Extrasolar planets, also known simply as exoplanets, are planets that exist outside of the solar system. Just like the solar system, exoplanetary-star systems consist of different planet types such as gas giants and terrestrial planets. Gas giant planets are mostly made up of Helium & Hydrogen. They are much larger in size and tend to not have any solid surfaces. Terrestrial planets are mostly composed of a solid surface made of rocks and metals and, in comparison to gas giants, are much smaller in size.
Starlight provides most of the information about an orbiting planet. Any light radiated by the planet will be too faint compared to its parent star when observed from Earth. For this reason, starlight is the primary source of information about an exoplanetary star system. As a reference the sun is about a few billion times brighter than the Earth.
The luminosity of the Sun is Lsun = 3.8×1026 W (using its full surface area).
The luminosity of the Earth can be determined by considering the flux of sunlight incident on its surface (assuming it is a disk) and then calculating how much of this is reflected back into space:
Learth = Lsun(π)(Rearth)2(reflection coefficient known as albedo)
Therefore,
3.8×1026(π)(6.4×106)(6.4×106)(0.3) = 5.3×1016 W.
Hence,
Lsun/Learth = 3.8×1026/5.3×1016 = 7.2×109
Terrestrial exoplanets come in variety of different masses. Those with masses higher than Earth’s are referred to as super-Earths. These so-called super-Earths can have masses between 2-10 Earth masses1.
For a planet to host life, liquid water must be present. This can only be achieved if the planets orbit lies in the habitable zone of the parent star. A planets orbit in this special zone allows for its atmosphere to sufficiently warm the surface temperature enabling liquid water to exist1.
Detection of terrestrial planets could be the first step in search for intelligence life beyond the solar system.
The aim of this project is to understand the different detection mechanisms associated with terrestrial-like exoplanets that may exist in other star systems. The objectives to achieve this aim are:
- To examine the different techniques used to detect exoplanets and explore the most appropriate method for detecting terrestrial planets.
- Explore how the light detected from exoplanets could provide information on the nature of the type of planet.
- Analyze existing data of discovered exoplanets from online resources, establishing any trends in variables e.g. mass, radius.
Doppler spectroscopy (or radial velocity) method uses radial velocity measurements from the Doppler shifts of the light from the parent star of the planet. This method utilizes the fact that a parent star experiences a slight gravitational pull from the planet orbiting it, as a result the star does not remain stationary but rather moves in a circular motion. In this way when the star moves towards the Earth, the light spectrum is blue shifted and when moving away from the earth it becomes red shifted. Doppler spectroscopy allows the minimum mass to be found of the planet2.
When a planet passes in front of its parent star, from the observer’s point of view the light given out by the star dims slightly. This is the Transit method of detecting exoplanets2.
Direct imaging can be used to find exoplanets by taking pictures of the bright star and correspondingly using a coronograph to block out the stars light enabling the planet to become visible2.
Gravitational microlensing of light occurs when the gravity of a star bends the light coming from a more distant star. The distant star temporarily appears brighter due the light being focused by the gravity of the nearer star2.
Astrometry method of detection requires measuring the stars position in the sky in relation to other stars and observing how the position changes overtime. If the star has a planet orbiting it then the gravitational pull of the planet will cause the star to wobble and as a result its position in space will change2.
The Transit and Radial velocity methods have been the two most successful methods for detecting exoplanets.
By far the greatest number of planets found is by using the Transit method, 3126 planets2. Valuable information can be deduced from this method of detection such as the planets atmospheric composition as well as temperature. As the planet transits the parent star, some of the starlight passes through the planet’s atmosphere. Detecting and analyzing this light can lead to information about the planet’s composition e.g. water vapor, methane etc2.
There are three specific actions that will be carried out in order to achieve the main aim of the project. The three components are; study of different methods associated with the detection of exoplanets, study of the radiation emitted by exoplanets and study of existing databases of discovered exoplanets.
In order to complete the first component, a more in-depth study of the literature about the different detection techniques must be carried out. A more detailed knowledge of the mechanisms of detecting exoplanets particularly of terrestrial type. Study of the information from online planetary databases leading to being able to deduce on the nature of the exoplanet type e.g. terrestrial, gas giant etc.
- Determine the properties that define a ‘terrestrial’ exoplanet such as mass, radii, density, composition, atmosphere, distance to parent star and determine the upper and lower limits for each parameter from current understanding and observation.
- In-depth study of literature relating to different detection methods of exoplanets, in particular of terrestrial type. Determine which method is best suited to finding terrestrial exoplanets, now and in the future, from their properties in (1).
Light detected from exoplanetary systems will mostly be starlight as the light from the planet itself will always be too faint compared to the star. Studying starlight opens door a vast of information on the nature of the exoplanet in orbit. Study of line spectra enables to find out the chemical composition that makes up the star.
- Determine the limits for detecting light from the exoplanet directly and determine if any planet defined as a terrestrial exoplanet in (1) could be detected from its own reflected light.
- Examine how the study of starlight can help to determine if an exoplanet has an atmosphere and what molecules are present on such atmosphere. Investigate the line spectra of detected exoplanets and determine the atmospheric composition i.e. what molecules have been detected etc.
Examine simple orbital mechanics which will allow to determine properties such as mass and radius from remote locations. Online databases have properties of discovered exoplanets which can be accessed and investigated.
- Become familiar with online resources such as NASA Exoplanet database and the available data from such resources.
- Examine properties such as mass, radii, density, stellar type etc. of observed exoplanets and from (1) establish the average properties of terrestrial exoplanets. Determine how many possible terrestrial exoplanets have been discovered using the established average properties.
