Detecting an exoplanet transit
Dr Adrian Jannetta FRAS
Exoplanets are planets in orbit around other stars.
The first exoplanets were discovered in the 1990s and astronomers have catalogued more than 4,000 others to date!
Stars, including the Sun, are self-luminous and shine because of nuclear reactions within their cores. Planets, on the other hand, are not luminous. They are much smaller and shine by reflecting the light of their star. This makes searching for exoplanets very difficult! Even through large telescopes exoplanets are lost in the blaze of light from the star.
Astronomers have usually relied on indirect methods for detecting exoplanets:
- Radial velocity changes. The star will wobble because the gravitational pull of unseen companions. This wobble is detectable through periodic shifts in the absorption lines in the spectrum of the star.
- Brightness variations. Exoplanets may pass in front of their parent stars - an event called a transit. We have to be lucky; the orbit of an exoplanet be tilted at the correct angle to our solar system otherwise the exoplanet transit would not be visible.
In recent years improved telescope technology and image processing techniques have allowed the direct imaging of some exoplanets!
Exoplanet research is an ongoing area of active research. It is also an area to which amateur astronomers can contribute. The discovery of new planets is usually confirmed by subsequent observations. In some cases properties of the exoplanet orbits are determined with varying degrees certainty. Amateur astronomers are able to contribute observations of newly discovered exoplanets to improve and refine these initial orbits.
Planning to observe an exoplanet transit
In early 2021 I decided to try to measure an exoplanet transit using my own equipment. It wasn't clear whether I'd be successful. My telescope, with it's 8" aperture, was perhaps a little on the small side. The camera I use for taking images cannot be cooled to reduce thermal noise in the images.
It was in a spirit of nothing ventured, nothing gained, that I set out to find an appropriately "easy" transit to observe. Easy, in this case, meant a relatively big drop in starlight (roughly 2-3%) and a short transit (hopefully less than 2 hours). Fortunately finding candidates is quite an easy undertaking because predictions can be obtained from the Exoplanet Transit Database (ETD) website.
I identified a suitable candidate (TrES 3) on a day when clear weather was predicted for that night. My plan was to take pictures of the star before, during and after the predicted transit.
Imaging the transit of TrES 3b
The picture above was taken before the transit using a colour camera. It shows the target star TrES 3 at the centre of the frame.
For the actual transit observations I used a mono camera (a ZWO ASI178MM) at prime focus of my Meade LX10 8" telescope.
I decided on exposures of 2 minutes, beginning roughly 20 minutes before the transit began and extending just as long after the predicted end of the transit. At the end of the transit I took calibration frames (darks, bias and flat fields). My camera was not cooled but the ambient temperature was around 0 degrees C that night.
Extracting a light curve
The process of getting a light curve was relatively easy. I used free software called HOPS. In a mostly automated process - the calibration frames are applied to the images and alignment is performed.