Passing stars have changed Earth’s orbit – but we don’t know how

Contrary to previous assumptions, passing stars could cause changes in the orbits of planets, including Earth, large enough to affect climate, research suggests. By not taking this into account, we have overestimated our ability to calculate past orbital variations, and thus our ability to attribute past climate changes to variations in the Earth’s orbit at that time.

Compared to many asteroids and comets, Earth’s orbit is a monument to consistency; otherwise life might not have survived. Nevertheless, subtle changes still occur due to factors such as the gravitational influence of other planets. Some of these, known as the Milankovitch cycles, are known to have been the driving force behind recent cycles of ice ages and interglacial eras. These influences do not change Earth’s average distance from the Sun, but they can cause shifts between rounder and more elongated orbits, which can have a surprisingly large effect on the planet’s climate.

We understand the relationship between Earth and the other planets so well that it is thought we can calculate the orbital variation over the past 50 to 100 million years. Together with the movements of the continents, this has been used to explain what the geological record shows as warmer and colder periods, but we may be missing something.

Every massive object in the universe exerts a gravitational influence on every other object, but this is usually far too small to matter. We calculated the effects of nearby stars such as Alpha Centauri, and giant stars such as Eta Carinae, and found that their effects are microscopic – even offset by dwarf planets in the outer solar system. Consequently, stars beyond the Sun have been left out of attempts to turn back the clock and reveal Earth’s historical orbit.

Yet, despite what the ancients told you, the stars are not fixed. In a few thousand years, Alpha Centauri will be over a light-year closer than it is today. That still won’t be enough for gravity to affect us, but what if it got even closer, really close?

We know such things happen. The star HD 7977 is on a path through the Milky Way that is very different from the stately path of our own sun. It has been calculated that 2.8 million years ago it was an estimated 13,000 astronomical units from the Sun (about 0.2 light years) – there is a 5 percent chance that it was less than 4,000 AU.

Dr. Nathan Kaib and Dr. Sean Raymond of the Université de Bordeaux of the Planetary Science Institute tested whether HD 7977’s gravity tug would have been enough to provisionally incorporate its influence into calculations. They found that a passage of 13,000 AU would have no noticeable effects, but this is not true for a passage more than three times closer (and therefore with gravitational effects more than ten times greater).

Influence of passing star on Earth's orbit

How HD 7977’s close passage increases uncertainty about Earth’s orbit over the past 56 million years. The map represents the eccentricity of Earth’s orbit and the direction of perihelion. Results vary depending on HD 7977 distance estimates.

Image credits: N Kaib/PSI

Perhaps surprisingly, HD 7977 didn’t make much immediate difference to Earth. However, because they are further away from the Sun, the gas giants are more sensitive and in turn affect the Earth. If HD 7977 made such a close pass, Jupiter and Saturn would have settled into their new orbits, continuing to impact Earth in ways dictated by the pass long after the star had left.

Although the effect is indirect, HD 7977’s flyby means we can only turn back the clock on Earth’s orbit precisely by about 50 million years, Kaib and Raymond conclude, rather than about 60 million years as previously thought.

“The geological record shows that changes in the eccentricity of the Earth’s orbit are accompanied by fluctuations in the Earth’s climate. “If we want to best search for the causes of ancient climate anomalies, it is important to have an idea of ​​what Earth’s orbit looked like during those episodes,” Kaib said in a statement.

“An example of such an episode is the Paleocene-Eocene Thermal Maximum 56 million years ago, when global temperatures rose by 5 to 8 degrees Celsius. [9-14°F]. It has already been proposed that the eccentricity of Earth’s orbit was remarkably high during this event, but our results show that passing stars make detailed predictions of Earth’s past orbital evolution currently highly uncertain, and that a broader spectrum of job behavior is possible than previously thought. “

It is entirely possible that previous passes of other stars were even closer and more disruptive.

The research has been published open access in Astrophysical Journal Letters.

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