Astronomical transit is when a celestial body in orbit passes around the host that it orbits, and we can see it from Earth. A special example of this is an eclipse, such as the solar or lunar eclipse. In a solar eclipse, the moon passes in front of the sun, obscuring our view of the sun and temporarily casting a portion of the Earth’s surface into darkness. Within a lunar eclipse, the moon travels behind the Earth so that it is in the Earth’s shadow. The same principles can be used to detect planets.

Importance of the Transit Method

There are various ways in which astronomers look for new planets and exoplanets. When observing a star outside our planet, instruments can pick up minor astronomic transits whenever an orbiting exoplanet passes between the star and the Earth in its orbit. There are other methods, such as microlensing, radial velocity, and even direct imaging, but astronomic transit yields the most results, and the majority of new findings come from using this technique.

How the Transit Method is Used Outside Our Solar System

In our solar system, the only astronomical transits we will see are the ones between Earth and the Sun. Namely, Mercury and Venus. However, we can also use this method to track other planets further away and even detect the movements of their moons.

Going outside our solar system, the technique is highly effective for discovering new exoplanets. Hot Jupiters, or gas giants, are more likely to be picked up due to their larger mass, and to find Earth-sized planets, scientists mainly observe red dwarf stars. These are the smallest kind of stars.

Types of Contact in Astronomical Transit

Contact in transit is defined as when the outline of the smaller planet touches the edge of the star that it is orbiting. There are different types of contact, and these can be used to determine the moving planet’s mass, orbiting speed and relative distance from the star.

First Contact

The first contact in astronomical transit is when the smaller body’s outer edge touches the edge of the larger body before moving in front of the larger body. This marks the beginning of the transit.

Second Contact

This is when the smaller body has completely moved in front of the larger body, and its outer edge is now touching the outer edge of the bigger body.

Third Contact

Marking the beginning of the end of the transit, the smaller body has completely moved across the bigger planet and touches the other edge of the bigger body. This is the third contact.

Fourth Contact

The transit has been complete when the fourth contact is made, and this is when the edges meet but the orbiting planet is leaving the transit. The planet is moving outwards and going to orbit the planet in a full cycle before it makes any contact again.

Fifth Contact

Also called the greatest transit, the fifth contact is when the centres of both stars align, marking exactly the halfway point of the transit.

Other Methods Used to Discover Exoplanets

Astronomers can also use other techniques to detect planets or, in many cases, to verify the findings using the transit method. This proof is needed for astronomers to conclusively agree on the existence of any new stars.

Methods to Discover Exoplanets

Radial Velocity to Calculate the Wobble in Stars

When exoplanets orbit stars, they will also move the host star from its relative position. These small movements are called wobbles and can be measured using the radial velocity method. The movements will change the frequency of the lightwaves and through the Doppler effect, will change the colour of the lightwaves ever so slightly.

Explore the Darkness in Space with Gravitational Microlensing

Microlensing is a method that can accurately determine the mass of stars and planet stars, regardless of how much light they emit. Celestial bodies that emit little to no light are more easily determined. It works by using a massive lens, which bends the light of a bright background object and studies the light displacement intensity. Monitoring this over a time period using photometry can give us an accurate light curve.

Blocking Out Infrared Radiation Using Direct Imaging

Planets do not offer much light compared with stars. The light they emit tends to get swallowed up by the star’s intense light waves, but by using thermal emission imaging and coronagraphs to block out the infrared radiation, we can get clear images of exoplanets.

Calculating Acceleration and Deceleration with Transit Timing Variation

The data made by the transit method can also be used in the transit timing variation method. Observing the historical information of the transit, scientists can verify the existence of planets by seeing if the transit has a strict timeframe or whether there is a variation. As planets orbit stars and the magnetic field of both pull on each body, the orbiting planet may accelerate or decelerate during the transition.

The Importance of Finding New Exoplanets

There have been numerous missions to find new exoplanets using the transit method, including the Kepler Space Telescope, TESS, HATNet and HELT. These terrestrial telescopes are among the most successful in the world, and astronomers have more planned projects to further our knowledge of the universe.

The main incentive to discover new planets is to find out whether there are potential candidate planets out there that can sustain life. Any exoplanet that is a similar distance to its host sun as the Earth is to our sun is in what is called the Habitable Zone. These are the prime targets for astronomers, as they can potentially support liquid water and stable atmospheres.