I do not know if you have dedicated yourself to observing the planets in the sky. First, they have static light, unlike stars, whose light sparks. Another characteristic of them, if they have the patience to observe them for several months, as the ancients did, is that they show a strange motion, different from that of the stars. Typically, the planets move eastward with respect to the stars in motion called direct motion. Sometimes, they reverse their motion and move westward. This is their retrograde motion. After a period of retrograde motion, they change the direction of motion again and continue moving in the original direction. The time that passes between two successive retrogradations depends on the planet. For example, for Mercury, this happens every 116 days, and the planet moves retrogradely for approximately 21 days; for Jupiter, it moves every 399 days, and the retrograde motion lasts 121 days. The retrograde motion of Mars particularly disconcerted ancient astronomers because, in a geocentric system, Mars orbit, during retrogradation, appears to pierce that of the Sun. Because of their strange motion, the Greeks gave the planets the name of plànētes asteres, “wandering stars”. The Greeks knew 6 of the planets we know: Mercury, the messenger of the gods, Venus, the goddess of beauty, Mars, the god of war, Jupiter, the father of all gods, Saturn, the lord of time. Even the sun and the moon were planets for them. Over millennia, their number had increased to 9, with the addition of Uranus, Neptune, and Pluto. After 2000, objects located beyond Neptune, i.e., trans-Neptunian objects, were discovered: in 2003, Haumea was slightly smaller than Ceres, Sedna had a diameter of almost two thousand kilometers, and in 2005, the discovery of Eris with dimensions and masses similar to those of Pluto was announced. After these discoveries, Pluto’s status as a planet was rethought. One fine day in 2006, Pluto was demoted to a dwarf planet; therefore, today, the number of planets is 8. A definition of a planet was also introduced on August 24, 2006, by the International Astronomical Union, according to which a planet is a celestial body that orbits around a star and does not produce energy through nuclear fusion; its mass is large enough to give it a spheroidal shape, and owing to its gravitational force, it manages to keep the orbital belt free from other bodies of comparable or larger dimensions. Pluto does not satisfy the last condition. Now you might be wondering why I’m talking about planets given that our discussion is related to life and, in particular, to extraterrestrial life, given that you will surely have heard that in our solar system, there is life only on Earth. Well, to be honest, we’re not truly sure. If you remember, when talking about ALH84001, we said that perhaps it brought life from Mars to Earth. We have seen that there are some experiments that disagree on this, but we cannot be 100% sure that there was no Martian life in ALH84001. We will also discuss experiments carried out on Martian soil, which still spark debates about whether there is life on the red planet. Furthermore, as we will see, it is possible that there is life in some satellites of the gas giant planets (Jupiter, Saturn, etc.). The objective of this chapter is precisely to discuss whether life can exist in our solar system. Microbial life, obviously. We do not expect to find humanoid Martians. These traces can be revealed directly or through chemical or energetic manifestations. In other words, it is necessary to understand whether probiotic molecules are present or can be formed and whether there is a source of energy and an adequate liquid medium. Light energy is the most effective energy source for biological processes, and the sun provides this energy to all bodies in the solar system. A favorable chemical environment presupposes the existence of carbon and organic molecules, and the presence of compounds of oxygen, nitrogen, sulfur, and phosphorus is also important. Currently, carbon, hydrogen, oxygen and nitrogen are among the most abundant in the Universe. The probiotic compounds could come, as on Earth, from space or perhaps form on site. It is very likely that all the bodies in the solar system have a minimum of these probiotic compounds. For life to arise from these molecules, a liquid environment on the surface or below it is essential. The habitability of a “world” is closely linked to the presence of a liquid such as water or others such as ammonia, simple hydrocarbons, etc. There are several places in the solar system that meet this requirement. In 2001, the astrobiologist Schulze-Makuch and others proposed an index to evaluate the habitability of a planet and its moons, the PHI index (planetary habitability index). This parameter is based on the variety of chemical elements and physical characteristics, such as the presence of a solid substrate, the availability of energy, the presence of liquids and a favorable chemical environment. Earth has a PHI of 0.96, followed immediately by Titan with 0.64, Mars (0.59), and Europe (0.49). Another useful index is the Earth similarity index (ESI). By definition, the Earth has an ESI equal to 1, and then Mars has 0.70, Mercury 0.60, the Moon 0.56 and Venus 0.44. This index is less useful than the PHI for the bodies of our solar system, whereas it is more relevant for extrasolar planets for which there is not much data on habitability.

Life in the Wanderers of Sky

章节
发表日期：

2025年

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Antonino Del Popolo

摘要

I do not know if you have dedicated yourself to observing the planets in the sky. First, they have static light, unlike stars, whose light sparks. Another characteristic of them, if they have the patience to observe them for several months, as the ancients did, is that they show a strange motion, different from that of the stars. Typically, the planets move eastward with respect to the stars in motion called direct motion. Sometimes, they reverse their motion and move westward. This is their retrograde motion. After a period of retrograde motion, they change the direction of motion again and continue moving in the original direction. The time that passes between two successive retrogradations depends on the planet. For example, for Mercury, this happens every 116 days, and the planet moves retrogradely for approximately 21 days; for Jupiter, it moves every 399 days, and the retrograde motion lasts 121 days. The retrograde motion of Mars particularly disconcerted ancient astronomers because, in a geocentric system, Mars orbit, during retrogradation, appears to pierce that of the Sun. Because of their strange motion, the Greeks gave the planets the name of plànētes asteres, “wandering stars”. The Greeks knew 6 of the planets we know: Mercury, the messenger of the gods, Venus, the goddess of beauty, Mars, the god of war, Jupiter, the father of all gods, Saturn, the lord of time. Even the sun and the moon were planets for them. Over millennia, their number had increased to 9, with the addition of Uranus, Neptune, and Pluto. After 2000, objects located beyond Neptune, i.e., trans-Neptunian objects, were discovered: in 2003, Haumea was slightly smaller than Ceres, Sedna had a diameter of almost two thousand kilometers, and in 2005, the discovery of Eris with dimensions and masses similar to those of Pluto was announced. After these discoveries, Pluto’s status as a planet was rethought. One fine day in 2006, Pluto was demoted to a dwarf planet; therefore, today, the number of planets is 8. A definition of a planet was also introduced on August 24, 2006, by the International Astronomical Union, according to which a planet is a celestial body that orbits around a star and does not produce energy through nuclear fusion; its mass is large enough to give it a spheroidal shape, and owing to its gravitational force, it manages to keep the orbital belt free from other bodies of comparable or larger dimensions. Pluto does not satisfy the last condition. Now you might be wondering why I’m talking about planets given that our discussion is related to life and, in particular, to extraterrestrial life, given that you will surely have heard that in our solar system, there is life only on Earth. Well, to be honest, we’re not truly sure. If you remember, when talking about ALH84001, we said that perhaps it brought life from Mars to Earth. We have seen that there are some experiments that disagree on this, but we cannot be 100% sure that there was no Martian life in ALH84001. We will also discuss experiments carried out on Martian soil, which still spark debates about whether there is life on the red planet. Furthermore, as we will see, it is possible that there is life in some satellites of the gas giant planets (Jupiter, Saturn, etc.). The objective of this chapter is precisely to discuss whether life can exist in our solar system. Microbial life, obviously. We do not expect to find humanoid Martians. These traces can be revealed directly or through chemical or energetic manifestations. In other words, it is necessary to understand whether probiotic molecules are present or can be formed and whether there is a source of energy and an adequate liquid medium. Light energy is the most effective energy source for biological processes, and the sun provides this energy to all bodies in the solar system. A favorable chemical environment presupposes the existence of carbon and organic molecules, and the presence of compounds of oxygen, nitrogen, sulfur, and phosphorus is also important. Currently, carbon, hydrogen, oxygen and nitrogen are among the most abundant in the Universe. The probiotic compounds could come, as on Earth, from space or perhaps form on site. It is very likely that all the bodies in the solar system have a minimum of these probiotic compounds. For life to arise from these molecules, a liquid environment on the surface or below it is essential. The habitability of a “world” is closely linked to the presence of a liquid such as water or others such as ammonia, simple hydrocarbons, etc. There are several places in the solar system that meet this requirement. In 2001, the astrobiologist Schulze-Makuch and others proposed an index to evaluate the habitability of a planet and its moons, the PHI index (planetary habitability index). This parameter is based on the variety of chemical elements and physical characteristics, such as the presence of a solid substrate, the availability of energy, the presence of liquids and a favorable chemical environment. Earth has a PHI of 0.96, followed immediately by Titan with 0.64, Mars (0.59), and Europe (0.49). Another useful index is the Earth similarity index (ESI). By definition, the Earth has an ESI equal to 1, and then Mars has 0.70, Mercury 0.60, the Moon 0.56 and Venus 0.44. This index is less useful than the PHI for the bodies of our solar system, whereas it is more relevant for extrasolar planets for which there is not much data on habitability.