How to search for planets outside our Solar System

Looking for a tiny firefly near a lighthouse

Last month I dedicated my blog to the launch of the James Webb Space Telescope. One of the main science goals of Webb will be to study the atmospheres of exoplanets to search for biosignatures; the so-called building blocks of life. This is super cool! But before this so called, characterisation, we first have to find these exoplanets. As this semester I am a teaching assistant for a course called High contrast imaging, I thought it might be nice to elaborate a bit more on how we can find exoplanets circling around stars far, far away from our Solar System.

If you paid close attention to the astronomy news, you might have seen the headlines mentioning “NASA's Transiting Exoplanet Survey Satellite (TESS) has identified more than 5000 potential alien worlds” But then how is the counter of confirmed discoveries of exoplanets by astronomers only at 4908? Well, to call a potential alien world an exoplanet we need a validation of its existence by at least two detection methods. When planets are identified by only one single detection, we call them exoplanet candidates.

There are two well-established detection methods;

• Transit method; when a planet is orbiting a star it dims the light of the star by a measurable amount when it is passing in front of the star.
• Radial velocity method; stars start to wobble around their system’s centre of mass due to (a) planet(s) that is orbiting the star.¹

Why is finding an exoplanet so hard? Well, not only they are very far away, they also are very close to their super bright host stars. You can compare the observation of an exoplanet with looking for a firefly, circling around the light of a lighthouse in the Northern part of Norway while standing in the most Southern part of Spain. For a professional telescope the challenge does not depend too much on detecting smaller objects at these larger distances. The real challenge is spotting the firefly without being able to turn off the lighthouse’s light.

If you think about it: the two main detection methods mentioned above are both indirect detection methods. What I mean by that is that the planets are not detected by making a picture, but only thanks to the effect they have on their host star.

If there was only a way to dim the overwhelming glare of the star during an observation, we might be actually taking an image of the exoplanets themselves. This gives rise to another detection technique, so-called Direct imaging. We refer to the ratio of the star’s flux to the planet’s flux as the contrast ratio. This is why direct imaging is also known as High contrast imaging, or HCI.



Coronagraphs and their use in HCI • One of the main instruments of Webb, MIRI, offers coronagraphic imaging with four individual coronagraphs. One of the coronagraphs is based on the a so-called Lyot design; a very classical type of a coronagraph. The Lyot coronagraph uses a small metallic disk to block most of the light coming from the central bright star in the first instrumental focal plane. Light from the star is concentrated around the edges of the telescope pupil and forms rings around the edge of the aperture image and the secondary mirror image. Thereafter, a Lyot stop blocks out the remaining rings of light coming from the central star while light coming from objects next to the star passes through to the final image on the camera.²

Did it occur, that the images produced by reflector telescopes do not have holes or shadows in them? A question for you to understand the principle of a coronagraph would be: What is the difference between an occulting spot, part of a Lyot coronagraph, and a secondary mirror that is placed above a primary mirror, blocking a specific area of the sky?

Light rays from the unblocked parts of a primary mirror are all added together when they are focussed together. The focus is not on the blockage, but on the sky, far, far away. If you want to visualize this, you can do the following experiment. Look with both your eyes to an object or wall at a distance of about 5 meters. Straighten of your arms forward and point your pointing finger upwards. If you focus on the wall you notice that you can see the entire wall even though your finger is right in front. When focusing on your finger close by, you will see that you cannot see the background anymore. Could you answer the question? What would be the difference?

¹ Up until 2014 this was the most adapted method to find exoplanets. After the launch of the Kepler (Space Observatory), many more planets were found using the Transit method, resulting in the Transit method being the leading exoplanet detection method nowadays.
²Note this is an example of a Lyot Coronagraph. This type of coronagraph cannot reach a high contrast for object very close to a star. However, improvements to this original concept led to many high-performance coronagraph designs.