Stars passing by the solar system influenced the movement of the Earth

The solar system is located in the Milky Way, stars are constantly moving past it, which with their gravitational field can influence the eccentricity of the Earth. It turned out that this influence largely controls the climate of our planet and even provided the warmest era in its history.

The Earth moves around the Sun almost in a circle. Deviation from the ideal trajectory of motion is assessed using eccentricity (e), which takes a value from zero (circle) to one (elongated ellipse). Our planet’s degree of deviation from the circle is 0.0167, the leader of the Solar System is Mercury with e = 0.2056. Due to its relatively small eccentricity, there is no large temperature difference on Earth throughout the year.

Over the past 30-40 years, computer modeling has helped astrophysicists study the orbital evolution of the planets of the Solar System, pushing the horizon of research back millions of years. However, as the available time has increased, the number of conditions that affect the accuracy of the results has increased. The gravitational fields of giant planets and asteroids that act on the planets of the Solar System are one of the main conditions noted by scientists.

Two astrophysicists published a study in The Astrophysical Journal Letters that calculated how stars outside the solar system affect Earth's eccentricity. The authors point out that in previous studies, researchers viewed the solar system as isolated. This condition is erroneous, since the solar system is located in the Milky Way.

The astrophysicists used a modified version of the MERCURY hybrid integrator for the simulation. This allowed the researchers to include an arbitrary number of stellar-mass objects in the experiment, as well as take into account relativistic orbital precession. Scientists have generated situations in which stars within one parsec (3.26 light years) of the Solar System influence the motion of planets within the system. Distances greater than one parsec required extremely large masses and low velocities of stars. Also, astrophysicists did not take into account the influence of the Moon on the Earth, the compression of the Sun, tidal effects and solid body effects.

The authors concluded that the influence of stars reduces the time period for predicting the evolution of the Earth's orbit by 10%, which is more than the influence of asteroids. The orbital motion of the inner planets is influenced mainly by the giant planets, which, in turn, are affected by stars outside the Solar System. Jupiter makes the greatest contribution to the long-term evolution of the Earth's orbit.

Astrophysicists believe that the star HD 7977, with a mass slightly higher than the Sun, influenced the evolution of the Earth's orbital motion about 2.8 million years ago. The probability that the star was approximately 3900 astronomical units from the solar system is 5%. However, researchers emphasize the importance of this event.

“It should be noted that the distance of 3,900 astronomical units likely made HD 7977's solar system transit one of the 10 most influential in solar system history,” the study authors noted.

They also determined the possible maximum eccentricity of the Earth, which it reached 55-56 million years ago. The eccentricity value ranges from 0.0546 to 0.0555 (±0.00045) if the solar system was isolated in the experiments. And under the influence of stars, including HD 7977, the range of maximum eccentricity was 0.0500-0.0569 (±0.0035). In other words, the Earth's eccentricity was about three times greater.

Astrophysicists see a connection between the maximum eccentricity of the planet and the period with the Paleocene-Eocene thermal maximum. At this time, there was a sharp warming of the Earth's climate, the composition of the atmosphere changed, and the diversity of fossil species increased significantly (in particular, the first reliable remains of our primate ancestors date back to this period).

Scientists have come to a very non-trivial conclusion that the period of the most elongated orbit of our planet was at the same time the period of the warmest climate over the past half a billion years at least. Meanwhile, from a climatic point of view, it is unclear how much solar insolation varying throughout the year could lead to such a result. An explanation of this issue is potentially of great interest to the geosciences.

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