Did passing stars change Earth’s orbit?

The Planetary Science Institute published this story on February 14, 2024. Edits by EarthSky.

• Two scientists used computer simulations to study the way passing stars have altered Earth’s orbit over millions of years.
• They said this knowledge is important if we want to understand long-term changes in Earth’s climate. That is, some climate changes might be due to orbit changes.
• They said we can’t know Earth’s orbit for certain beyond 50 million years ago, due to the passage of a sunlike star called HD7977 2.8 million years ago.

The effect of passing stars on Earth’s orbit

Stars that pass by our solar system have altered the long-term orbital evolution of Earth, and, by extension, modified our climate.

That’s according to Nathan A. Kaib, senior scientist at the Planetary Science Institute in Tucson, Arizona. Kaib is lead author of Passing Stars as an Important Driver of Paleoclimate and the Solar System’s Orbital Evolution, a new paper published on February 14, 2024, in the peer-reviewed Astrophysical Journal Letters. His work shows how perturbations caused by passing stars – minor deviations in Earth’s path, caused by the pull of gravity from objects passing near – can alter Earth’s path over the long term. And he said:

One reason this is important is because the geologic record shows that changes in the Earth’s orbit accompany fluctuations in the Earth’s climate.

If we want to best search for the causes of ancient climate anomalies, it’s important to have an idea of what Earth’s orbit looked like during those episodes.

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Sean Raymond at the Laboratoire d’Astrophysique de Bordeaux also contributed to this work. And, according to their study:

One example of such an episode is the Paleocene-Eocene thermal maximum 56 million years ago, where the Earth’s temperature rose 5-8 degrees Celsius (9-14 Fahrenheit). It has already been proposed that Earth’s orbital eccentricity – how much Earth’s orbit deviates from a perfect circle – was notably high during this event.

But our results show that passing stars make detailed predictions of Earth’s past orbital evolution at this time highly uncertain, and a broader spectrum of orbital behavior is possible than previously thought.

Simulations predict the past orbit of Earth

Kaib and Raymond used backward-running computer simulations to predict the past orbital evolution of Earth and the sun’s other planets. Like weather forecasting, this technique gets less accurate the longer the period of the forecast. Previously, these scientists did not consider the effects of stars passing near the sun in these “backwards forecasts.”

As the sun and other stars orbit the center of the Milky Way, they inevitably can pass near one another, sometimes within tens of thousands of astronomical units (AU or Earth-sun units). These events are are known as stellar encounters.

For instance, a star is thought to pass within 50,000 AU of our sun about every 1 million years. And a star is thought to pass within 10,000 AU of our sun about every 20 million years. This study’s simulations include these types of events. In contrast, Kaib and Raymond said, most prior similar simulations did not.

Also, one major reason the Earth’s orbital eccentricity fluctuates over time is because Earth is regularly pulled upon by the giant planets of our solar system: Jupiter, Saturn, Uranus, and Neptune. As stars pass near our solar system, they perturb the giant planet’s orbits, which consequently alters the orbital trajectory of the Earth.

Thus, the giant planets serve as a link between the Earth and passing stars.

Dealing with orbital uncertainties

Kaib indicated that – when simulations include stellar passages – orbital uncertainties grow even faster. So, the time horizon beyond which these backwards simulations’ predictions become unreliable is more recent than previously thought.

This means two things. First, there are past epochs in Earth’s history where our confidence in what Earth’s orbit looked like (for example, its eccentricity, or degree of circularity) has been too high. At times in the past, the real state of Earth’s orbit is unknown. And, second, the effects of passing stars make regimes of orbital evolution (extended periods of particularly high or low eccentricity) possible and not predicted by past models. Kaib said:

Given these results, we have also identified one known recent stellar passage, the sun-like star HD 7977 which occurred 2.8 million years ago, that is potentially powerful enough to alter simulations’ predictions of what Earth’s orbit was like beyond approximately 50 million years ago.

And the current observational uncertainty of HD 7977’s closest encounter distance is large, ranging from 4,000 AU to 31,000 AU. So Kaib concluded:

For larger encounter distances, HD 7977 would not have a significant impact on Earth’s encounter distance. Near the smaller end of the range, however, it would likely alter our predictions of Earth’s past orbit.

Star HD 7977 Can Alter Earth’s Orbit

Details about the top image: Each point’s distance from the center corresponds to the degree of ellipticity (eccentricity) of Earth’s orbit. And the angle corresponds to the direction pointing to Earth’s perihelion, or closest distance to the sun. Scientists used 100 different simulations (each with a unique color) sampled every 1,000 years for 600,000 years to construct the figure in the top image. Every simulation is consistent with the modern solar system’s conditions.

Bottom line: We long suspected that passing stars are what send distant comets sunward. But could passing – and did – passing stars also change Earth’s orbit? A recent study says yes.

Source: Passing Stars as an Important Driver of Paleoclimate and the Solar System’s Orbital Evolution

Via Planetary Science Institute