Transit Method

When a planet passes directly between its star and an observer on Earth, the amount of light we observe with astronomical instruments decreases.

Image of the Sun as Venus passes between the Sun and the Earth. Venus is the small black circle that appears on the surface of the Sun in the upper left corner. Credit: SDO/NASA

The transit method is very similar to a solar eclipse, a well-known astronomical phenomenon on Earth. It occurs when the Sun and Moon are perfectly aligned. The Moon will then cover the Sun and completely block out its light.

When an exoplanet passes directly between its star and an observer on Earth, some of the star’s light is blocked. For a short period of time, the star dims slightly, as if it were winking at us.

The graph we see in the video is what we call a light curve. This is what is recorded by the instruments used by astronomers. We can see that, as the planet passes in front of its star, the light curve shows a decrease in brightness.

Credit : NASA

A larger planet will block out a bigger portion of the star, and so the decrease in brightness will be bigger. On the other hand, a small planet will cause a small decrease in the star’s light as measured by astronomical instruments.

The transit method is very effective in detecting exoplanets, as demonstrated by NASA’s Kepler Space Telescope that discovered thousands of exoplanets. This method allows us to determine their orbital period (the length of their year) and their size.

At the iREx, several researchers are using this technique to find new planets or confirm their existence. Jason Rowe, professor at Bishop’s University, specialises in the search and characterisation of exoplanets and is an expert in the transit method.