Exoplanets — The Other Worlds
Searching for the habitable planets
Universe has no end. It continues forever. There are many stars. How many stars do you think are there in our galaxy alone?
— Some astronomers believe there are about 200 billion stars in our Milkyway galaxy. Scientists made assumptions that there are 11 billion potentially habitable Earth-sized planets in our Milky Way galaxy alone.
Similarly, how many galaxies do you think are there in our universe?
— It’s about 100 billion galaxies. And do you still believe there won’t be any trace of life in the entire universe?
Our world is not the only world in the entire cosmos. We believe because we inhabit it. Our search for similarity led to the search for similar earth-like planets. There are several other planets apart from the earth that are located in different parts of the universe. Our search for such worlds led us to develop futuristic technologies — some of them are Transiting Exoplanet Survey Satellite, Kepler, CHEOPS, and many more. It’s always been the human ambition to search for similarities. In the world of all other similarities, one of them is extrasolar planets or simply exoplanets. They’re everywhere. There are more planets than stars. Some of them are closer to their stars. Some are far. Some of them are even revolving around multiple stars.
What is an exoplanet?
An exoplanet is any planetary body that is outside the solar system and that usually orbits a star other than the Sun.
They are extremely far away. We can’t send a spacecraft on them. Exoplanets aren’t easily detectable as they’re fainter than the stars they revolve around. The very first evidence of the possibility of the existence of an exoplanet, orbiting Van Maanen 2, was noted in 1917 but was not recognized. The first confirmed detection of exoplanets came in the year 1992.
Most known exoplanets orbit stars similar to the Sun —the main-sequence stars of spectral categories F, G, or K.
Scientists say, “Some of these exoplanets are as (relatively) small as the moon, while others are as massive as Jupiter. Unlike Earth, most of the exoplanets are not tightly bound to stars, so they’re actually wandering through space or loosely orbiting between stars. We can estimate that the number of planets in this [faraway] galaxy is more than a trillion.”
How many exoplanets have been found?
All these years, there has been a continuous search for planets that are beyond the vicinity of our solar system. Up to now, in the present year 2022, the 5000th exoplanet has been firmly confirmed.
Habitable Zones
The habitable zone is also called the “Goldilocks Zone”. It’s a region that is neither extremely cold nor extremely hot. For a planet, the habitable zone is the distance from the nearby star that allows liquid water to exist on its surface — as long as that planet has a suitable atmosphere. For larger, hotter stars, the zone is farther away; for smaller, cooler stars, it can be very closer. In simple words, this region is the prime region in terms of search for extraterrestrial life. There are certain regions identified by scientists. There are many earth-sized exoplanets found in a specific location, around a star called TRAPPIST-1. It’s a red-dwarf star that is located approximately 40 light-years far away from earth. Scientists have predicted the presence of water on these exoplanets. With water comes life. That’s the exact reason, they’ve predicted the presence of life on these planets. There’s another planet called Proxima centauri b — that is the closest exoplanet where the possibility of finding traces of water is maximum.
Exoplanet Types
- Gas giants — These are huge exoplanets of the size of Jupiter or Saturn. These types of exoplanets might be in the closed orbits around their stars so they have high temperatures. Example — Kepler 16b
- Super earth — They may or may not have atmospheres. Their size lies in between Earth and Neptune. Example — Kepler 186f
- Neptune-like — Their atmospheres are dominated by hydrogen or helium. Example — OGLE-2005-BLG-390L b
- Terrestrial — They are of the size of the earth or are smaller in size. These planets are composed of rocks and metals. Some of these exoplanets have oceans or atmospheres or maybe habitable. Example — TRAPPIST-1 e
Methods used for studying exoplanets
Radial velocity method — There’s always a tug of war between stars and planets. We can’t perceive exoplanets we can see and study the star around which it’s revolving. With the use of highly sensitive spectrographs, scientists can track a star’s spectrum to find out periodic shifts. Initially, the spectrum seems blueshifted and then red shifted. In case, we find that the shifts seem to repeat at regular fixed intervals — days, months, or even years.
Next in the line of detecting exoplanets comes another method called the Transit method. Most exoplanets have been discovered using this specific method. A transit occurs when a planet comes in between the star and its observer. The dimming of the light of the host star when the planet comes in between gives rise to transit method observations.
What can we predict using these methods?
For example, take the case of the transit method. This method uses NASA was successful in finding out a plethora of exoplanets using this specific method. With the help of transit, we can predict certain characteristics of a planet. These include:
- The size of the exoplanet’s orbit can be calculated.
- The size of the planet can be calculated using the technique of measuring a star’s brightness especially when it’s lowered.
- Exoplanet’s atmosphere can be predicted using the transit method.
- The temperature of the planet can be predicted.
Some of the other methods to search for exoplanets
Direct Imaging — It’s a direct method while others are indirect methods. This method involves taking pictures of exoplanets by removing the overwhelming glare of the stars they orbit. It’s rare for astronomers to perceive an exoplanet using their telescopes as we might see Saturn through a telescope from Earth. Just a limited number of exoplanets have been found in this manner (and these tend to be young gas giant planets orbiting very far from their stars).
Gravitational microlensing — This method is an astronomical phenomenon due to the gravitational lens effect. This method can be used to detect objects ranging from the mass of a planet to the mass of a star, regardless of the light they emit. Light from a distant star is bent and focused by gravity as a planet passes between the star and Earth.
Astrometry — The orbit of a planet can cause a star to wobble around in space in relation to nearby stars in the sky
How to study the atmospheres of exoplanets?
This is done with the help of studying light. Light from a nearby star passes through an exoplanet’s sodium-rich atmosphere. The function of our telescopes in turn is to capture the light. The next process is to split the light into a rainbow spectrum using a spectrometer.
Similar other missions
- Kepler(now retired)was the mission that was used earlier for the detection of exoplanets.
- Hubble Space Telescope
- CHEOPS (Characterising Exoplanet Satellite)
- James Webb Space Telescope (There’s a special method that JWST would use for searching life on these planets — the method is called — Transmission Spectroscopy Method.)
- TESS(Transiting Exoplanet Survey Satellite) — TESS’ prime mission ended on July 4, 2020 and it’s presently in an extended mission. The prime mission of TESS included sky survey using giant mosaics produced as a result of twenty-six sector images. In capturing this giant mosaic, TESS found 66 new exoplanets, or worlds beyond our solar system, as well as nearly 2,100 candidates astronomers are working to confirm.
Future exoplanet missions
In 2025, Roman Space Telescope would be launched to detect earth-sized exoplanets.
European Space Agency mission ARIEL, launching in 2029, will observe exoplanet atmospheres.
Exoplanets are the future of human civilisations. Studying these planets would unlock the unsolved mysteries related to life and its diverse forms.
Our Milky Way galaxy alone has plenty of stars, planets, and would have had time to develop super-intelligent lifeforms. But when we look we don’t find any clue about such lifeforms. Forget about Milky Way, our universe has 100 billion galaxies. And, let’s assume at least one lifeform in half of these galaxies(each galaxy has billions of stars) — then there might be atleast 50 billion planets(the smallest number that can be assumed) inhabited by intelligent life. But the real question is, “where are they?”…..
