by Ludwig Scheibe (TU Berlin), October 2024
Imaging an exoplanet directly is a difficult process that is only doable in a select few cases. Thus, we need indirect methods of detecting and studying these elusive worlds. The most successful one by sheer number found is this: the transit method.
Looking at the brightness
Imagine a star with a planet orbiting around it. If we as observers from Earth happen to look at this system edge-on, the planet will periodically pass between us and its star. That passing is called ‘transit’. We cannot finely resolve this process with our instruments, cannot take a “photo” of the planet in front of the star as is possible when Mercury or Venus transits the Sun. That is because even in really good telescopes, the distant star is just a dot of light that illuminates only a few pixels. However, when the planet is between us and the star, it blocks a little bit of the star’s light. So if we measure the star’s total brightness with our telescopes, it will go down during the transit. This is what we have to look for: regularly repeating dips in a planet’s light curve. The following animation demonstrates this principle ( Credit: ESA)
From the very first transiting exoplanet …
The first time we could measure this reoccuring transit signal was for the planet HD 209458 b, later unofficially dubbed ‘Osiris’. It had been discovered via the radial velocity method in 1999, and immediately afterwards two separate teams could publish their observation of the transit dips in the star’s brightness (Charbonneau et al. 2000, Henry et al. 2000).
This was a big deal: Up until then, exoplanets had only been found using the aforementioned radial velocity method, and there were still skeptics that doubted if these signals were indeed planet. Now, scientists had independent verification of a planet’s existence, not merely by having different observations, but also by using different methods.
Artist’s impression of “Osiris”, HD 209458 b. It is a hot Jupiter that is so close to its star that it is believed to be in the process of losing its atmosphere. Credit: European Space Agency and Alfred Vidal-Madjar (CNRS)
… to a catalogue of thousands
The second reason the transit method was so important is: While the radial velocity method gives us a planet’s minimum mass, the transit method can give us its size. Combining both measurements together gives us an estimate of its density. This, then, can give us a clue about the planet’s composition: Is it composed of a very dense, heavy material like iron, or a very light material like hydrogen?
In addition, transiting planets offer an opportunity to find out if a planet has an atmosphere and what that is made of. During transit, the starlight falling through the “ring” made by the atmosphere around the planet, is changed by the atmosphere and we can measure this change.
In the 2000s and 2010s, a large number of transit surveys were initiated, for example the CoRoT and the Kepler space telescope, and as of 2024, the vast majority of the ~6000 known exoplanets very discovered using transits.
Interesting questions about the transit method:
What can we learn about a planet using the transit method?
There are two things we can learn directly from observing some transits of an exoplanet:
1) Of course, a larger planet blocks more light than a smaller one. That means that measuring how much the star’s brightness is reduced during the transit tells us about the planet’s size. To be precise, what we learn is the relation between the planet’s and the star’s size, because a planet around a small star will block a larger proportion of the light than a same-size planet around a larger star. So, if we have the transit depth and and the stellar radius – from stellar astrophysics – we can determine the planet’s radius.
2) The transit occurs once for every orbit the planet completes. Therefore, measuring the time between two or more transits gives us the planet’s orbital period.
Additionally, during the transit the planet’s atmosphere alters the starlight passing through it by absorbing some wavelengths stronger than others. Thus, using sufficiently precise measurements of the stellar spectrum both during and outside the transit, we can find out about the planet’s atmosphere. More on that here.
Which planets are particularly suitable for the transit method?
The bigger the planet compared to its star, the more it obscures and the easier it is to measure the periodic darkening. Thus, big planets like Jupiter (11 times the radius of Earth) are easier to discover than Earth-sized ones or smaller. That is an important reason why we have mostly found big gas giant planets in the early days of exoplanet exploration.
Additionally, planets that are close to their star can be found particularly well. This has several reasons: For one, a small orbital distance means small orbit periods, and thus we do only have to observe its star for a relatively short time – sometimes days are enough – to see the recurring transit. Conversely, a distant observer looking for Earth could only see one transit a year, and for further out planets like Jupiter even less. Second, we can only detect the transits for planet that pass between us and their star, so planets whose orbits we see edge-on. For planets that are very close to their star, there are more angles at which a planet still passes between us and their star (see graphic below). So, planets close to their star have a higher probabibility that we can detect their transit.
Schematic illustration how a planet on a narrow orbit might transit, while a planet on a wider orbit with the same angle might not transit. Image by L. Scheibe
Learn more about the radial velocity method here, the other main method for exoplanet hunting.
Recommended book (scientific textbook):
Transiting Exoplanets
Carol Haswell
Publisher: Cambridge University Press
ISBN: 9780521191838
Find more books to the topic exoplanets and astronomy for children, amateurs and scientists in our booklist.
