How to understand if a planet is suitable for life? Telescopes capable of seeing a living creature on another planet, even if it is very large, are not expected in the near future.
The first thing that comes to mind is whether the temperature on the planet is right for water to exist on it in a liquid state. This is how the concept of "habitable zone" appeared. Astronomers search for and find planets in the habitable zones of different stars.
And, of course, it is very important to understand whether there are substances necessary for life on the planet. For example, it is the rich chemistry of Saturn's moon Titan that makes scientists suspect that life is already there now or may arise in the future. For the sake of the potential prosperity of this life, the still functioning Cassini apparatus was even destroyed.
But you can't send a probe with chemical analyzers to the planet of an alien star (at least not yet). How, then, do you know the chemistry of the distant world? Of course, by examining the spectrum of radiation coming from a distant planet. (By the way, this was the way Hubble discovered water on earth-like planets not so long ago.) Moreover, one should not search in the visible range. Infrared astronomy, which we talked about recently, is much more effective in helping in this matter.
Many chemical compounds have their own "signatures" in the infrared range - "peaks" and "dips" on the spectrum graph (corresponding to the emission and absorption of radiation of a certain wavelength and similar to thin lines). It is on these traces that the necessary substances can be identified.
Of course, scientists are primarily interested in oxygen, methane, ozone, water vapor, carbon dioxide and nitrous oxide (N2O). By the way, it was living organisms that once made the "oxygen revolution" on Earth. However, it is difficult to observe the lines of these gases; it requires many days of observation time. And astronomers are always in short supply, because there are far fewer tools and funding than interesting tasks.
A team of scientists led by Vladimir Airapetian of NASA has proposed a new way to search for planets with habitable chemistry. It uses "signatures" that are much easier to detect. But, as usual, there is a nuance: they appear only due to star storms.
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We wrote in detail about the coronal mass ejections that the Sun regularly "spits" with. Due to its magnetic field, they have little effect on the Earth (by the way, Mars was less fortunate). But this is because our luminary is mature and calm.
Young active stars, similar to the Sun or slightly colder (spectral classes G and K), have stronger and more frequent ejections. Therefore, charged particles, breaking into the atmosphere of the planet, are able to make a rustle in it. This is what the new method relies on.
Under the action of stellar wind particles, water molecules (H2O) are converted into hydroxyl molecules (OH-). At the same time, atmospheric nitrogen is oxidized to its monoxide (NO). These two substances have very clear infrared signatures.
Are there enough of them to be "seen" from the Earth? To answer this question, the researchers adapted a model that has been used in atmospheric physics for decades. This model predicts the response of the Earth's ozone layer to coronal mass emissions. Hayrapetyan's group managed to adapt it to the new task. In addition, the authors used data from the TIMED spacecraft, which studies the composition and temperature of the Earth's atmosphere.
Computer modeling with the use of "space" data has shown that in the atmosphere of a planet similar to the Earth, but revolving around an active star, a lot of "signal substances" are formed. So many that an infrared telescope with a diameter of only 15-25 centimeters could distinguish them in the atmosphere of nearby exoplanets in two hours of observation.
Such a signal would mean that oxygen, nitrogen (the most important component of proteins) and water vapor are present on the planet, and even atmospheric pressure is close to Earth's. This means that such a world can be inhabited.
However, if the signal is too strong, this does not bode well. After all, this means that the planet does not have a magnetic field that would hold back almost all the stellar wind (and this is necessary so that living beings on the planet do not die from radiation). Therefore, a "moderate" signal indicates that the planet is suitable for life.
A scientific article with the results of the research was published in the journal Scientific Reports.