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Potentially Habitable Exoplanets

Updated: Sep 13, 2021


Ötegezegen ve Gök taşları

The only place in the vast Universe that we know contains life – for now – is our planet Earth. And in such a paradise of endless possibilities, isn't there another Earth? It is the question that encourages scientists to find new worlds. Till now, they have discovered dozens of them so far. NASA's Kepler project, which aims only for this purpose, achieves very successful results. The news about the discoveries makes us feel like we're ready to visit them next year but even their appearance is limited to "Representative" pictures for now.

Dünya ve 452B
Artist's concept of Earth (left) and Kepler-452b, which is about 60 percent larger in diameter. NASA/JPL-Caltech/T. Pyle

In this article, I will share various information about the discovery methods of Potentially Habitable Earth-Like Exoplanets which are physically far away but scientifically close.


How Are Planets Outside the Solar System Discovered?


Planets outside our Solar System are called exoplanets for short. These planets can be discovered by various methods, but today, the largest number of Exoplanet discoveries are made by a method called Transit. The reason of this result is that Kepler's main purpose is to discover exoplanets using this method.


So, let's first take a look at the Transit Method, and then briefly at other methods.


Transit Method

We can define this method as “measuring the light curve effect of an exoplanet orbiting a target star as it passes between the star and our viewpoint”. To put it simply, the basis of this method is the study of the shadow formed by the exoplanet as it passes in front of its star.


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Transit Method – NASA/Ames

When the light curve is examined for a star whose size has been determined, the amount of brightness change shows the size (radius) of the exoplanet; the frequency of the change in the light curve shows the rotation period of the exoplanet in orbit of its star (1 exoplanet year). The determination of the orbital period helps to determine the approximate distance of the exoplanet to its star.


Brightness Change Frequency - NASA/Ames/Kepler Mission


In order to make a discovery with this method, the orbit of the exoplanet and its star must intersect with our point of view. For this, it is advantageous if the star is large and the exoplanet orbit is small. It is also necessary for the exoplanet orbital plane to be parallel to our point of view. The probability of a star/exoplanet system meeting these conditions is very low. Fortunately, there are billions of stars in our galaxy and we have many of them which respond to this need.


Uzay Gözlemevi ve Samanyolu
The Milky Way and an Observatory in Chile (ESO / S. Brunier)

Another disadvantage of this method is the high probability of error. Because, stars with different properties can create regular light fluctuations due to their structure. This can mislead scientists who study the light curve. A lot of detailed analysis is required to confirm the discoveries. A 2012 study on the subject suggests that the probability of false discovery may be around 35%.

However, no matter how difficult the conditions may seem, NASA continues its research with the Kepler space telescope, which was sent to space in 2009.


Kepler Uzay Teleskobu
Artist's concept shows the Kepler spacecraft in space. - NASA/Ames/JPL-Caltech/T Pyle

Other Discovery Methods

For those who are interested in the subject, it is useful to briefly introduce other methods used in exoplanet discovery.


Direct Imaging Method: With this method, very large (even more massive than Jupiter) exoplanets close to their star are directly imaged to provide exploration.


Doğrudan Görüntüleme Yöntemiyle Keşfedilen Ötegezegenler
HR 8799 (center) with HR 8799 e (right), HR 8799 d (lower right), HR 8799 c (upper right), HR 8799 b (upper left) from - W. M. Keck Observatory

Astrometry Method: An exoplanet orbiting its star exerts a gravitational force on its star, just as the star itself exerts. This causes the star to vibrate in a small orbit. This method enables exoplanet discovery by tracking this movement in the star.


Radial Velocity Method: "The movement of the star in a small orbit", which is mentioned in the astrometry method; It causes certain changes in the speed of the star relative to the Earth. By examining this velocity variation model, an exoplanet can be discovered around its star.


Çekim Etkisi ve Radyal Hızı Gösteren Grafik
Gravitational effect of the exoplanet over its star and the Radial Velocity change

Polarimetry Method: As a result of the absorption of the light emitted from the star by the components in the atmosphere of the exoplanet, refractions are observed at certain wavelengths in the light spectrum. These refractions give information about both the existence of an exoplanet and the compounds in that exoplanet's atmosphere. Although this method can be used in theory alone in exoplanet discovery, in practice it is used to determine the particle compositions in the atmosphere of a discovered exoplanet.


Işık Spektrum Grafiği
Örnek Işık Spektrumu Bileşen Soğurmaları – edX/Cosmology

Gravitational Lensing Method: Stars cause the light rays around them to bend, thanks to their strong gravity. This causes a star to act as a lens for another star behind it. Meanwhile, an exoplanet orbiting the star, which acts as a lens, causes aberrations on the lensed image. With this method, exoplanet discovery is carried out by examining lens image aberrations.


Kütleçekimsel Merceklemeyi Açıklayan Çizim
Gravitational Lensing Method – Nature/Vol.439/Num.7075/p.400

What Are the Criteria for Being an Earth-Like Planet?


Being Earth-Like is not that easy! Exoplanets discovered by the above methods qualify as Earth-Like after filtering through certain criteria. Let's briefly talk about these criteria.


Dimension

Although not very sharply defined, it is preferred that our Earth-Like candidate has a size of 0.8 to 2.0 Earth-wide, mostly determined by the Transit Method. The smaller ones are called Sub-Earth and the larger ones are called Super-Earth.


Structure

Our Earth-Like candidate is required to have a rocky surface. For this, in addition to the diameter of the object, its mass, which is mostly determined by the Radial Velocity method, is taken into account and it is expected to have a density close to the Earth's. This means that a value of around 5.5 gr/cm3 will be accepted. In addition, the atmospheric composition of the exoplanet, if any, takes its place among the criteria as a plus regarding its structure.


Star and Orbital Similarity

Since there are many different sizes and types of stars in our galaxy, the star and orbit similarity criterion is expressed by the term Habitable Zone. The stellar features and the orbital area within which liquid water can be found are determined. Our Earth-Like candidate is also expected to be located in this orbital area.


Gliese581 Ötegezegenlerinin Yaşanabilir Bölge Grafiği
Comparison of the Habitable zone of Gliese581 and the Solar System - European Southern Observatory

If an Exoplanet successfully passes all these challenging stages, it is proudly announced by its explorer as an Earth-Like exoplanet. As of September 2021, there are over 50 exoplanets with this title. The distance of these exoplanets to us varies between 5 and 5000 light years. So it is not possible for us to visit them for now.


But who knows, maybe it will be possible to travel at the speed of light in the coming ages. These travels may answer the question “Are We Alone in the Universe?” And perhaps one of these exoplanets will be our new home after our depleted Earth.

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