One of the open questions in planetary dynamics that most puzzle specialists is the origin of Mercury, the smallest planet in our solar system. Its relatively minuscule mass, almost 20 times smaller than that of Earth, and its unique orbit around the Sun, the most elongated and tilted of all the planets in the system, cannot be explained by most of the models of planetary formation. Until the mid 1990s, the most accepted explanation was that all of the planets in our solar system formed at more or less the same position in which they are currently located. With the confirmed discovery, over the last 20 years, of almost 3,000 planets orbiting other stars, known as exoplanets, forming systems different from ours, Mercury’s unique situation increasingly appears to be an exception in the galaxy—and new explanations for its orbit have been attempted.
A recent study published by planetary scientists Fernando Roig and Sandro Ricardo de Souza, of the National Observatory (NO) in Rio de Janeiro, and the Czech researcher David Nesvorný, of the Southwest Research Institute in Colorado, defends a new hypothesis to justify the strange location of Mercury, whose orbit is tilted 7 degrees with respect to the average orbital plane of the other planets. Based on computer simulations of the dynamics of our solar system more than 4 billion years ago, the researchers suggest that the planet’s orbit became so elongated and tilted because of an extreme event. At some time during the first 500 million years of our solar system, the gravitational interaction between a hypothetical giant gas planet of the size of Uranus, and Jupiter—also a gas giant—could have altered local conditions. Per this hypothesis, the unknown planet would have been ejected from the system and caused Jupiter to be suddenly displaced towards the Sun. Jupiter’s jump would have pushed Mercury into its current position (see infographic).
This hypothetical event is known as Jumping Jupiter. In accordance with this theory, Jupiter’s displacement could have given rise to Mercury’s current orbit, and also ensured the stability of the trajectory of all of the rocky planets, including Earth, around the Sun. “It seems counter-intuitive,” says Roig, “but everything indicates that the giant gas planets had to have passed through an unstable phase in order to stabilize the rocky planets.” In the simulations, the jump in Jupiter’s orbit caused by the expulsion of a hypothetical planet resulted in almost no changes in the orbits of the rocky planets, except for Mercury. Roig explains that, if Jupiter had moved slower, instead of jumping towards the Sun, Mercury’s orbit could have become even more elongated and tilted than it is today. If this had happened, Mercury could have been ejected from the solar system or collided with its neighbor, Venus. According to the astrophysicist, this encounter could have provoked a domino effect that would have destroyed all of the rocky planets. Jupiter needed to have jumped in order to ensure the survival of the rocky planets,” suggests Roig.
A little over 20 years ago, most researchers believed that the planets in the solar system formed in approximately the same positions they occupy currently, through a slow, smooth process of accumulation of gas and dust. These models predicted that other stars should give rise to planetary systems similar to ours, with two distinct types of planets: the rocky planets, with sizes similar to that of Earth, near the star; and the gas giants, such as Jupiter or Saturn, farther away. “The discovery of exoplanets radically changed this idea,” explains Roig. “We saw that there is a variety of planetary configurations very different from our solar system.”
Statistical analyses of the characteristics of all exoplanet systems discovered up until now suggest that stars similar to the Sun tend to have very different planetary systems. Many of them contain rocky planets two to three times larger than Earth, with orbits closer to their suns than Mercury is to the Sun. Jupiter’s orbit, which is almost circular and far from the Sun, also diverges from what we observe in many exoplanetary systems.
A primordial cloud of gas and dust
Astronomers agree that the Sun and its planets began to form 4.6 billion years ago, when a gigantic cloud of gas and dust in interstellar space collapsed due to the gravitational force of its own mass. At that time there was a spherical core of gas that gave rise to the Sun, surrounded by a disk of material from which the planets took shape. The first worlds to form are believed to have been the gas giants — Jupiter, Saturn, Uranus and Neptune — and a few tens of millions of years later the rocky planets Mercury, Venus, Earth and Mars. Some researchers speculate that Mercury could have originated from the fragments of a first generation of larger rocky planets, with masses comparable to that of Earth, closer to the Sun than Mercury is currently.
The gas giant formation process is believed to have lasted less than 10 million years. At that time, in the space between the planets, there was still a reasonable quantity of gas remaining from the material in the disk from which they originated. The drag from the gas led the planets to migrate to locations closer to the Sun. At some point, however, the mutual gravitational attraction between Jupiter and Saturn could have inverted the direction in which the two gas giants were migrating, leading them away from the Sun. This back-and-forth movement of the gas giants is called the grand tack by the researchers, an allusion to the maneuvering of sailboats when sailing into the wind. Shortly after the grand tack, the current rocky planets were supposedly already formed or close to forming more or less in their present positions.
The orbits of the gas giants should be very different. Jupiter should be further from the Sun than it is, while the others should be much closer to Jupiter and to each other. The gas giants might have remained in this more compact configuration for up to 500 million years. When very close together, however, they must have been constantly affected by their mutual gravitational forces. Additionally, the orbits of these planets could also have been altered by the presence of many nearby smaller bodies, known as planetesimals.
The giants shook off these planetesimals slowly, pushing them to the outer limits of the solar system, where the Kuiper Belt—whose most famous body is Pluto—and the Oort cloud are located. In 2005, astronomers Hal Levison, Alessandro Morbidelli, Kleomentis Tsiganis and Rodney Gomes, the latter also a researcher at the National Observatory, presented computer simulations showing how, starting with this initial unstable situation, the gas giants could have slowly drawn away from each other, migrating to their current positions over millions of years.
Known as the Nice model, because it was developed while the researchers worked together at the Nice Observatory, in the French city, it stood out because it explained the current layout of the gas planets. However, in 2009 the Dutch astronomer Ramon Brasser noted that the slow migration of the gas giants predicted in the Nice model would have had a large probability of causing a series of planetary collisions. The hypothetical movement of the gas giants could have resulted in the expulsion of one of them—most likely Uranus—from the solar system.
In an attempt to reconcile this inconsistency, astronomer David Nesvorný, of the Southwest Research Institute, who presently collaborates with Roig as a visiting researcher at the National Observatory, proposed, in 2011, that the solar system could have had a fifth giant gas planet, with a size similar to that of Uranus or Neptune. Nesvorný calculated that the ejection of this hypothetical planet would have caused the distance of Jupiter’s orbit around the Sun to change from 5.5 times the distance between the Earth and the Sun to 5.2 times this value over a period of less than 100,000 years. “On the timescale of the formation of the solar system, this change in orbit would have occurred quickly. This is why we describe it as Jupiter’s jump,” explains Roig. “The Nice model works well to explain the gas giants, but one soon notices that the smooth migration of the gas giants predicted by this theory would have made the formation of the rocky planets difficult,” justifies astronomer Othon Winter, a specialist in planetary dynamics at São Paulo State University (Unesp), Guaratinguetá. “So far, Jumping Jupiter is the only known solution to this problem.”
Together with other researchers, including astronomer Valerio Carruba, from Unesp, Roig and Nesvorný recently argued that the Jumping Jupiter scenario would also explain some characteristics of the asteroid belt between Mars and Jupiter. The result of simulations published in March 2016 in the journal Icarus offers an explanation for why the astronomers were unable to observe any evidence in the belt of large collisions prior to 4 billion years ago between asteroids. In the article, the authors state that the presence of the hypothetical fifth gas giant, and its later expulsion from the system, would have reshuffled the orbits of the asteroids to the extent that any evidence of these collisions would be lost.
The idea that one giant gas planet escaped the solar system and its star is not as crazy at it may seem. Roig reminds us that astronomers have already observed the effect of a gravitational lens on the light of stars that could be attributed to the passage of giant planets wandering through interstellar space. Some researchers estimate that there are thousands of wandering worlds in the Milky Way. “These bodies have no way of forming far from stars,” explains Roig. “They must have formed in a planetary system and were then ejected.”
Secular families (nº 2014/06762-2); Grant Mechanism Regular Research Project; Principal Investigator Valerio Carruba (Unesp); Investment R$31,200.00.
ROIG, F. et al. Jumping Jupiter can explain Mercury’s orbit. Astrophysical Journal Letters. V. 820, No. 2, March 24, 2016.
BRASIL, P. I. O. et al. Dynamical dispersal of primordial asteroid families. Icarus. V. 266, p. 142-151, March 1, 2016.
ROIG, F. & NESVORNÝ, D. The evolution of asteroids in the jumping-Jupiter migration model. The Astrophysical Journal. V. 150, No. 6. December 1, 2015.