Earth, Mars, Venus and an unknown planet harmoniously "waltzed" around the Sun, — modeling based on exoplanet observations

Chinese researchers have found, that four planets of the Earth group in the solar system, including the Earth and a long-lost planet, were probably formed by orbiting a parent star in a certain rhythm.

The results of the study also suggest, that these planets formed earlier than previously thought. Astronomers are increasingly interested, how planetary systems change position on cosmic time scales. This was facilitated by a number of recent discoveries of several exoplanet systems, including a group of seven exoplanets orbiting the tiny star TRAPPIST-1.

Previous research has shown, that early planetary systems form pairs, or triple systems, or systems with more planets, orbiting in a similar rhythm, called resonance, around a parent star. The orbital periods of the planets in resonance represent the ratio of two integers, that is, the result of dividing one integer by another, which is also an integer. For example, in the TRAPPIST-1 system, the largest inner planet, TRAPPIST-1 b, makes eight orbits around its five closest neighbors.

The resonance occurs among planets that formed in the protoplanetary disk from debris surrounding the newborn star and contain gas. These planets interact with the gas, exchanging rotational motion with it, which often leads to their movement closer to the parent star. Most of these planets can get so close that their orbital periods “resonate” or become integer multiples.

Currently planets of the solar system are not in resonance, although Venus and Mars are close to it, having an orbital period ratio of 3.05:1. In 2025, astronomers demonstrated, that Jupiter and Saturn were in resonance shortly after formation. However, their resonance abruptly stopped 4.4 billion years ago, when the protoplanetary gas disk began to evaporate, pushing Saturn, Uranus, and Neptune outward in a phenomenon, called giant planetary instability.

According to the co-author of the new study, scientist from Tsinghua University in China, Chris Ormel, no one has ever investigated whether the Earth’s planets were in resonance. In his opinion, this is due to the fact, that an alternative theory about the formation of planets as a result of a series of large-scale collisions was considered adequate to explain their current behavior. 

A study conducted in 2013, based on the analysis of Martian isotopes, demonstrated that the Earth-like planets could have formed at a time when the protoplanetary disk still had a lot of gas, approximately 10 million years after the formation of the solar system. This means, that the terrestrial planets could have been in resonance at that time.

To test this hypothesis, the researchers created computer models of the early Solar System. Each model included Jupiter and Saturn — as well as the four rocky worlds: Mars, Theia (a hypothetical Mars-sized object whose collision with the early Earth resulted in the formation of our Moon), the early Earth (before the collision with Theia), and Venus. Due to the widespread theory that Mercury was formed as a result of giant collisions, scientists did not include this planet in the study. 

In all models, Saturn was closer to Jupiter than it is now. Scientists have suggested, that rocky planets are enlarged by the accumulation of either pebbles or larger stone blocks weighing trillions of tons. In most models Venus, Earth, Theia, and Mars formed a 2:3:4:6 resonance chain over a million years of simulated time.

After that, the researchers conducted 13,200 simulations of potential planetary movements over 100 million years, taking into account the gravitational influence of each planet on the others. At the 10 million-year mark, the scientists made Saturn move outward to simulate instability of the giant planet. 

Astrophysicists have learned, that based on the selected parameters, about half of the simulations reproduced the current configuration of the Earth group planets. This included such aspects, as the presence of a single collision between Theia and the Earth and the ratio of the orbital periods of Venus and Mars of 3.05:1.

The data also show, that the planets formed in a gas-filled protoplanetary disk within the first 10 million years after the emergence of the Solar System. This is at least 20 million years earlier than current models predict.

One of the planets that can confirm the age of rocky inner worlds is Venus. Unlike Earth and Mars, Venus was not subjected to large-scale bombardment by large space objects during collisions, and its mantle reflects its ancient origins. 

The results also indicate that outer giant planets can significantly destabilize their inner satellites. The authors argue that this may explain why resonant systems like TRAPPIST-1 do not have external giant planets.

The results of the study are published in the journal The Astrophysical Journal

Source: LiveScience