How to detect exoplanets from your back garden

Use the transit method to detect a planet orbiting a distant star.

How to detect an exoplanet

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.

Detecting exoplanets using the transit method

Determine the orbital period of an exoplanet by measuring the elapsed time between transits.

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 above 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 four-hour 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.

You can see the results by downloading our Exoplanet Transit Detection PDF.

How to detect an exoplanet

Credit: ESO/L. Calçada

Here, I’ll look at the basic process you need to follow to detect an exoplanet for yourself.

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 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 computer 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, or HOPS software from ExoWorld Spies is free.

Detecting an exoplanet, step by step


Step 1

detect exoplanet step 01

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

detect exoplanet step 02

To find a target, pick an event by using an online resource such as the Exoplanet Transit Database. 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

detect exoplanet step 03

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

detect exoplanet step 04

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

detect exoplanet step 05

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

detect exoplanet step 06

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!


This guide originally appeared in the June 2022 issue of BBC Sky at Night Magazine.