Of all the places we have ever looked at in the universe, only Earth has provided us with evidence for the existence of life. But why? Because life is rare, and it requires us all the conditions that we have on Earth in order to be sustained? Or because life is ubiquitous, but we found it here because it was the easiest place to find it?
Since everything on Earth is arranged as it is, we are accustomed to believe that if we had a planet and a star with the same properties as the Earth and the Sun - with the same age, with the same orbital distances, sizes and masses, from the same materials, then we would get life again. We also assume that other combinations are less likely. But all of our assumptions may be wrong. Earth can be as rare as life.
In 2015, NASA announced the discovery of Kepler-452b and called it “the most Earth-like exoplanet” ever discovered. Of course, she had many similarities with the Earth, and her star had many similarities with the Sun:
- Its home star is very similar to the Sun in terms of temperature, mass and size: it is a G2 star, about the same brightness and overall lifespan.
- It rotates at almost the same distance and with approximately the same period as our planet around the Sun: 385 days instead of 365.
- The star around which it revolves is not much more developed than our Sun: older by 1.5 billion years, which means it is 20% more energetically powerful and 10% colder.
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- The planet itself is not much larger than our Earth, and its radius is 60% larger.
And although these conditions may seem to you "similar to earthly", the discovered world, of course, has nothing to do with the Earth.
In our solar system, the difference between Earth and Venus is tiny: about 5% in radius. By comparison, the difference between Earth and Uranus or Neptune is huge: these worlds are four times the size of Earth in radius. Therefore, 60% more may not seem like an exaggeration, but there is a high probability that we will find a solid planet with a thin atmosphere, which will have the properties of a gas giant: a large shell of light atmospheric gases. In fact, there is a very narrow window that should be considered "terrestrial" in terms of planet size, and a deviation of more than 10-20% from terrestrial size would be too large.
However, there is every reason to believe that terrestrial planets are quite common. The latest results from the Kepler telescope show that there are at least 17 billion Earth-sized planets in the Milky Way's disk, and at least a few percent of stars will have at least one Earth-like world nearby. While our ultimate goal is, of course, to find a world with advanced biological life - preferably a world with life during the Cambrian explosion - our thoughts always return to Earth's twin. But such a double, even if it exists, may not be the best place to look.
Our Sun is a 4.6 billion year old G-class star. Although we think it is one of the most common, it is not: our star is more massive than 95% of all stars. M dwarfs, small red stars, are the most common type of stars in the universe: three quarters of all stars are M dwarfs. The oceans on our planet will boil in a billion years, but M stars will burn at a stable temperature for tens of trillions of years.
"Kepler" found many terrestrial planets near these M-stars, which were located in suitable places for water on their surface in a liquid state and whose mass was quite suitable for terrestrial definition. And while M stars are more likely to emit flares, and planets should be closer to them, they also provide a more stable environment for their planets, with less ultraviolet radiation and increased protection from the violent manifestations of interplanetary and interstellar space. The tidal forces from their stars are also stronger, and their shortened orbital periods provide them with an easy way to generate a large magnetic field, possibly protecting against flares.
These systems are fairly common, but Earth twin systems are not. What do we need for a true "double"? First of all we need a star like the Sun. This means that the star should not only be of the same temperature and spectral type, but also approximately the same age. It takes time for life to evolve and develop into something interesting, which means we need a stellar system that is many billions of years old. But we cannot wait too long, because as the stars age, the region of the core that connects hydrogen with helium grows, and the output power increases (and with it the brightness and temperature). Eventually, planets (like Earth) that were once habitable will become too hot, boiling water and preventing life from developing.
Let's say we have a window of 1-2 billion years, which is about 10% of the life of a star. There are about 200-400 billion stars in our galaxy, and about 7.6% of them are G-class stars, like our Sun. Although our Sun is more accurately classified as a G2V star, it still follows that about 10% of all G-class stars will be of the same type as our Sun. At the top, there are 400 billion stars, 7.6% of which are G-class, 10% of which are of the same subclass as the Sun, 10% of which are the right age for interesting life. These are 300 million stars. But even then, not all of them will have enough heavy elements to create the earthly world.
Above you can see the spectrum of the Sun. In other words, the lines you see represent a wide variety of atoms and their relationships. There are many of them on the Sun, and they have very specific relationships. The indicator that is not hydrogen or helium, but synthesizes materials in the Sun, astronomers call metallicity. If we want a terrestrial planet, we need stars with solar-like metallicities. It's not so bad; up to 25% of the stars that formed at the same time as our Sun were intermediate stars of population I, and many of them (perhaps about 15%) have the same metallicity as our Sun.
It turns out that in our galaxy there are 11 million stars like ours, with the same index of heavy elements. How many of these 11 million solar "twins" have Earth twins in their habitable zones?
We need to form a solid planet of the right size, with enough elements, the right amount of water, and in the right place to be considered the twin of Earth. All these problems are interconnected. One would think that if the central star had the correct elemental abundance, then the resulting planets should have the same density to radius ratios as in our solar system. But if your planet has 20% more radius than Earth, you will surely get an envelope of light gases - hydrogen and helium - that will shelter your planet, even if you are in the inner part of the solar system.
A world that is 60% larger than Earth will be five times its mass, which is too much to be a solid planet with a thin atmosphere. If we scroll through all the estimates again, we get from forty to a hundred thousand terrestrial planets with terrestrial-type orbits around solar-type stars. With 400 billion stars, the odds are extremely slim.
And remember that the real purpose of finding such planets is to find worlds that can support Earth-like life. And if that is the goal, don't look for the Earth's "double"; it is better to look for smaller planets near M-class stars. Better to look for terrestrial worlds in potentially habitable zones near stars. There will be many more options.
ILYA KHEL