Question

Explain the astrometric method that astronomers use to search for other planetary systems that orbit a star.

NOTE: Please do not hand-write the answer as it is often difficult to read.

Answer 1

Their are many methods which can be used to detect the exoplanets , some of them are given below.

1. Transit method - The technique that has discovered the most exoplanets , the transit method, involves looking at dips in the observed brightness of stars caused by a planet passing in front of them and blocking the light. The amount in which the brightness drops can tell us how much of the star is being blocked, which lets us know the size of the planet. Since the planet will only block its star once every orbit, by looking at how often this happens we can also determine the planet's orbital period.

2. Radial velocity -  A star with a planet will move in its own small orbit in response to the planet's gravity. This leads to variations in the speed with which the star moves toward or away from Earth, i.e. the variations are in the radial velocity of the star with respect to Earth. The radial velocity can be deduced from the displacement in the parent star's spectral lines due to the Doppler effect. The radial-velocity method measures these variations in order to confirm the presence of the planet using the binary mass function.

3. Relativistic beaming - A separate novel method to detect exoplanets from light variations uses relativistic beaming of the observed flux from the star due to its motion. It is also known as Doppler beaming or Doppler boosting . As the planet tugs the star with its gravitation, the density of photons and therefore the apparent brightness of the star changes from observer's viewpoint. Like the radial velocity method, it can be used to determine the orbital eccentricity and the minimum mass of the planet. With this method, it is easier to detect massive planets close to their stars as these factors increase the star's motion. Unlike the radial velocity method, it does not require an accurate spectrum of a star, and therefore can be used more easily to find planets around fast-rotating stars and more distant stars.