much more challenging and sophisticated task than finding a star. That is because, in comparison to stars, planets are inherently much smaller and extremely dimmer objects. There is about a billion-fold difference in brightness between a star like the Sun and the light reflected by any of the planets orbiting it. To overcome the difficulty in detecting these much fainter objects, astronomers have come up with some clever ways that enable them to “see” the exoplanets.
1) Looking for Wobbles (Radial Velocity Method)
One of the most used detection methods for exoplanets is called Radial Velocity. This indirect way of detecting exoplanets is based on looking for the slight changes in the colour of the starlight we see, which look like wobbles in the wavelength of the starlight. In general, when we talk about a star-planet system, we often picture a planet orbiting around a star located at the centre; however, this statement only tells a part of the story. In reality, when two moving objects are held together by gravity, they actually both move around a centre-of-mass point. While it appears that a planet goes in a circle around the star, both the star and planet are in fact moving around a common point located in between them. However, since the star is a much heavier object than a planet, its movement is considerably smaller compared to the planet’s. Nevertheless, modern instruments are able to pick up this small movement of the star by looking at slight changes in the wavelength of the starlight.
In simple terms, when a light-emitting source moves towards or away from the observer, the wavelength of the light will change depending on the direction of movement. As shown in Fig. 2, when a star is moving towards us, the light we see will have a shorter wavelength (appears bluer – blueshift), but if it’s moving away from us the light will have a longer wavelength (appears redder – redshift). This is analogous for how an ambulance siren changes to higher and lower pitches when it is driving towards or away from us, respectively. By observing the blueshifts and redshifts of the starlight, and determining the rate at which these shifts happen, astronomers can indirectly infer whether or not there is a planet present around the star.
2) Searching for Shadow (Transit Photometry)
Another prominent way of detecting exoplanets is a technique called transit photometry. An indirect detection method as well, this technique is based on searching for the “shadow” of exoplanets. In astronomy, the term transit refers to a phenomenon when a celestial body passes in front of another in the line of sight of an observer. An example of such an instance is the solar transit commonly known as the solar eclipse – an astronomical event during which the moon passes in front of the sun and covers it either partially or completely.
Similarly, the solar eclipse analogy can also be applied when an exoplanet passes in front of its host star in our line-of-sight. The only difference here is that, unlike the Sun-Moon scenario where we can clearly observe the moon’s shadow, it is much more difficult to see the exoplanet’s shadow because of how far away the star-planet system is from Earth.
An easy way to understand why this is the case is to imagine a bug flying in front of a light bulb (see Fig. 3 – left). When seeing it up close, both the bug and the light bulb can be distinctly identified, and it is very easy to see the shadow of the bug as it covers parts of the light bulbs. However, if we increase the distance between us and the bug-light bulb (see Fig. 3 – mid), it starts to become harder to distinguish between them. And if we continue to increase the distance even further (see Fig. 3 – right), at some point, it becomes virtually impossible to tell them apart. The outlines of the bug and the light bulb have vanished, merging together into a single point. In fact, this is exactly what astronomers see when observing the stars – there is no outline of the stars themselves, let alone seeing the planets passing across.
So then, how do we know if there is a planet at all, you may ask? Actually, the concept is quite simple. Astronomers search for planets by tracking many stars and see if there is a change or dip in their brightness over some time (see Fig. 4). In other words, they are hunting for eclipses on other stars! All in all, although it may seem like we are observing starlight, what transit photometry is really about is searching for the shadows of planets.
3) Taking a picture (Direct Imaging)
At this point you might wonder, is it at all possible to detect exoplanets directly? The answer is: yes, but it is very challenging to do. As mentioned before, seeing an exoplanet orbiting its host star directly is very difficult because a) the star is much brighter than the planet and b) they are very far away. However, there are some instances where it is possible to see an exoplanet orbiting in action with very sophisticated techniques and an advanced instrument.
Fig. 5 shows an example of such an instance where astronomers were able to block out the starlight of a close-by star-planet system. That being said, the technology is still being developed and improving, and with more capable telescopes being built in coming missions, we should expect more real pictures of exoplanets being taken in the near future.
Astrobite edited by Jessie Thwaites
Featured image credit: Made by BZZRINCANTATION and edited by Jim Shih, via Flaticon.
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