In the same orbit as the Earth. According to the giant impact theory, the collision with the Earth led to the formation of the Moon. It probably existed for hundreds of millions of years from the formation of the Solar System (~ 4.6 Gigalets) until the moment of collision with the Earth (~ 4.5 Gigalets).
The object was formed at the Lagrange point (L4 or L5) in the Earth-Sun two-body system. Theia's mass was approximately the same as (1/10 of the Earth). The planet is named after the titan Theia - the mother of Selene (goddess).
According to some planetesimal data, Theia probably existed 30-50 million years from the formation of the Solar System and collided with the proto-Earth 4.53 Gigalet (billion years) ago. According to the results of a comparative analysis of the distribution of rubidium and strontium isotopes on the Moon and Earth carried out in 2008, the collision occurred 4.48 ± 0.02? Gigalet. The latter number is in good agreement with the date of 4.46 ± 0.04 Gigalet, which was previously obtained based on the loss of lead and the formation of the lunar crust. Thus, Theia could exist for 70-110 Megayears (millions of years).
The proto-Earth, at the time of the collision, already had almost modern mass. The initial speed of the collision was insignificant, in the astronomical sense - 4 km/sec. Theia's incidence angle was acute, approximately 45°. Theia's iron core sank toward the Earth's core, while most of Theia's mantle and a significant portion of the Earth's mantle were ejected into space, where they formed an accretion disk. From the accretion disk, in a very short time (within a century, perhaps even one month), the planet’s satellite, the Moon, was formed.
The convergence and collision of Theia and the Earth, and the formation of the Moon.
Computer simulations show that significant bodies or accumulations of debris could exist at the Trojan Lagrange points of the Earth-Moon system for long periods of time up to 100 Megaliths.
As a result of the collision, the Earth received significant angular momentum; the day lasted about five hours. Subsequently, due to the removal of the Moon, the rotation of the Earth slowed down to the existing twenty-four hours in a day.
According to modern views, the isotopic distribution of planets depends significantly on the distance to the Sun. The Moon and Earth, having a similar distribution of isotopes, could not have formed in different orbits, but the fact that there were no heavy elements on Sputnik was difficult to explain by the simultaneous formation of both bodies in the same orbit.
The “big impact” or “big splash” hypothesis was first proposed in 1975 by a group of American astrophysicists Al Cameron, William Ward and William Hartmann. Thus, it was relatively easy to justify the almost complete absence of heavy elements, such as iron, on the satellite. However, the resulting object must have a significantly different composition, and the newly formed Moon would have had a different isotopic composition, for example, oxygen isotopes.
The place of origin of Theia has long remained a weak point of the theory. In the newly formed Solar System there were no places where such a significant object as Theia with an isotopic composition similar to Earth could form. After all, in order to accumulate such a mass, a certain period of existence in a stable orbit had to pass. In 2004, based on computer modeling, two Princeton University researchers, Richard Gott and Edward Balbruno, illustrated that at one of the Trojan Lagrange points, which are 60° from Earth, a planetoid could form that would have enough time to grow into the Martian mass.
After approximately a hundred million years, the object was shaken by the giant and gradually came closer and collided with the Earth at a low speed. Since both the Earth and Theia were formed in the same orbit, their isotopic composition is similar.
On February 25, 2011, the discovery of two planets in the same orbit in the KOI-730 planetary system was announced. Both planets are at Trojan points relative to each other.
The existence of such objects is further confirmation of the plausibility of Theia's existence. According to orbital calculations, both planets will have stable orbits for at least the next 2.2 million years.
Outer space as imagined by an artist
©NASA
At one time, Neptune was one of the hypothetical planets: astronomers predicted its existence, although for a long time it remained invisible to telescopes. Many hypotheses have been refuted, others are still awaiting confirmation.
Planet X
At the beginning of the 19th century, astronomers, using Newton's laws, predicted the existence of another planet, whose gravitational force influenced the trajectory of Uranus. It turned out to be Neptune. However, its mass, according to scientists' calculations, was insufficient to explain the orbit of Uranus.
There should have been another, ninth planet in the solar system, which the American astronomer Percival Lowell dubbed Planet X. However, the search for the mysterious planet was not successful. Even the later discovery of Pluto did not have sufficient mass to exert the necessary influence on the orbit of Uranus.
The search for Planet X ended only in 1989, when the Voyager 2 spacecraft accurately measured Neptune's mass. Its value turned out to be much greater than scientists predicted, which fully explained the shift in the orbit of Uranus.
©NASA, ESA and G. Bacon (STScI)
Planet between Mars and Jupiter
In the 16th century, Johannes Kepler drew attention to the huge gap between the orbits of Mars and Jupiter. According to his assumption, there should have been another planet hiding in it. Many astronomers supported his assumption.
The orbit of the invisible planet was precisely calculated, and scientists systematically searched the sky for it by looking through their telescopes. In 1801, a celestial object was actually discovered, whose orbit coincided with the predicted one, but its size turned out to be too small for a full-fledged planet.
We are talking about Ceres, which for many years was classified as an asteroid. It is currently considered a dwarf planet, like Pluto.
An artist's impression of water vapor on Ceres
©IMCCE-Observatoire de Paris/CNRS/Y.Gominet, B. Carry
Theia
Theia is a hypothetical planet, similar in size to Mars, whose collision with Earth 4.4 billion years ago led to the formation of the Moon.
The name was given to her by the English geochemist Alex Halliday in honor of the Titanide, who, according to Greek mythology, gave birth to Selene, the goddess of the Moon.
It should be admitted that the origin of the Earth's natural satellite still remains a mystery to scientists. The theory of a giant collision between the Earth and Theia is one of the most likely hypotheses. However, there are others.
It is possible, for example, that the Earth and the Moon formed in pairs at the birth of the Solar System, or that the Moon was attracted to our planet by gravitational forces.
©NASA
Volcano
Uranus wasn't the only planet whose trajectory didn't match theoretical predictions. An anomalous shift in Mercury's perihelion, discovered in 1859, led astronomers to search for a hypothetical planet Vulcan within the orbit of the smallest member of the planetary family.
This task was very difficult due to the bright sunlight. Many scientists mistook dark spots on the Sun for the mysterious Vulcan.
The problem was solved only in 1915 thanks to Einstein's general theory of relativity (GTR). Due to the adjustments made by General Relativity to the calculations of the orbit of Mercury, the need for an additional planet disappeared.
©listverse.com
Phaeton
The discovery of a second large asteroid, Pallas, the year after the discovery of Ceres, led German astronomer Heinrich Olbers to propose that both asteroids were fragments of an ancient planet destroyed by a comet impact.
But in this case, between the orbits of Mars and Jupiter there should have been many more fragments of the destroyed planet. The discovery of Juno and Vesta a few years later confirmed this hypothesis. The ancient planet was christened Phaeton in honor of the mythological son of the Sun god, who crashed in his father’s chariot.
However, the mass of all the bodies in the asteroid belt is too small for a planet. In addition, the asteroids themselves are very different from each other, so most scientists believe that the asteroid belt was formed as a result of the attraction of small fragments.
Planet V
Another hypothetical planet that should have existed 4 billion years ago between the asteroid belt and Mars. It was predicted by NASA specialists Jack Lisso and John Chambers.
According to their calculations, the orbit of Planet V was extremely unstable and eccentric. The fifth planet was supposed to die as a result of meteorite bombardment, ultimately falling into the Sun. However, its death has nothing to do with the formation of the asteroid belt.
Artist's impression of the planet from the surface
©NASA
Fifth gas giant
One of the explanations for the meteorite bombardment, which resulted in the formation of many craters on the Moon, as well as on several planets, is provided by the so-called Nice model (it was developed in the famous city on the Côte d'Azur of France).
According to this model, the orbits of the outer gas giants - Saturn, Uranus and Neptune - were initially much smaller. After the protoplanetary disk of gas dissipated, these planets moved to their current positions.
Planetary migration successfully explains many phenomena discovered in the solar system, but it requires one additional gas giant to occur. According to scientists, as a result of cosmic cataclysms, Planet V was eventually thrown out of the solar system.
The collision of the Earth with the hypothetical planet Theia probably formed the Moon in a completely different way than previously thought: the powerful impact evaporated most of the solid rocks of our planet, sharply inflating it in size, and it was from the outer layers of this vapor that our natural satellite arose.
American scientists have developed a new method for determining the concentration of potassium isotopes and, on its basis, created an exotic theory of the formation of the Moon, which had never before been considered by the scientific community. The corresponding article was published in the journal Nature.
Since the 1970s, it has been generally accepted that the Moon formed when a hypothetical Mars-sized planet (Theia) struck the proto-Earth 4.5 billion years ago. However, over the past 15 years, a number of data have been inconsistent with this idea. Almost any model of such an impact shows that the Moon must be at least 60 percent formed from Theia. But an analysis of the composition of the lunar soil - both Soviet and American - indicated that there is the same ratio of oxygen isotopes as on Earth. It is also known that the chemical composition of planets formed in different regions of the solar system should differ. American rovers record that the isotopic composition for Mars is completely different from that for Earth.
The generally accepted model of the formation of the Moon.
To explain this contradiction, a new model was proposed in 2015, according to which the collision of the bodies was “head-on” and so powerful that the bulk of both planets evaporated from heating. The rocks became gas, but its temperature was so high that instead of a silicate atmosphere, a continuous cover of silicate supercritical fluid appeared above the planet's core. This is the name given to the state of a substance when the temperature and pressure in it are above the critical point. Because of this, it simultaneously has the properties of both a gas and a liquid. For example, a supercritical fluid easily penetrates obstacles like a gas, but also dissolves solids like a liquid.
In such an environment, the matter of Theia and the proto-Earth could quickly mix and become chemically homogeneous in a short time. The hypothesis had two main flaws. Firstly, if this was so, at first glance it was impossible to either refute or convincingly prove it. After all, the composition of the Earth and the Moon would then be the same. Secondly, the script turned out to be too exotic. It required the evaporation of the main part of our planet after the impact and its increase in volume by 500 times. The diameter of the planet could then reach 100,000 kilometers (almost like Saturn). That's about eight times larger than today, and more like a gas giant planet than the Earth we know.
However, now scientists from the USA, having created a more accurate method of analysis for potassium isotopes, have found that lunar rocks contain slightly more potassium-41 than terrestrial ones (by 4 ten-thousandths). The only scenario that can correctly explain such a difference is the different rate of condensation of potassium-41 from the hot vapor cloud. The outer layers of the proto-Earth, swollen after the impact, would have been tens of thousands of kilometers from its center and began to cool earlier. As it cooled, heavier potassium-41 was deposited in the outer layers more intensively than in the inner ones. Since the outer layers later became the Moon, and the inner layers became the present-day Earth, the satellite naturally ended up with slightly more potassium-41 than on our planet.
If this process took place in a vacuum, it would give a big difference in the concentration of potassium-41. Since the differences are still quite small, calculations show that the condensation of potassium-41 in the substance of the future Moon took place at a pressure of 10 atmospheres. This is a fairly large value, which indicates that the hypothesis of the evaporation of the proto-Earth after the collision with Theia is most likely correct. No matter how difficult it might be to imagine today, in the area where the future Moon was formed there existed a supercritical fluid from the evaporated solid rocks of our planet. Over time, it gradually crystallized into the rocks of the modern Moon. And the rest of the “excess” matter settled back on our planet, forming its outer layers.
Science
The planet Neptune used to be also classified as hypothetical; it had never been seen, but its existence was assumed.
In fact, scientists have assumed and continue to assume the existence of more planets.
Some fall off this list over time, others may have actually existed in the past, and may even still exist today.
10. Planet X
In the early 1800s, astronomers knew of the existence of all the major planets in our solar system except Neptune. They were also familiar with Newton's laws of motion and gravity, which were used to predict the movements of planets.
When correlating these predictions with the actual observed movement, it was noticed that Uranus did not "go" where it was predicted. Then the French astronomer Alexis Bouvard asked the question: could the gravity of an invisible planet shift Uranus from its intended course.
After the discovery of Neptune in 1846, many astronomers decided to test whether its gravitational force was strong enough to explain the observed motion of Uranus. The answer turned out to be negative.
Perhaps there is another invisible planet? The existence of a ninth planet has been proposed by many astronomers. The most meticulous searcher for the ninth planet was the American astronomer Percival Lowell, who named the wanted object “Planet X”.
Lowell built an observatory with the goal of finding Planet X, but never found it. 14 years after his death, astronomers discovered Pluto, but its gravitational force was also not strong enough to explain the observed movement of Uranus, so The scientific world continued to search for Planet X.
The search continued until Voyager 2 passed Neptune in 1989. It was then discovered that Neptune's mass had been measured incorrectly. Updated mass calculations explain Uranus' motion.
Unknown planet
9. Planet between Mars and Jupiter
In the 16th century, Johannes Kepler noticed the existence of a huge gap between the orbits of Mars and Jupiter. He assumed that there maybe a planet, but did not look for her.
After Kepler, many astronomers began to notice patterns in the orbits of planets. The approximate sizes of the orbits from Mercury to Saturn are 4, 7, 10, 16, 52, 100. If you subtract 4 from each of these numbers, you get 0, 3, 6, 12, 48 and 96.
It is noteworthy that 6 =3+3, 12=6+6, 96=48+48. Between 12 and 48 there remains a strange emptiness.
Astronomers were puzzled by the question of whether they had missed a planet, which, according to calculations, should be located between Mars and Jupiter. As the German astronomer Elert Bode wrote: “After Mars, a huge space was discovered in which not a single planet had yet been identified. Can we believe that the founder of the Universe left this space empty? Of course not".
When Uranus was discovered in 1781, the size of its orbit fit neatly into the pattern described above. This seemed like a law of nature, which later became known as Bode's law or Titius-Bode's law, however, the notorious gap between Mars and Jupiter still remained.
Elert Bode
A Hungarian astronomer named Baron Franz von Zach also became convinced that Bode's law works, which means that There is an undiscovered planet between Mars and Jupiter.
He spent several years searching, but never found anything. In 1800, he organized a group of several astronomers who systematically carried out research. One of them was the Italian Catholic priest Giuseppe Piazzi, who in 1801 discovered an object whose orbit exactly the same size.
However, the object named Ceres, turned out to be too small to be called a planet. In fact, Ceres was considered an asteroid for many years because it was the largest in the main asteroid belt.
Today, Ceres is classified as a dwarf planet, as is Pluto. It is worth adding that Bode's law stopped working when Neptune was found because the size of its orbit did not fit the accepted pattern.
Galaxy: unknown planets
8. Theia
Theia is the name given to a hypothetical, Mars-sized planet that likely collided with Earth about 4.4 billion years ago, possibly resulting in the formation of the Moon. It is assumed that the name of the planet was given by the English geochemist Alex Halliday. This was the name of the mythological Greek titan who gave life to the moon goddess Selene.
It is worth noting that the origin and formation of the Moon is still unknown. the subject of active scientific discussion. While the above story is the main version (Giant Impact Hypothesis), it is not the only one.
Perhaps the moon was somehow "captured" by the Earth's gravitational field. Or maybe the Earth and the Moon formed in pairs at about the same time. It is important to add that the Earth, at the very beginning of its formation, probably suffered from collisions with many large celestial bodies.
7. Vulcan
Uranus was not the only planet whose observed motion did not match predictions. Another planet had such a problem - Mercury.
The discrepancy was first discovered by mathematician Urban Le Verrier, who discovered that the lowest point in Mercury's elliptical orbit (perihelion) was moving around the Sun faster than his calculations showed.
The discrepancy was minor, but additional observations showed that the mathematician was right. He suggested that the discrepancies are caused by the gravitational field of an undiscovered planet orbiting within Mercury's orbit, which he named Vulcan.
Urban Le Verrier
This was followed by numerous "observations" of Vulcan. Some observations turned out to be simply sunspots, but there were others made by respected astronomers that seemed plausible.
When Le Verrier died in 1877, he believed that Vulcan's existence confirmed. However, in 1915, Einstein's general theory of relativity was published, and it turned out that Mercury's motion was predicted correctly.
The volcano disappeared, but people continued to look for objects orbiting the Sun inside the orbit of Mercury. Of course, there is nothing “planet-like” there, but asteroid-sized objects that have been called “live” there may well “live” volcanoids."
6. Phaeton
German astronomer and physician Heinrich Olbers discovered the second known asteroid, named Pallas, in 1802. He suggested that the two asteroids found could be fragments of an ancient planet, which was destroyed under the influence of some internal forces or during a collision with a comet.
It was implied that there were more objects besides Ceres and Pallas, and indeed, two more were soon discovered - Juno in 1804 and Vesta in 1807.
The planet that supposedly broke up to form the main asteroid belt became known as Phaeton, named after the character in Greek mythology who drove the sun chariot.
However, the Phaeton hypothesis ran into problems. For example, the sum of the masses of all the main belt asteroids is much less than the mass of the planet. In addition, there are many differences between asteroids. How could they come from the same "parent"?
Today, most planetary scientists believe that asteroids form due to the gradual sticking together of small fragments.
The unknown in space
5. Planet V
This is another hypothetical planet between Mars and Jupiter, but the reasons why it is believed to have once existed are completely different from the above.
The story begins with the Apollo mission to the moon. The Apollo astronauts brought many moon rocks to Earth, some of which were formed by the melting of rocks during the period when something like an asteroid collided with the Moon and generated enough heat to melt the stone.
Scientists have used radiometric dating to reveal when these rocks cooled. They concluded that the coldest period is approximately 3.8 - 4 billion years ago.
It appears that many comets and asteroids collided with the Moon during this period of time. This period is known as the "Late Heavy Bombardment" (LTB). "Late" because it happened after most of the others.
Previously, collisions in the solar system occurred with enviable regularity, but now time has passed. In this regard, the question arises: what happened to the temporarily increased number of asteroids hitting the Moon?
About 10 years ago, John Chambers and Jack J. Lissauer suggested that the cause may have been a long-lost planet they called " Planet V".
According to their theory, Planet V was between the orbit of Mars and the main asteroid belt before the gravity of the inner planets forced Planet V into the asteroid belt, where it threw off the trajectories of many of them, ultimately leading to their collision with Moon.
It is also assumed that Planet V collided with the Sun. This hypothesis has been met with criticism because not everyone agrees that a PTB occurred, but even if it did, there must be other possible explanations other than the presence of Planet V.
4. Fifth gas giant
Another explanation for the PTB is the so-called Nice model, named after the French city where it was first developed. According to this model, Saturn, Uranus and Neptune are outer gas giants– originated in small orbits surrounded by a cloud of asteroid-sized objects.
Over time, some of these smaller objects passed near gas giants. Such close encounters contributed to the expansion orbits of the gas giants, although at a very slow pace.
Jupiter's orbit actually became smaller. At some point, the orbits of Jupiter and Saturn entered into resonance, as a result of which Jupiter began to revolve around the Sun twice, while Saturn had time only once. This caused chaos.
By solar system standards, everything happened very quickly. The nearly circular orbits of Jupiter and Saturn tightened, and Saturn, Uranus, and Neptune collided several times. The cloud of small objects was also agitated.
In total this led to PTB. After everything passed, Jupiter, Saturn, Uranus and Neptune “acquired” the orbits that they have to this day.
This model can also be used to describe other features of the solar system, such as the Trojan asteroids of Jupiter, however, the original model does not explain everything. It needs modification.
Theia is a hypothetical planet that arose, according to the giant impact theory, 4.6 billion years ago (together with other planets of the Solar System). It is believed that its collision with the Earth led to the formation of the Moon. Presumably, Theia also moved along the Earth’s orbit, but at some point, under the influence of the gravitational forces of the Earth and the Sun, it switched to a chaotic orbit, approached our planet at a critical distance and literally crashed into it.
Since the collision occurred almost tangentially and at a relatively low speed, most of the substance of the impacted celestial body and part of the substance of the earth's mantle were thrown into low-Earth orbit. From these debris the Moon was formed, which began to revolve in a circular path. As a result of the collision, our planet received a sharp increase in rotation speed and a noticeable tilt of the rotation axis. Computer simulations showed the possibility of such a scenario, in which case the Moon acquired its spherical shape within one hundred years after the giant impact.
The giant impact version well explains the increased angular momentum of the Earth-Moon system, as well as the lower iron content in our satellite, since it is assumed that the impact occurred after the formation of the Earth’s core. True, it is currently impossible to prove that already 4.5 billion years ago a heavy iron core was released on the planet and a silicate mantle was formed. In general, this theory does not contradict almost all known information about the chemical composition and structure of the Moon. The only fundamental problem is the depletion of the Earth's natural satellite in volatile elements.
During the era of American lunar expeditions of the 1960-1970s, samples of lunar soil were delivered to our planet, from which the geochemical properties of the satellite were studied. However, some details of this geochemical analysis cast doubt on the hypothesis of an Earth collision with a protoplanet. Chemical examination of the samples revealed no volatile compounds or any light elements.
It is believed that they were all simply evaporated during the extremely intense heat that accompanied the formation of these rocks. But in accordance with the version of the collision, the Moon was formed as a result of the ejection of molten matter into near-Earth orbit. And even if we assume that part of this substance could have evaporated at the moment, nevertheless, during evaporation, the light isotope always precedes the heavy one, which means that the residual substance should have been enriched with the heavy isotope of the element that was lost. At the same time, no traces of isotopic fractionation of volatile elements were found in the lunar substance. In addition, according to NASA Ames Center scientist Jack J. Lissauer, most of the material ejected during a collision with a protoplanet would have fallen back to Earth. He believed:
“The process of accretion of matter in the “lunar disk” formed after the impact could not occur with great efficiency. To form the Moon, much more material must have been ejected into orbit and at a greater distance from Earth than previously thought." Another important circumstance is the identity of the ratio of oxygen isotopes in terrestrial and lunar rocks, which, as noted above, indicates the formation of the Moon and Earth at the same distance from the Sun. How does this fit into the generally accepted collision theory? Indeed, in this case, a planet the size of Mars would have to move in the same orbit with the Earth and exist in this state for many millions of years before the notorious collision. Thus, the version of the origin of the Moon described above is also not without serious shortcomings. The study of samples of lunar rock delivered by the American Apollo spacecraft and Soviet unmanned probes brought quite unexpected results. It turned out that the rocks collected on the surface of the Moon are much older than those discovered by scientists on Earth. 
In particular, samples from the Moon are believed to be 4.5 billion years old, which is very close to the age of our Solar System. Therefore, by studying the Moon, you can learn a lot about the earliest episodes in the history of our planet. The surface of our satellite is all cut up with craters, which indicate a powerful meteorite bombardment. This suggests that, having a more powerful gravitational field, our planet in the first 700 million years of the existence of the Solar System was subjected to an even more intense attack than the Moon itself. But the active geological processes on Earth that followed completely hid from us all evidence of that large-scale meteorite fall.
The permanent and only satellite of the Earth has an important influence on many events on our planet. Since the Moon has a fairly large mass and is not so far from the Earth, we can observe the gravitational interaction between them. This is expressed in the form of ebbs and flows, which can be recorded not only on the coasts of oceans or seas, but also in closed reservoirs and the earth’s crust.
Under the influence of gravity, waves run across the earth's surface, which stretch the Earth's shell about 50 cm towards the Moon. This causes not only periodic fluctuations in sea level, but also changes in the magnetic properties of the earth's atmosphere. In the earliest period of the history of our planet, when the young Moon was located only a few tens of thousands of kilometers from the Earth, its influence was apparently even more significant. It was the powerful tidal forces that slowed down the rotation and heated the interior of the planet.
Whether the Earth actually collided with the mythical protoplanet Theia cannot be said with certainty. But, as scientists believe, the gravity of the Moon contributed to active volcanic activity and the emergence of the primary basalt layer of the Earth. The only satellite smoothes out the vibrations of the earth's axis, making the climate on the Blue Planet more favorable for the development of living organisms.
