1. PART 2: The transit method is only one of many methods astronomers use to discover an exoplanet.

The orbit of an exoplanet may vary a lot, in terms of orbital distance and inclination.

A transiting exoplanet Is a planet which passes infront of the hoststar as seen from Earth.

When this happens a portion of the light from the star is blocked out and a decrease in the number of photons is measured.

Measuring the photon number or photon flux over time allows astronomers to make what is known as a light curve.

It is this light curve with it's characteristic shape and depth which lets astronomers characterise the exoplanet and in some cases its atmosphere.

It is truly remarkable that from observations of an unresolved point of light astronomers are able to infer precise values for the properties of the
star-planet system.

Assuming the star is of uniform brightness, the change in flux will depend on both the radius of the exoplanet, and the radius of the star.

If the radius of the host star is known, one also knows the radius of the planet.

For this to work we assume the exoplanet system is viewed from an interstellar distance so great that the distance to the exoplanet or host star can be considered equal.

Astronomers can estimate the inclination of the exoplanet orbit depending on how the exoplanet crosses the host star.

From a method known as the Radial velocity method, one has an expression for what is known as the minimum mass.

Knowing the inclination of the exoplanet orbit and knowing the minimum mass, one can calculate the mass of the exoplanet itself, assuming the stellar mass is known.

Having an estimate of both the radius and the mass of the exoplanet, one can easily calculate the average density.

With an average density astronomers can estimate what sort of planet it is. If the planet is a gasseous planet like saturn or jupiter or rocky like our own earth.

2. PART 1: The Universe is such a vast place that it is beyond the capability of mankind to understand it's true extent. The fact that there are on the order of 100 billion galaxies out there, each containing close to 100 billion stars is a mindboggling thought. Our star, the Sun, has 8 planets that orbit around it. This observation has naturally lead mankind to wonder how many other stars out there in the vast universe might also have planets orbiting around them. Although a relatively old question, it was not until 1995 (Mayor & Queloz 1995) that astronomers first discovered evidence of a new planet, orbiting a sun like star, outside of our solar system. Since then astronomers have discovered loads other planets. These planets outside our solar system, referred to as extra-solar planets or in short exoplanets, have been detected using many different methods. One of the main techniques, which will be covered in the next film, is the transit method.

# Exoplanets

Paul Anthony Wilson