Scientists Say There¡¯s Only One Earth-Like Planet In Our Galaxy

The idea of finding other Earth-like planets teeming with alien life has always excited the astronomers and the common folks alike. And that curiosity led to the discovery of the first exoplanet in the 1990s -- a planet outside our solar system, usually orbiting another star -- and since then that number has exploded to more than 4,000 confirmations, with thousands of other "candidates¡± awaiting confirmation.

NASA

These exoplanets come in a wide variety of sizes, masses, orbital distances and have compositions ranging from very rocky (like Earth and Venus) to very gas-rich (like Jupiter and Saturn). Some planets may be dominated by water or ice, while others are dominated by iron or carbon. A paper published in The Astronomical Journal used data from NASA¡¯s now-retired Kepler space telescope to conclude that our galaxy holds an estimated 300 million of these potentially habitable -- a rocky planet capable of supporting liquid water on its surface.

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But as it turns out, it takes a lot more than water for an alien exoplanet to harbour life as we know it.

Earth-like biospheres may be rare

You don¡¯t need Marvel superheroes to tell you how special the Earth is. All the world around us -- the planets and moons in our solar system -- and this is the only habitable planet, bountiful and wondrous. Life, as we know it, was birthed here and has since sustained, thrived and transformed the global ecosystem, as well as became far more complex, intelligent and technologically advanced. And therefore, when scientists say they are looking for Earth-like exoplanets, they are looking for conditions similar to what we have here on the Blue Planet.

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And part of what makes Earth a habitable planet is a biochemical process called Photosynthesis, which you may recall from your class VII NCERT textbook. It¡¯s the process by which plants use sunlight, water and carbon dioxide to create food and release oxygen into the atmosphere. This is critical in enabling complex biospheres of the type found on Earth, and for an exoplanet to be potentially habitable therefore means it would develop an oxygen-based atmosphere.

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A new research published in the Monthly Notices of the Royal Astronomical Society found that the planets that are both Earth-like and have just the right temperature for liquid water to exist on the surface are much less common. In fact, none of the several known rocky and potentially habitable exoplanets has the theoretical conditions to sustain an Earth-like biosphere by means of ¡®oxygenic¡¯ photosynthesis.

Except one: Kepler?442b

The study, which looked in detail at how much energy (radiation) an exoplanet receives from its host star, found that only one -- Kepler?442b -- came close to receiving enough sunlight necessary to sustain a large biosphere like Earth.

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Discovered in 2015, Kepler?442b is a rocky planet about twice the mass of the Earth that¡¯s orbiting a moderately hot star around 1,200 light years away in the constellation of Lyra.

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It¡¯s a super-Earth exoplanet that orbits its host star at a distance of about 0.409 AU (61 million km) and takes 112 days to complete one orbit of its star.

How the study was carried out

The scientists calculated the amount of photosynthetically active radiation (PAR) received by a planet from its star and discovered that stars around half the temperature of our Sun can¡¯t sustain Earth-like biospheres. This is because they do not provide enough energy in the correct wavelength range. Oxygenic photosynthesis would still be possible, but such planets could not sustain a rich biosphere.

NASA

Planets around even smaller and cooler stars known as red dwarfs couldn¡¯t receive enough sunlight to even activate photosynthesis. Stars that are hotter than our Sun are much brighter, and emit up to ten times more radiation in the necessary range for effective photosynthesis than red dwarfs, however generally do not live long enough for complex life to evolve.

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¡°Since red dwarfs are by far the most common type of star in our galaxy, this result indicates that Earth-like conditions on other planets may be much less common than we might hope,¡± lead author Giovanni Covone of the University of Naples said in a release.

¡°This study puts strong constraints on the parameter space for complex life, so unfortunately it appears that the ¡®sweet spot¡¯ for hosting a rich Earth-like biosphere is not so wide.¡±