Astrobiology: Introduction to Exoplanets
- Dev
- Sep 15, 2021
- 6 min read
Updated: Sep 22, 2021
This article is written for The Curiou Project, a social enterprise project to increase the accessibility and transparency of science for the general public. You can view their website and read more science at www.curiou.co.uk."
Hello everyone and welcome to Astronosmia! In today's article, I will be talking about exoplanets. Do you know what are exoplanets? If yes, great! But no worries if you don't! I will be covering these topics as we venture out into the vastness of the universe, to look for planets that might host human life in the future! That being said, let's begin!
Artist's impressions of few potentially habitable exoplanets: Kepler 22b, Ross 128b, Kepler 452b and Trappist 1e. Credit: NASA/JPL/ESA/Caltech
CONTENTS
1) What are exoplanets?
2) The search for potentially habitable exoplanets.
3) Different categories of exoplanets.
1) WHAT ARE EXOPLANETS?
As the name suggests, "exo-planets" means those planets which exist outside of our solar system and orbit another star. This means that planet(s) orbiting Proxima Centauri, which is the nearest star to our Sun will be called an exoplanet. Similarly, if that planet has a moon as the Earth has, it will be called an exomoon. This also implies to other stars.
For example, Proxima Centauri B, which is 4.2 light-years away from the Earth orbits Proxima Centauri. Therefore, it is an exoplanet.
Did you know?
Proxima Centauri B is a potentially habitable exoplanet and a mission is proposed to examine the habitability conditions on these planets? This mission is called the 2069 Alpha Centauri mission. You may read about it here.
BONUS: THE FIRST EXOPLANET DISCOVERY!
On October 6, 1995, scientists discovered the first exoplanet orbiting another star. This exoplanet is called 51 Pegasi b which is around 50 light-years away from the Earth in the Pegasus constellation.
Few other potentially habitable exoplanets include Gliese 667Cc, Trappist 1e, Ross 128b, Kepler 22b, Kepler 442b, Kepler 452b, etc.
So how do astronomers look for and identify potentially habitable exoplanets? Let's discuss this below!
2) LOOKING FOR POTENTIALLY HABITABLE EXOPLANETS
So what does make an exoplanet potentially habitable? There are several things scientists look at when determining the habitability of an exoplanet. Factors such as the habitable zone of the star, location of the planet with respect to its parent star, the spectral type of the star are taken into account while determining a potentially habitable exoplanet.
So, going further, the first thing in the determination of the habitable factor is the spectral type of the star.
Classification of Stars
There are over seven classes of stars used in astronomy. These are O, B, A, F, G, K and M with O-type stars being the hottest and most luminous and M-type stars being the coolest and less luminous when compared with O-type stars.
Astronomers preferably look for M-type stars (red dwarfs) as they are the most abundant type of stars in the universe and they have a larger life span lasting billions of years as compared to other types of stars. This is because M-type stars having a bigger lifespan and cooler temperatures will allow complex life to evolve on the planet's surface unlike O, A or B type stars which are too hot and have a smaller lifespan.
#BonusFact: Our Sun is a G-type star with a temperature of over 5,000 K (Kelvin). You can convert this temperature into Celcius by subtracting 273.15 from Kelvin (C+273.15=K).
The Habitable Zone
A planet's location with respect to its parent star is very important for its classification as a habitable planet. The planet should not be too close nor too far from its parent star. Astronomers look for traces of water in a planet's atmosphere to examine its habitability. Here comes the concept of the "Habitable Zone", also called the goldilocks zone.
The habitable zone is the range of the orbits around a star where liquid water can exist on the surface of the planet.
In the above image, the green zone is the habitable zone of the star, the red area is the zone where the temperatures are too high and the existence of life is unlikely, whereas the blue zone signifies the zone where temperatures are too cold.
The planet should be in the habitable zone because if the planet is too near to its parent star, it might be tidally locked which would mean one side of it would be facing the star permanently while the other side will be facing eternal darkness. This may prevent life from forming as one side of the star would be too hot and the other one too cold - difficult for liquid water to exist on both sides. Even if the planet is not tidally locked, temperatures would be too hot for liquid water to exist.
Similarly, if the planet is too far away from its parent star, it will be too cold for life to evolve on that kind of planet considering the planet will not have internal sources of heating to heat its surface.
3) DIFFERENT CATEGORIES OF EXOPLANETS
Our universe is very diverse. There are exoplanets of different characteristics. Below are some of the categories of exoplanets found to date:-
A) GAS GIANTS
There have been over 1,400+ discoveries of gas giants to date. Gas giants are planets that do not have a rocky surface (like the Earth or Mars) but a gaseous atmosphere (like Jupiter and Saturn). These planets are mainly composed of hydrogen and helium with a rocky core. The sizes of these planets vary a lot. Some can be huge while some could be of smaller size. The first exoplanet to be discovered, 51 Pegasi b, is a gas giant.
A sub-category of gas giants is called "Hot Jupiters". These planets are gas giants orbiting very close to their parent stars with fast orbital periods.
In our solar system, there are two gas giants, namely, Jupiter and Saturn. To learn more about gas giants, click here.
B) NEPTUNE-LIKE PLANETS
Neptunian exoplanets are exoplanets that have sizes similar to Neptune and Uranus. These planets have a hydrogen and helium dominated atmosphere with a rocky core. These planets are also sometimes also referred to as "ice giants" because compounds like water, ammonia and methane are also found inside Neptune and Uranus. Since these chemicals exist in the outer solar system in the form of frozen ices, these planets are called "ice giants".
There have been over 1,500+ discoveries of Neptune-like exoplanets in our universe. A sub-category of these types of exoplanets is what scientists call "mini-Neptunes". Mini-Neptunes are planets smaller than Neptune but bigger than the Earth.
Few examples of Neptune-like exoplanets are GJ 436 b and HAT-P-26 b.
C) SUPER-EARTHS
Super-Earths are exoplanets that are bigger than Earth and are potentially rocky worlds. However, super-earths are smaller and lighter than Neptune-like planets. These planets could be rocky, gaseous or a mix of both. These planets could be up to 10x the mass of the Earth and may have diverse characteristics. Some might be water worlds, some might be snowball planets or some maybe planets with a gaseous atmosphere.
One prominent super-earth is 55 Cancri e, which is also called a "diamond planet". This planet is mostly composed of carbon and is probably tidally locked. As per speculations, about one-third of the planet's surface may be carbon in the form of diamonds. The planet is located 40 light-years away from Earth.
Other super-earths such as Kepler 186f, Kepler 22b, Kepler 442b etc are considered to be potentially habitable exoplanets as they lie in or around the habitable zones of their parent stars. Want to read more about super-earths? Click here.
4) TERRESTRIAL PLANETS
Terrestrial exoplanets are planets that have a rocky surface and are sized about half of the size of Earth or twice its radius.
In our solar system, there are four terrestrial planets. These are Mercury, Venus, Earth and Mars.
As per NASA's exoplanet website, there have been a total of 166 confirmed terrestrial exoplanet discoveries to date. One such terrestrial exoplanet is Trappist 1e, located about 39.46 light-years away from Earth.
TRAPPIST 1e orbits the ultra-cool red-dwarf star TRAPPIST 1 and has an orbital period of 146 hours or about 6 days. The planet is inside the habitable zone of its parent star, hence, is considered a strong candidate for habitability. TRAPPIST 1e might have a temperature range of about (-27.5°C to 3.5 °C).
For more information on terrestrial planets, check out NASA's official exoplanet website here!
Thank you for reading this article till the end! I hope you had a good time reading the article and you now know briefly about exoplanets. Subscribe to my blog to get the latest updates from me!
P.S.: Want to explore exoplanets and the universe? Click here and you will be transferred into space! ;)
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