Overview of the Exoplanets-1

Planets orbiting a star other than the Sun are called exoplanets. Although scientists have predicted the existence of exoplanets for centuries, their discovery is relatively new and exciting. The reasons for the delay in the discovery of exoplanets are that their detection requires quite complex and innovative approaches. These methods are diverse.

There is much to be said about exoplanets. But the first question that comes to mind may be: Why are “Homo sapiens” so eager to search exoplanets?

The evolution process, which built humans step by step with the mechanisms of natural selection, gave him the ability to keep up with change, analyze, and, most importantly, a sense of curiosity.

Although the sense of curiosity is not unique to humans, we know that it plays a significant role in the development of humanity. The person who sets out with this curiosity is looking for answers to the questions in his mind.

• Are we alone in the universe?
• Where does life originate from?
• How did the earth we live on come to be?

We know that what we know and learn about exoplanets can lead us to the secrets of the universe’s existence and the origin of life. To better understand the evolutionary process that forms the skeleton of life, it is of great importance to investigate possible life on these planets.

Accessing Information About Exoplanets

As of November 13, 2022, there are 5,206 confirmed exoplanets discovered. There are also 9,084 candidate exoplanets (NASA, 2021).

Newly discovered exoplanets can be followed up-to-date on the exoplanets platform of the American National Aeronautics and Space Administration-NASA. The platform also has an advanced filtering system in itself. For instance;

It allows the filter out of planets discovered by the Kepler Space Telescope. In addition, filters can be made on the type of planets and the discovery method. Here you will notice that some methods have led to the discovery of many planets. For example, the ‘transit method’ pioneered the discovery of 3,314 exoplanets.

Figure 1. A section from the detailed chart on the NASA exoplanets platform. (Image Credit: NASA, 2021)

From left to right, planet names, how many light years away from the earth, the planet’s mass, brightness, and the year of discovery are given. Clicking on the planet names here brings up a page describing the details of the relevant planet.

One of the pioneering studies on exoplanets is NASA’s Exoplanet Exploration Program (ExEP).

ExEP; is a study that organizes NASA’s exoplanet research and research outputs and supports scientific and technological studies about exoplanets (NASA, 2022).

The report “New Worlds, New Horizons in Astronomy and Astrophysics” was published as an ExEP study in 2010 (National Research Council, 2010). This report is presented as a publication with detailed information about exoplanets, emphasizing the intersections of space studies with physics, chemistry, biology, and computer science and predicting future scientific opportunities.

The European Space Agency (ESA) has a platform that presents its data to cover exoplanets and future studies (ESA, 2022). You can access a detailed archive about exoplanets from the database created by the California Institute of Technology (Caltech, 2022).

Discovery of Exoplanets

Before discussing the properties of exoplanets, it is necessary to discuss their discovery methods’ details.

As might be expected, the first planets found are usually quite large compared to the Earth, as it is easier to detect large objects. Hot Jupiter and Super-Earth can be examples of giant planets (Mustill et al., 2016).

Finding habitable, Earth-sized planets is one of astrobiology’s most important research topics. The methods used to detect exoplanets are pretty diverse.

The book “Astrobiology” (David C. Catling, 2019), which has an essential place in the astrobiology literature, separates the most frequently used methods and divides them into indirect and direct detection methods.

In indirect detection methods, astronomers detect planets by looking at stars’ brightness and position characteristics. In direct methods, the image of the planets or the spectrum of the light they emit are used.

Figure-2. Exoplanet detection methods according to David C. Catling’s classification. (Image Credit: David C. Catling, 2019).

It is necessary to examine the graph below, which presents data on the methods that led to the discovery of the most significant number of exoplanets to understand David C. Catling’s classification system (Figure 3).

Figure-3. The number of exoplanet detections depending on the discovery year is given on the graph. (Image Credit: Caltech, 2022).2).

The graph is color-coded depending on the exploration methods.

• Blue: Direct imaging,
• Orange: Gravitational microlensing,
• Green: Transition Method,
• Red: Doppler Shift,
• Purple: Timing Method

Indirect Detection Methods

Explaining the mentioned methods is necessary to make sense of the underlying mechanisms.

A determined star’s movements are studied with a telescope’s help in astrometry. XParallax viu is supported by tools such as Astrometrica (Astrometrica) and Astrometry.net (Astrometry). The use and development of such tools are also a part of the field of astrobioinformatics (Computational Astrobiology).

Many scientific resources are compiled on astrometry, which forms the basis of what we know about the universe (Soffel, 1989; Kovalevsky & Seidelmann, 2004). The positions, sizes, and shapes of celestial bodies can be determined with astrometric methods. The primary purpose of astrometry is to use this information to describe the movements of objects.

After data such as position and size are collected, they are evaluated kinematically (for example, in stellar kinematics, a correlation is tried to be established with the motion by making use of the properties of the star such as chemical composition and age) and dynamically (movements are examined in terms of the sources that indirectly cause them) (Kovalevsky, 2002).

Another method, Doppler shift (also called radial velocity), is studying a planet’s star to determine its position in the light spectrum. It is similar to the Doppler effect in sound waves.

Blue and redshifts occur in the spectra of stars. These shifts indicate that there is a planet around that star. Two factors are taken into account when performing the analysis in question: The size and speed of the slip. The glide size indicates the planet’s mass, and its velocity indicates the time it takes to complete the orbit (Fischer et al., 2015).

The principle on which the transition method is based: when planets pass in front of their stars, their light decreases. The method that measures this decrease is called the “transition method” (Wright & Gaudi, 2012).

Transits can help identify exoplanet features. The size of the exoplanet’s orbit can be calculated from how long it takes to orbit once, and the planet’s size can be calculated from how much the star’s brightness dims.

Microlensing is based on Einstein’s “Theory of relativity.” According to the theory, if matter passes between the star and the observer, the gravitational field of the matter will cause the light to bend.

This forward material will focus on the light, and the star’s brightness will gradually increase. Although very useful for detecting exoplanets far from the solar system and orbiting far from their stars, it still cannot detect multiple planets in a single application. (Bennett et al., 2004).

Direct Detection Methods

The Hubble Space Telescope successfully detects exoplanets utilizing the coronagraph, which blocks the light of the stars. Another feature in telescopes on Earth is adaptive optics, which corrects distortions caused by the movements of the Earth’s atmosphere.

In addition, one of the methods used to eliminate the light of the stars is interferometers. It minimizes the wave and peak points with multiple telescope mirrors (Jones, 2008; David C. Catling, 2019).

The coronagraph is for dimming the overwhelming glow of the stars to reveal orbiting planets, and all this happens inside the telescope.

Masks, prisms, and detectors that come together to suppress the starlight are the essential components of this system. It contains self-flexing mirrors with thousands of tiny, piston-like actuators that flex in real-time when the telescope captures light that has traveled tens of light years from an exoplanet.

These “deformable mirrors” compensate for subtle imperfections in telescope optics to suppress starlight and make the planet’s light clearer.