Practical astronomy projects for every level of expertise
DIY ASTRONOMY
Detect exoplanets from your back garden
Use the ‘transit method’ to capture and plot a distant star’s dimming caused by a nearby world
Exoplanets are planets that orbit stars other than the Sun. It’s incredible to think that before 1992 it was still uncertain whether exoplanets existed. Today, the number of known exoplanets is just over 5,000 and this total is increasing each day Here, I will show you that even amateurs can detect these faraway worlds from their back gardens.
There are several indirect ways by which exoplanets are found, as only a handful have been directly observed by very large telescopes. By far the most productive method is known as the ‘transit’ method, which works as follows. If there is a planet orbiting a star in such a way that the planet passes in front of the star as seen from Earth, the light from the star will dim by a small amount. The graphic (right) illustrates the dimming effect produced by the transit.
It is within the capabilities of amateur astronomers to measure this dimming and to detect the existence of this unseen body. The amount of dimming is important because it is tiny and often quoted in millimagnitudes. However, there are many exoplanets you can hunt down with dips in the light curve of 20 or 30 millimagnitudes, which is within the capabilities of an amateur. For example, in a fourhour run of 45-second exposures, where I captured a system called KELT-23A with an exoplanet called KELT-23Ab, the dip depth was 20 millimagnitudes (see ‘More Online’ box for more details ).
Here, I’ll look at the basic process you need to follow. First, select a host star and a transit event and then capture images of it from before the start (the ingress) until after the end (the egress). You’ll need to choose an exposure so that the star will not saturate the camera detector, but will give a good signal above background noise (unwanted artefacts). You can experiment by viewing the images in a software program that will show the brightness of the pixels.
When the event has been recorded, you need to use one of many software packages to perform a process called ‘aperture photometry’ on each image to come up with a brightness, or magnitude value of the star in each image. Note that you will need calibration frames for your detector with good flat frames being important for high accuracy. Calibration Time frames are vital: good flats reduce image gradients and help with checking brightness measurements against fixed-brightness stars. More software can be used to plot the resulting light curve in which you hope to see the transit event. Finally, check your PC clock is accurate and you too can detect a planet orbiting another star!
What you’ll need
A polar-aligned mount capable of tracking the stars. Autoguiding is useful, but not mandatory as a typical exposure is around a minute; less for the brighter host stars.
You can use whatever telescope you have! Smaller apertures can still work when trained on brighter host stars.
A detector; the best is a cooled mono CCD camera with filters. CMOS cameras are fine and a DSLR will also get the job done.
Use image capture software of your choice, as long as it will take a sequence. Save in FIT format as this can be read by photometry software.
Software for the photometry and plotting: there is plenty to choose from, including AstroImageJ, which is free (www.astro.louisville.edu/software/astroimagej/) or HOPS software from ExoWorld Spies (www.exoworldsspies.com/en/software/) is free.
Step by step
Step 1
To prepare, you’ll need a clear night. Try to avoid a very bright Moon, as this will cast a gradient and decrease the accuracy of your photometry. If you have a portable rig, make sure to get it set up and polar-aligned as early as possible.
Step 2
To find a target, pick an event by using an online resource such as the Exoplanet Transit Database (bit.ly/3DrNuBD). Enter your location and look for an event. The listings show the magnitude of the star and the dip, and how to set up a plan to image the event.
Step 3
Centre on the star (plate-solving will help here) and take a sequence of exposures to cover the expected transit, while allowing at least 30 minutes either side. WASP-35 is the star at the centre of the field of view (above).
Step 4
You may already have calibration frames, depending on your camera. If not, take bias, dark and flat frames. If you are new to this there are plenty of helpful resources. Good flat frames are essential in order to obtain the most accurate magnitude values.
Step 5
The next step is to perform the photometry. The software will require you to review the images and choose comparison stars. These should be of a similar magnitude to the target star and not variable. The photometry example (above) is in HOPS software.
Step 6
You can plot a graph on paper, or use the features of software such as HOPS or AstroImageJ (AIJ). The example shows how you can see just the ingress (start) of a transit by exoplanet WASP-35b. The rest was clouded out, but an exoplanet was detected!
Ian Sharp is based in West Sussex and has been an amateur astronomer for over 50 years