The number of discovered exoplanets is constantly growing. And it is no longer so important for us to find another exoplanet, how to find one where there may be life, from where brothers in mind can fly to us, or where we ourselves, perhaps, will fly when the Earth becomes cramped for us or unsuitable for life. How to find such an exoplanet?
These and most other planets are considered potentially habitable primarily because they are located in the habitable zones of their stars - that is, where the planet can receive enough energy to keep the bulk of the water on the planet in liquid form. This zone is calculated based on the size and luminosity of the star.
Atmospheric composition and biomarkers
But what allows, looking from afar, to say that life nevertheless took advantage of its chance and woke up on a once lifeless planet? First of all, the presence in the atmosphere of the planet of certain chemical compounds - biomarkers, which say, for example, that life on the planet "breathes", that is, some biological processes are taking place on it. Take oxygen and carbon dioxide, for example. The first is secreted by plants as a result of photosynthesis and is consumed by animals in the process of respiration, the second is exhaled by animals and absorbed by plants. But this is just one example.
In total, five biomarkers are distinguished: water, carbon dioxide, methane, oxygen and ozone. Of course, each of them can have its own natural, not related to life, origin. But if they are found together, and even on a planet similar to Earth, then the likelihood that it is inhabited will be high.
There are several ways to find out the chemical composition of an exoplanet's atmosphere. The first is during the transit of the planet. The method is called "transmission spectroscopy". The planet passes between the observer on Earth and its star. The light of the star passes through the exoplanet's atmosphere and reaches the observer. But in this case, part of the light in the planet's atmosphere will be absorbed. If you carry out spectral analysis, you can find the chemical elements that participated in this. By breaking the light of a star into a rainbow spectrum, you can pay attention to dips - dark narrow spectral lines, each of which corresponds to a specific chemical element.
The tasks of searching for potential exoplanets will be faced by the replacements of the modern Kepler and Hubble telescopes. The replacement of the first - TESS - will search for exoplanets, the successor of the second - "James Webb" - to study in detail each of the found.
There are also two more promising ways. We see the planets of the solar system because they reflect the light of our star. All planets shine with reflected light. Including exoplanets. We can already see the light coming from some planets. And here we can again build a spectrum and try to find biomarkers. But, in addition, as it may seem surprising, the planets can "shine" and their own light. In this case, we are talking about infrared radiation invisible to the human eye. Both of these methods involve direct study of the planet, and not the light coming from the star. And here it does not matter how the plane of its orbit is turned towards our planet. But serious discoveries in this area are still ahead. We do not yet have sufficiently powerful telescopes.
The bowels of the planet
It would seem that the chemical composition of exoplanets is hardly possible to determine, but scientists are trying to do this. Thus, two of the five exoplanets discovered in 2012 near the star Tau Ceti were rushed to be recorded as potentially habitable. But in 2015, astrophysicists from the University of Arizona in Tucson (USA) determined the chemical composition of the star. They came to the conclusion that the bowels of the Tau Ceti contain much more magnesium than our Sun.
The star and the planets orbiting it formed from the same cloud of gas and dust. Consequently, according to scientists, the upper and deep layers of the mantle of these planets contain significant excess of magnesium-containing rocks - olivine and ferropericlase. Being more flexible and fluid than the rocks that dominate in the bowels of our planet, they will over time hinder the formation of lithospheric plates and the formation of the crust.
Despite the fact that Kepler-438b is already very similar to Earth (well, at least from afar - we are separated by about 470 light years), its star does not look like our calm Sun. Kepler-438 is a red dwarf, half the mass and size of our star. And it belongs to flare (variable) stars, which are capable of sharply and non-periodically increasing the luminosity several times. Studying the star, scientists found that flares on Kepler-438 occur quite often: once every several hundred days. Their power is ten times that of solar. These outbreaks are likely associated with coronal mass ejections, which can have serious devastating consequences for the planet's habitability. In such a turbulent environment, it is difficult for the planet to have an atmosphere, as it is exposed to excessively dangerous radiation and, most likely, is a place unsuitable for life.
If life could appear on such a planet, then, probably, its life would be short-lived. Scientists, of course, hope that Kepler-438b can have a magnetic field similar to Earth, but it is likely that even in such conditions, it will not help the planet.
The presence of a global magnetic field is a necessary condition for the existence of life. It protects the planet from cosmic radiation and prevents the solar wind from blowing away the atmosphere. How can you find it?
Being on the surface of the Earth, we know about its existence thanks to the compass. The magnetic needle, freely turning around its axis, is located along the lines of force of the Earth's magnetic field. Another sign of the existence of a magnetic field is auroras. They are caused by streams of solar wind entering the polar ionosphere. Terrestrial auroras are clearly visible from space, for example, from the International Space Station. But at considerable distances they can no longer be distinguished.
But it doesn't matter. The fact is that, in addition to radiation in the visible range, auroras also generate low-frequency radio waves. But they spread beautifully in space, it is much easier to detect them than the radiance itself. For example, the aurora on Jupiter was first recorded in this way - thanks to radio emission.
In addition, this method will make it possible to discover exoplanets that have not been previously discovered by other methods, to establish the length of the day on the planet, the inclination of the axis relative to the plane of the orbit and the inclination of the magnetic field relative to the axis of rotation of the planet, the period of its rotation and orbital period, and in some cases even the presence of satellites. Well, in fact, determine the parameters of the magnetic field.
Among the tools astrophysicists rely on are the LOFAR and SKA low-frequency ground-based radio telescopes. And in the future - space radio observatories and even telescopes on the Moon, which are perfect for this purpose.
Night illumination of alien cities and other "exotic" signs
Let's go back to how our, of course, habitable planet looks from space, while inhabited by representatives of intelligent life.Already on approaching it, a hypothetical alien could see the lights of our cities on the unlit side of the planet, receive our radio signals and even decipher them, and also watch our TV programs, in advance, even before arriving on the planet, getting acquainted with local life. All this could be done from distant space with the appropriate equipment. So earthly scientists have already thought, but not to look for signs of other civilizations by artificial illumination of their settlements in distant worlds?
Two famous American astrophysicists - Abraham Loeb from Harvard University and Edwin Turner from Princeton - suggested looking for artificially illuminated objects comparable in full brightness to a large land city on the outskirts of the solar system, in particular, in the Kuiper belt, and later, as they improve optical telescopes, extend this method beyond the solar system. Due to the different spectral composition of artificial illumination, it will be quite easy to separate it from the light of the parent star, which is reflected by the planet.
But Lisa Kaltenegger from Harvard University proposes to expand the list of biomarkers with substances that are exclusively of artificial origin. That is, those that should not be formed in nature, and primitive organisms do not produce them. For example, chlorofluorocarbons. They are good at absorbing infrared rays of the spectrum, which means they can be found in the atmosphere of other planets. If we ever find them, then we can say with confidence that somewhere in space there are still living beings that have evolved to such a level that they began to "civilized" pollute their planet.
In general, we can say that the number of signs by which we can judge the potential habitability of planets will only grow. Too many conditions must be met for life to appear on the planet. And they all need to be identified to be sure: the planet can be inhabited. But for this we need new, better tools.