by Ludwig Scheibe (TU Berlin), October 2024
Without a lot of prior knowledge, upon hearing “discovering planets around other stars” most people would probably think something like this: Take a powerful telescope, ‘zoom in’ really closely to a distant star, and then you see the planet as a much smaller dot next to it. However, in many cases this direct approach does not work, and most planets are found with indirect methods, particularly via transit and radial velocity measurement.
Why is that? What challenges does the method of direct imaging have, and what can be done to overcome them?
Resolving things
The first problem you run into when trying to directly image a planet is that of resolution. compared to interstellar distances of light years, typical star-planet separations of a few astronomical units are almost negligibly small. An astronomical unit – the distance between Earth and the Sun – is about 60 000 times smaller than a light year after all. In essence, in a lot of telescopes, the star and its planet would be imaged almost on top of each other. This can be overcome by building telescopes with larger apertures, or by combining the lights from different far apart telescopes all looking in the same direction – a process known as interferometry. However, building large telescopes or several very finely attuned ones is a very complex and expensive endeavour. This limitation one of the reasons why directly imaged exoplanets tend to be far out from their stars, where it is easier to resolve the two.
Image of the binary star pair b Centauri A and B (top left), and their giant planet companion (bottom right). The planet orbits at about 500 times the distance of Earth to our Sun. The third dot in the upper right is a background star. The picture was captured using the VLT in Chile. It was taken in the infrared wavelength range and converted to visible colors. Credit: ESO/Janson et al., with added labels
Hidden in the glare
The second problem is called glare. To illustrate it, imagine trying to detect a firefly, but it’s next to a powerful spotlight. Planets are much, much fainter than most stars. The starlight drowns out the – usually reflected – light from the planet. This effect is of course less noticeable for further out planets, as in the previous paragraph. However, since usually for evolved star systems, planets mostly reflect light and do not give it off on their own, far out planets are faint, making the problem of brightness contrast worse again. There are several ways this is counteracted. For one, planets form very hot, so a young planet radiates comparably strongly, at least in the infrared part of the spectrum. So young stars are a good target to look for directly imaged planets.
An important step to counteract glare is coronagraphy. A component is introduced into the optics of a telescope in order to block out the central star, similar to how you shade your eyes if you want to see something against the sun’s glare. This is why in a lot of direct imaging pictures, the center, where the star would be, is dark or it is represented by a later-added symbol, as in the following picture:
The planet HIP 65246 b, imaged by four different instrument on the James Webb telescope in the infrared. The star is blocked out and denoted by the white star-symbol. Credit: NASA/ESA/CSA, A Carter (UCSC), the ERS 1386 team, and A. Pagan (STScI)
The first direct imaging detection of an exoplanet
In 2004, 9 years after the discovery of a planet around another sun, a planet around brown dwarf 2M1207 was discovered via direct imaging, and subsequently verified for a year. It was simultaneously the first exoplanet found around a brown dwarf and the first found via direct imaging. As laid out above, it is a far out, giant planet, 5 times as heavy as Jupiter, and orbits at 42 times the orbital distance of the Earth around the sun.
Image of brown dwarf 1M1207 (center) and its planet companion (lower left). It was imaged using ESO’s VLT in Chile in 2004, using the infrared part of the spectrum and converted to visible colors. was imaged the first time by the VLT in 2004. Credit: ESO
The ground-breaking observations were made at European Southern Observatory‘s Very Large Telescope in Chile.
Composite image of all ESO observatories and facilities in Chile. The four seperate telescopes of the VLT can be seen in the centre. Credit: ESO/M. Kornmesser
Interesting questions on the topic:
What can we learn about a planet using direct imaging?
The image itself allows us to directly measure the orbital distance of the planet, and, if we observe for long enough, the orbital period (or at least a good guess).
Measuring the planet’s spectrum gives us its temperature, and some sense about its atmospheric composition.Furthermore, observing the planet’s brightness and having its temperature, we can make a good attempt at estimating its radius.
Which planets are particularly suitable for the direct imaging method?
Planets far from their star are particularly well-suited for this method, since they are not as easily covered by the stars glare and are more easily resolved seperately from their star. However, this also means that they reflect only very little starlight, so we usually need young planets that are still hot and thus luminous.
Furthermore, big planets are better suited than small ones, since with their big area they can emit more radiation, or reflect mire starlight.
As opposed to many other methods of exoplanet detection, direct imaging is better suited to observe systems where we look on “from above”, since there the star cannot obscure the planet.
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