'Intruder' stars have changed Earth's climate over the eons. Here's how.

Stars intruding into the sun's cosmic backyard could have shifted Earth's orbit in the distant past, triggering major climate events in our planet's history.  

The gravitational influence of these intruder stars has also impacted the orbit of other planets in the solar system, causing minor deviations called perturbations. 

New research looks at the effect of these encounters between the sun and other stars on backward forecasts of Earth's orbit around its star and climate effects arising from shifts in that orbit and our planet's orientation. 

"Perturbations — a minor deviation in the course of a celestial body, caused by the gravitational attraction of a neighboring body — from passing stars alter the long-term orbital evolution of the sun’s planets, including Earth," research team leader Nathan Kaib, a senior scientist at the Planetary Science Institute in Tucson, Arizona, said in a statement. 

"One reason this is important is because the geologic record shows that changes in the Earth’s orbital eccentricity accompany fluctuations in the Earth’s climate," Kaib added. "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."

Related: Climate change: Causes and effects

As the sun and other stars orbit the center of the Milky Way galaxy, they occasionally pass each other relatively closely, cosmically speaking.

The sun is thought to come within 50,000 AU of another star every million years or so, and within 20,000 AU of a neighbor every 20 million years or so, on average. (One AU, or astronomical unit, is the average Earth-sun distance — about 93 million miles, or 150 million kilometers.)

So, over the course of the 4.6 billion years that the solar system has existed, it has been influenced by many of these stellar encounters. The new research is the first to factor such events into the "backward forecasts" of our planet's orbit and climate used to predict the past orbital evolution of Earth and the other solar system planets.

Earth's orbital eccentricity — the amount by which the circle it draws around the sun is flattened — is strongly affected by the orbits of the solar system's giant planets, Jupiter, Saturn, Uranus, and Neptune.

Stars passing the solar system perturb the orbits of these giant planets and this, in turn, alters Earth's trajectory around the sun. This means that the giant planets act as links between passing stars and the orbit of Earth. 

Like forward-looking weather forecasts, the more time that backward climate models attempt to cover, the larger uncertainties grow. Kaib and his team found that, when factoring in stellar encounters, orbital uncertainties grow rapidly and models quickly become unreliable. 

There are two major outcomes of this, according to the researchers. First, scientists have been too confident when predicting Earth's orbit and its eccentricity accurately at certain points in our planet's history. Second, there are points during Earth's history when stellar encounters have made possible certain orbital regimes — extended periods of particularly high or low eccentricity — that aren't seen in current models. 

Kaib said that one example of such an episode could be the Paleocene-Eocene Thermal Maximum that occurred around 56 million years ago, during which Earth’s temperature rose by 9 to 14 degrees Fahrenheit (5 to 8 degrees Celsius). 

"It has already been proposed that Earth’s orbital eccentricity 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," Kaib explained. 

Kaib and colleagues were also able to identify a recent stellar encounter between the sun and a star entering the cosmic neighborhood of the solar system. There is a large uncertainty in the distance at which the star HD 7977 passed the sun around 3 million years ago, with estimated distances ranging from just 4,000 AU to 31,000 AU. But, if the encounter was a close one, it may make backward forecasts of Earth's orbit even more unreliable. 

That encounter was "potentially powerful enough to alter simulations’ predictions of what Earth’s orbit was like beyond approximately 50 million years ago," 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."

The team's research was published online Feb. 14 in Astrophysical Journal Letters.