A study that seeks to reconstruct the process of formation of the dwarf planet Ceres was published by researchers from Unesp (Universidade Estadual Paulista) and collaborators in the journal Icarus.
The work was conducted by Rafael Ribeiro de Sousa, professor at the Graduate Program in Physics, Guaratinguetá campus. Professor Ernesto Vieira Neto, who supervised Ribeiro de Sousa in his doctoral research, and researchers from Université Côte d’Azur, in France, also signed the article; from Rice University, in the United States; and the National Observatory, in Rio de Janeiro.
As the authors explain, Ceres is a member of the asteroid belt, a collection of celestial bodies located between the orbits of Mars and Jupiter. Approximately spherical in shape, it is the largest object in the Belt, concentrating one third of its total mass. Its diameter, at almost 1,000 km, is just over a third of the diameter of the Moon. With an eccentricity of 0.09, it has a nearly circular orbit. And the inclination of its orbit with respect to the invariant plane of the Solar System, less than 10 degrees, is much greater than the inclination of the Earth’s orbit, which is 1.57 degrees.
Ceres’ mass is too small to hold an atmosphere by gravitational attraction. But a notable fact is that the ammonia and water ices that exist under its surface evaporate with the incidence of sunlight. And the fog formed disperses into outer space. Ice deposits shine brightly at the bottom of craters. The hypothesis that they may harbor some primitive form of life is not excluded. NASA’s Dawn Mission, the US space agency, which came very close to the asteroids Ceres and Vesta, mapped these craters. A very interesting video of the Occator crater, composed with images captured by the spacecraft, can be watched on the mission’s website.
The dwarf planet’s core is probably composed of heavy material: iron and silicates. But what sets Ceres apart from neighboring objects is its ammonia and water ice sheet. Since most bodies in the asteroid belt do not have ammonia, it is hypothesized that Ceres was formed outside, in the cooler region that extends beyond Jupiter’s orbit, and then launched into the belt’s midzone due to great gravitational instability caused by the formation of the gas giant planets Jupiter and Saturn.
“The presence of ammonia ice is strong observational evidence that Ceres may have formed in the coldest region of the Solar System, beyond the so-called Ice Line, where temperatures were low enough for condensation and melting of water and substances to occur. volatiles such as carbon monoxide [CO]carbon dioxide [CO2] and ammonia [NH3]”, says Ribeiro de Sousa.
Today, the Ice Line is located very close to Jupiter’s orbit. However, when the Solar System was in formation, 4.5 billion years ago, the position of this zone varied according to the evolution of the protoplanetary gas disk and the formation of the giant planets.
“The strong gravitational disturbance caused by the growth of these planets could have altered the density, pressure and temperature of the protoplanetary disk, which would have shifted the Ice Line. This disturbance in the disk of protoplanetary gas would have caused planets to grow, while acquire gas and solids, migrate to orbits closer to the Sun”, explains Professor Vieira Neto.
“In our work, we proposed a scenario to explain why Ceres is so different from neighboring asteroids. In this scenario, Ceres would have started its formation in an orbit beyond Saturn, where ammonia was abundant. During the growth of the giant planets, was pulled into the asteroid belt, as a retreatant from the outer Solar System, and has survived until today, for 4.5 billion years”, says Ribeiro de Sousa.
To prove this hypothesis, Ribeiro de Sousa and collaborators performed a large number of computer simulations of the phase of formation of giant planets within the disk of protoplanetary gas that surrounded the Sun. In the model, the presence of Jupiter, Saturn, planetary embryos (precursors of Uranus and Neptune) and a collection of objects similar in size and chemical composition to Ceres were considered in the disk. The assumption was that Ceres would be a planetesimal type object. These are considered the “building blocks” of planets and other bodies in the Solar System, such as asteroids, comets, etc.
“In our simulations, we verified that the phase of formation of the giant planets was not at all smooth. It was characterized by gigantic collisions between the precursors of Uranus and Neptune, by the ejection of planets out of the Solar System and even by the invasion of the inner region. by planets with masses greater than three times the mass of Earth. In addition, the strong gravitational perturbation scattered Ceres-like objects far and wide. Some, with a certain probability, reached the asteroid belt region and acquired stable orbits, able to survive other events”, says the researcher.
According to Ribeiro de Sousa, three main mechanisms acted to preserve these objects in the region: the action of the gas, which dampened the eccentricities and inclinations of their orbits; the resonances of their average motions with Jupiter, which protected them from ejections and collisions caused by this giant planet; and close encounters with the invading planets, which spread the planetesimals to more stable, inner regions of the asteroid belt.
“Our main result indicates that, in the past, there were at least 3,500 Ceres-like objects beyond the orbit of Saturn. And that, with that number of objects, our model showed that one of them managed to be transported and captured in the asteroid belt, in an orbit very similar to the current orbit of Ceres”, highlights the researcher.
This number, 3,500 Ceres-type objects, had already been estimated by other studies, based on the observation of craters and the sizes of other populations of stars, located beyond Saturn, such as those that make up the Kuiper Belt, where Pluto orbits. and other small planets. “With our scenario, we were able to confirm this number and explain the orbital and chemical properties of Ceres. This work has a point in favor of the most recent models of the formation of the Solar System”, summarizes Ribeiro de Sousa.
A little about planetary formation
A scenario on the planetary formation of the Solar System, composed from the most up-to-date information available, allows a better understanding of the study in question, placing Ceres in the general picture.
“From observational evidence, it is known that any planetary system — not just the Solar System — is formed from a disk of gas and dust that surrounds a newly formed star. study, but the consensus so far is that it is born from the gravitational collapse of a giant molecular cloud”, says Ribeiro de Sousa.
The existence of protoplanetary disks is not a mere assumption. On the contrary, it is supported by robust observations. This is the case of the images obtained by the European Space Agency using the Alma radio observatory (Atacama Large Millimeter Array), a system made up of 66 antennas located in the Atacama Desert in Chile. With impressive resolution and richness of detail, these images show protoplanetary disks around very young stars.
“In the case of the Solar System, the data we have suggests that the protoplanetary disk was made up of 99% gas and 1% dust. This would come from older stars, which ended their life cycle and threw heavy material into space. The dust that accumulated around the Sun was enough to form at least the small bodies, the terrestrial planets and the cores of the large gaseous planets. The first solids that condense in the protoplanetary disk are called CAIs. Calcium Aluminum rich-inclusions). As the name implies, they were rich in calcium and aluminum. They were found as inclusions in meteorites. And their oldest ages have been dated at 4.568 billion years,” the researcher reports.
Several young stars, observed in environments characterized as cradles of planet formation, have been dated with ages varying between 1 and 10 million years. This data provided very important information, because it showed that the formation of gaseous planets (such as Jupiter and Saturn) or that have at least one gaseous envelope (such as Uranus and Neptune) must occur, at most, in the first 10 million years of life. of the star. After that, the protoplanetary disks no longer have enough gas.
Rocky, terrestrial-type planets could appear sooner or later — it is not known. But other available information shows that the formation of the Earth and the Moon was one of the latest events in the genesis of the Solar System, occurring around 4.543 billion years ago. As for the small bodies that make up the system (dwarf planets, satellites, comets, asteroids, dust, etc.) the gas, collisions, gravitational captures and others.
The process of planetary formation is quite complex. The stages range from dust, with sizes on the order of a micron (10−6 m), to planets several times larger than Jupiter.
“Dust accumulates by adhesions and collisions within the protoplanetary disk. The gravitational attraction between these particles is not relevant. But the gravitational attraction exerted by the Sun causes the gas to rotate more slowly than the dust. And this produces aerodynamic drag. very strong on the dust. The drag force takes the particles to the plane of the gas disk and moves them radially towards the Sun”, says Ribeiro de Sousa.
“When the dust reaches sizes of the order of a few centimeters, pebbles are formed, which make all the difference in the planetary growth process. Because they influence the rotation speed of the gas. When the gas and pebbles speeds are equal, the drag of gas becomes virtually nil, which gives the pebbles a chance to concentrate enough to form planetesimals — bodies ranging in size from 10 to 1,000 km, which become the building blocks of planets and the precursors of small bodies.” , concludes.
In the next stage, larger and larger objects are formed, either by gravitational capture of pebbles and dust or by collisions. When an object grows large enough to have a mass of three to ten Earths, the gravitational disturbance it produces in the gas disk causes it to migrate to orbits closer to the star. When it grows above ten Earths, it starts to accumulate around it an envelope of gas. And, from the accumulation of the gas, its growth becomes very fast.
“The formation of the giant planets Jupiter and Saturn produced such a gravitational perturbation that it shaped the disk of gas and triggered a new type of planetary migration. This violent phase caused planets to collide and planets to be ejected out of the Solar System, until the swing gravity enabled the system as a whole to acquire a certain degree of stability”, concludes Ribeiro de Sousa.
The recently released study was funded by Fapesp through a doctoral scholarship and a research internship abroad granted to Ribeiro de Sousa. It also received support through the thematic project “The relevance of small bodies in orbital dynamics”.
