Five short articles to better understand astronomy

Sometimes articles on space leave more questions than answers. So that you will not be lost in the abundance of scientific terms, I will introduce some astronomical concepts with the help of small related articles.

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How many planets are in the solar system?

The idea of what a planet is and how many of them in the solar system has changed over the centuries. Ancient astronomers did not have telescopes, and the only key characteristic that helped distinguish planets from other celestial bodies was that they moved across the sky relative to other stars. For them there existed stars and stars-wanderers-planets. Sometimes the Sun and Moon were also referred to the planets. The very word "planet", which in translation from ancient Greek means "wandering", "wandering", it allowed.

The geocentric system of the world assumed that in the center of the universe there is a fixed Earth, and the Sun, the Moon and planets revolve around it. But Copernicus placed the Sun in the center of the world. After that it turned out that the Earth, like other planets, also revolves around it. And if so, then the Earth began to be considered a planet, because it was no longer motionless, but moved around in a circle around the Sun.

After the final approval of the heliocentric system of Copernicus, the Moon remained the only companion revolving around our planet. But in 1610 the Galilean satellites of Jupiter were discovered. And then satellites found Saturn. At first, many different terms were used to designate the satellites of the planets: they were called moons, stars, secondary planets, and also simply planets. But over time the term "satellite" still ousted all the others.

The number of planets began to grow again by the middle of the 19th century. The status of the planet was assigned to any object orbiting the Sun, with the exception of comets. The list of planets was replenished at the expense of Ceres, Pallas, Vesta and Juno. And by this time, in addition to the planets, known since ancient times, added and Uranus. And in 1846 - Neptune. Since Ceres and similar objects were small in comparison with earlier known planets and were in one area of the Solar System, later called the asteroid belt, they decided to separate them into one separate group and called asteroids.

The growth of the number of planets ceased with the discovery in 1930 of Pluto. He became the 9th planet of the solar system. It was in this form that she was familiar to all of us. But by the end of the last century, the possibilities of astronomy increased. And we were on the verge of discovering new planets behind the orbit of Pluto. But the growth in the number of planets did not happen. The astronomical community, faced with the dilemma of assigning the status of the planet to new open celestial bodies or depriving Pluto of such status, chose the latter. In general terms, the situation of the 19th century was repeated. For newly discovered bodies (for today it is Eris, Haumea, Makemake) and for the Pluto and Ceres discovered earlier, a new category was introduced-dwarf planets.

Thus, today there are eight planets in the solar system, five dwarf planets. Among the eight "big" planets, four - Mercury, Venus, Earth and Mars - are called planets of the terrestrial group, and Jupiter, Saturn, Uranus and Neptune are giant planets. The latter are also called gas giants, two of which - Uranus and Neptune - are singled out as subclass of ice giants.

Objects called small planets (there is such an informal concept), several thousand. The small planets catalog is maintained by the Center for Small Planets, which is located at the Smithsonian Astrophysical Observatory. Among them there are many notable objects. These, for example, are such candidates in dwarf planets, like Kvar and Sedna.

But we are talking about open planets. The dimensions of our solar system allow us to accommodate more planets. In any case, Michael Brown, the "killer" of Pluto, is sure that there is one more, ninth planet in the solar system.

Why Pluto is not like other planets

Pluto has always been different. It is small, and its orbit is not the same as that of other planets. But the youngest in the family was forgiven. So why did not they forgive Pluto, depriving him of an honorary status?

©Depositphotos / Pluto

So, the first condition for being considered a planet is that the celestial body must orbit around the Sun. This condition is outside the definition of the satellites of the planets, although some of them are quite comparable in size to the planets, for example, the satellite of Jupiter Ganymede, which has a diameter exceeding the diameter of Mercury. Secondly, the celestial body must have sufficient gravity to have a spherical shape. Formless objects disappear, such as, for example, the asteroids of Pallas, Vesta and Juno. But still there is their neighbor on the belt of asteroids Ceres, which, although the smallest of the dwarf planets, is massive enough to allow it to acquire the shape of a ball. Finally, the third condition - near the orbit - should be a space free from other bodies.

Neither Ceres, located in the asteroid belt, nor Pluto, located in the Kuiper belt, could not clear the vicinity of its orbit from other objects.

At the same time, the requirements of small eccentricity of the orbit (circular orbit) and a small inclination of the orbit to the ecliptic plane did not fall into the list of conditions. Perhaps this is due to the fact that the orbit of the hypothetical new ninth planet will not correspond to these conditions.

Ecliptic and Zodiac

One of the key characteristics of any celestial body is the inclination of its orbit. For planets and other bodies revolving around the Sun, the inclination of the orbit, or more precisely, the plane of the orbit to the plane of the ecliptic, is taken into account. This allows us to understand how the celestial body moves in the solar system.

The plane of the ecliptic in the solar system is the plane of the Earth's orbit. If you know the value of the slope, you can imagine where to look for the object in the sky.

The orbits of all the planets lie near the plane of the ecliptic. Mercury stands out a little, its maximum angle to the ecliptic is 7.01 °. For comparison, the inclination of the orbit of Pluto, once the former ninth planet, is 17.14 °.

At the dawn of the solar system, the planets formed from a protoplanetary gas-dust disk. This explains why all planets revolve around the sun in the same plane. But there are celestial bodies in our system, whose angle of inclination is even greater, but about them later.

Where the ecliptic is, there is a zodiac. The ecliptic itself is a large circle of the celestial sphere, along which the visible annual motion of the Sun takes place. If we could see the stars and constellations during the day, we would observe the Sun in one of the zodiac constellations for a year. In May-June, for example, the Sun is in the constellation Gemini. Uranus will be in the constellation Pisces this June, and Neptune will spend a month in Aquarius. Neither the Sun nor the planets go beyond the "zodiacal belt".

It would seem that if everything in the solar system was formed from a protoplanetary disk, then the orbits of all bodies should lie in the same plane, but not. The inclination of the orbit of the comet Hale-Bopp, which arrived at the end of the last century to us from the Oort cloud, is 89.43 °. In 1997, it approached the Sun almost perpendicular to the ecliptic plane.

Sedna, "Voyager" and the edge of the solar system

Since the launch of the first artificial Earth satellite, 59 years have passed. During this time, we achieved a lot in cosmonautics. But dreams of science fiction about interstellar flights have not come true yet. In question, even going beyond the solar system. On the one hand, the speed of our spacecraft is unacceptably small, on the other - it is not entirely clear where this border is.

©Depositphotos / Voyager 1
The space probe Voyager-1 is the farthest object from the Earth, which was created by man. Launched in 1977 for the study of Jupiter and Saturn, in 39 years he retired from the Sun to 135 astronomical units. In terrestrial measures, the length is more than 20 billion kilometers. But to measure distance within the solar system, terrestrial measures are not entirely convenient.

The distances in the solar system and systems of other stars are measured in astronomical units. One astronomical unit is approximately equal to the average distance from the Earth to the Sun. This is almost 149.5 million kilometers. Thus, Voyager 1 left the Sun for a distance equal to 135 distances from the Sun to our planet.

For example, the average distance from Mars to the Sun is 1.52 a. e., from Neptune to the Sun - 30.1 a. e. Pluto's orbit, unlike the almost circular orbits of "large" planets, has a greater eccentricity, that is, an ellipse. For such celestial bodies, specifying the average distances does not make much sense. At perihelion (the nearest point of the orbit to the Sun), Pluto approaches our star at a distance of 29.7 a. e., in aphelion (the farthest point from the Sun) is removed by 49.3 a. e.

But these distances are nothing in comparison with the orbital characteristics of Sedna - the trans-neptune object, the next candidate for the title of a dwarf planet. Its orbit is even more elongated than the orbit of Pluto. The nearest point to the Sun is at a distance of 76 a from it. e. At the same time, the farthest point of the orbit is at a distance of 900 a. e. - almost 7 times farther than what is now Voyager 1.

Earlier several times there were reports that "Voyager 1" went beyond the solar system. Finally, NASA clarified the question - the spacecraft came out into interstellar space, but he did not leave the solar system. And consequently, this is not the same thing.

"Voyager 1" reached the heliopause, the boundaries of the heliosphere, the place where the final braking of the solar wind takes place. But the boundary of the solar system, according to scientists, should be considered the place where the forces of gravity of the Sun will be zero. Up to such a limit the probe will fly for another 300 years. For the Sun, such a boundary, according to modern estimates, is about 2 light-years away. Within these limits is, for example, the Oort cloud, from where the already mentioned comet Hale-Bopp flew to us.

Nemesis is a hypothetical companion of the Sun

But behind the Oort cloud, we can expect surprises. It's about Nemesis - a hypothetical star, a possible and not yet discovered companion of the Sun. Maybe no star, of course, and no. But in the vicinity of the Sun about half of the stars are double, there is a high probability that the Sun is also a part of the double star system.

The distance to Nemesis, if it exists, of course, is 50-100 thousand astronomical units. This, however, is an order of magnitude beyond the extreme point of Sedna's orbit. It is worth noting that Michael Brown, who is her pioneer, explaining Sedna's long orbit, suggests as one of the hypotheses the influence of the gravity of an undiscovered large planet beyond the orbit of Neptune. But the astronomer Walter Krattenden expresses the opinion that the undiscovered star of Nemesis influenced the orbit of a small planet.

But if we have a star so close to us, why have not we discovered it yet? It is explained simply. Stars are different, and not all of them are bright enough. Scientists who are looking for nemesis, suggest that this mysterious star can be a brown, red or white dwarf. By the way, the Sun is considered to be a yellow dwarf.

We are used to that the stars are such luminous giants in the cosmic abyss. Even giant planets, such as Jupiter and Saturn, look very small compared to them. But those stars, which belong to the above classes, are by no means the same. White dwarfs fit the size of our planet. Brown dwarfs can be compared in size to Jupiter.

Due to the small size and very low luminosity, such stars are difficult to detect, and if the Nemesis exists, this is one of the reasons that we have not yet found it. The question arises: what is the small and dull hypothetical Nemesis and similar famous stars, and the stars in general are different from the planets?

In the depths of stars, unlike planets, there are (or have occurred earlier) reactions of thermonuclear fusion. And for the beginning of a thermonuclear reaction, a substantial mass is needed. So, according to some estimates, Jupiter, which consists of hydrogen and helium, the same elements as the stars, in order to become a star, you need to increase the mass 47 times. We add that if there is a thermonuclear reaction, then there are luminosity and significant temperatures, which the planets do not have.