Contrary to the "announcements" of many Russian media outlets, no alien life has been discovered. However, the real number of potentially habitable planets in the TRAPPIST-1 system is not three, but seven. However, almost certainly all of them are not yet inhabited.
Yesterday's news of the discovery of a system of seven Earth-like planets near an ultracold red dwarf, 40 light years away, was a real sensation. And it raises a number of very difficult questions. How is it that one red dwarf has so many solid planets? Why are they huddled so closely together that three of them fell into the formal habitat zone at once? How many of them are actually in it? And are there oceans?
Seven planets are just a minimum score
The publication in Nature on this topic is replete with unexpected details. For example, seven planets is actually a very modest estimate. The main source of new data on their number is the Spitzer space telescope, and it was used for only one twenty-day observation cycle. Transit detection requires at least one passage of the exoplanet between the disk and the terrestrial observer. If a star has more planets, but their year lasts longer than 20 Earth days, then they could not be seen in this way. To clarify the issue, it took much longer to observe. Let's take the solar system: in order to find the Earth by the transit method, it would be good for alien astronomers to look at the solar disk for a year. And to see the hypothetical ninth planet - from 10 to 20 thousand years. Had they observed 20 days, they would not have discovered even Mercury. Therefore, the new seven planets are thosewhat was easiest to find, and not the entire planetary population of a distant system.
Such a large number of planets is not very common, as TRAPPIST-1 is 12 times lighter than the Sun. Yes, there are more heavy elements (109% of the solar level). Solid planets and the cores of gas giants are "made" of them. So with such a "metallic" composition, multiplanetaryism is not surprising. Another thing is striking: usually red dwarfs do not show such "metallicity". In this regard, the star is completely atypical, apparently, it arose in a region abnormally saturated with heavy elements.
The tightness is like in a communal apartment - and that's good
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The first planet in the system, TRAPPIST-1b, is about 1.6 million kilometers from its star (four Earth-to-Moon distances), and the seventh planet, TRAPPIST-1h, is less than 10 million kilometers. This is not a planetary system, but a "communal apartment" in which the density of the planets is ten times higher than ours. The moons of Jupiter are located at a distance of up to 30 million kilometers from their planet, and in fact its gravity is much weaker. The authors of the work believe that the planets in such tightness simply could not arise, they would not have enough material for the protoplanetary disk. For the large satellites of Jupiter, it was shown quite a long time ago that they arose farther from the planet than they are now, and then gradually migrated to it.
The fact that TRAPPIST-1 exoplanets are migrants is a very good sign. In the first tens of millions of years of existence, a red dwarf can produce very strong flares, tearing off and carrying away from the planets light elements, the atmosphere and the hydrosphere - something without which there will be no terrestrial life. The discovered seven celestial bodies escaped this fate by spending their "youth" in safer distant orbits.
Three inhabited, seven or more?
The authors of the article in Nature tried to be very moderate in their assessments. They considered that only TRAPPIST-1e, f and g are in the habitable zone of the system, which receive 0.66, 0.38 and 0.26 from the radiation that the Earth gets from the star. The density of TRAPPIST-1e is 20% lower than that of the Earth, f is 40%, and g is 6%. That is, they all contain more light elements, gases and, possibly, water. A denser atmosphere retains heat better. The tidal impact of a nearby star on planets and planets on each other additionally heats up their cores. Because of this, there are frequent volcanic eruptions and a higher heat flow from the center of the planet. So, despite the low illumination, life there, in theory, will not freeze.
But the first three planets receive 4.25, 2.27 and 1.14 times more radiation from their star. If today's Earth was even 14% more warm, the oceans would quickly boil on it. However, one should not compare the conditions in a "communal apartment" near the TRAPPIST-1 star and in a "decent apartment" (near the Sun). All seven open planets of TRAPPIST-1 are tidal. That is, they always look at the star with one side, showing the other to the rest of the cosmos.
Planetary scientists have long simulated conditions on such exotic planets. And it turns out that if there are no particularly dense atmospheres, then it is hot on the day side, cold on the night side, and on the terminator, with its eternal dawn, the conditions are intermediate. This means that the first three planets may well have liquid water on the surface in the region of the terminators, which the researchers honestly discuss.
For the same reason, the seventh planet can be potentially "alive". Yes, she receives only 0.13 of the energy that reaches the Earth. But on the day side, that means 0.26 of the Earth level. With a thick atmosphere and a core heated by tidal interaction, this is a lot. The authors admit that if there is hydrogen in the planet's atmosphere (a very effective greenhouse gas), then there will be liquid water on the surface even at low insolation.
As we can see, for a planet that is always looking at its sun, the concept of a "habitable zone", that is, such a distance from the star, at which life is possible, is still rather vague. On the day side, it is one, on the night side, another, on the terminator - the third. Only with a very dense atmosphere, such as that on Titan, will the temperature equalize at all points of the exoplanet's surface.
Without learning more about the atmospheres of new worlds, it will be difficult to determine the habitability zones there. One thing is for sure: the estimate of "three potentially inhabited" is the lowest possible. In practice, there may be seven of them, and maybe much more. It is well known that planets can have very large satellites, on which (the same Titan) the atmosphere is denser than the earth's and the seas are also quite extensive. Telescopes of the next generation are needed to better learn about the satellites of these seven planets, and until then life cannot be ruled out there.
Seven Worlds: Hardly Green, Hardly Ice
On Earth, green vegetation is associated with life. However, this was not always the case - more recently, photosynthetic organisms on our planet were very often red (purple bacteria). If TRAPPIST1 has its own vegetation on exoplanets, it is unlikely that we will like it in appearance. 95% of the radiation energy of a red dwarf lies in the infrared part of the spectrum. Local plants will have to use it, and they will also reflect the non-green part of the visible spectrum in order to avoid overheating. Among astrobiologists, the question of what color will be is still controversial, but most talk about black or red, or their combinations.
However, there is some good news. Ice caps are practically not formed on such worlds. Ice at the poles of the Earth does not melt in the summer only because it reflects light well. In the infrared part of the spectrum, ice absorbs almost everything, so there it must melt very quickly. This means that the onset of regular ice ages, as on Earth in the last few million years, is extremely unlikely there.
Should you look for alien pen pals?
As you know, a session of black magic should be followed by its exposure. Here it is: you shouldn't send radio signals there. The age of the star TRAPPIST-1, according to astronomers, is only 500 million years, and part of that time its planets have spent much farther from their star than they are now. This means that in this planetary system it is simply too early to wait for the emergence of complex life, or even life in general. The Earth, according to today's concepts, became an inhabited planet 3.5-3.8 billion years ago, 0.7-1.0 billion years after its formation. The most radical date put forward is 4.1 billion years ago. It is unlikely that a radio was invented there in 100 million years.
Potentially, the system 40 light years away is very, very suitable for life, but it is probably too early to hope for contact with its inhabitants. However, their future looks more confident than ours. The red dwarf is less massive than the yellow ones (the Sun), and therefore lives much longer. Earth's oceans will boil away in a couple of billion years, and then the Sun will become a white dwarf. Whereas TRAPPIST-1 will be able to properly shine for the inhabitants of its system for another 4-5 trillion years - a thousand times longer than the time allowed for earthly life.
Lysyakov Ivan