Galaxies look fixed in astronomical photos, but of course they’re dynamic systems ever in motion. The closest stars to Earth at Alpha Centauri will eventually close to within about 3 light years if we wait thirty thousand years or so. After that, as the system moves away again, Ross 248 will emerge as the closest interstellar target, closing to about the same distance before moving off into the night. But that will require waiting a bit longer, on the order of another 6000 years. We might also keep an eye on Gliese 445, which in 46,000 years or so will close to less than 4 light years of the Sun.

So everything is moving all the time, and we can say something more about future encounters. The REsearch Consortium On Nearby Stars (RECONS) has found that within the local 10-parsec volume, 81 percent of the 357 main sequence stars in its stellar census are less than half as massive as the Sun. That’s about the current estimate for the percentage of stars in the galaxy that are M-dwarfs, and we might expect that most close passages to our system have been and will be with this type of star.

The average distance between these stars is 3.85 light years, which again is close to what we see locally, since the Alpha Centauri stars are not far beyond that range from us. In this collection of stars, 232 single-star systems appear, and 85 multiples. This has led to interesting speculations, such as Bradley Hansen and Ben Zuckerman’s work at UCLA arguing that if stars get close enough, interstellar flight between them is more feasible. Suppose a star at 14,000 AU and the prospect seems more workable.

Stars do indeed get that close, as the example of Gliese 710 shows. If we’re patient, we can wait out the 1.3 million years it is projected for this to happen, for this star, on the borderline between M-dwarf and K-class, is headed our way from its current vantage in the constellation Serpens Cauda. As it will eventually be well inside the Oort Cloud, we can imagine quite an impact on cometary orbits and planetary ones as well over the long haul, as the paper I’m about to discuss shows. But before leaving Hansen and Zuckerman, let me mention that they calculate that if we widen the timeframe to a billion years, the likelihood of a star getting as close as 5000 AU is 81 percent.

Now let’s flip the question backward in time. Scholz’s Star (WISE J072003.20-084651.2) moved near our system about 70,000 years ago, reaching by current estimates somewhere between 52,000 AU and 68,000 AU from the Sun. Here we’ve got a binary consisting of an M-dwarf and a probable brown dwarf companion at 0.8 AU.

Image: Now about 20 light years away, Scholz’s Star (red star in the center) and its brown dwarf companion passed close to the Solar System and remained within 100,000 AU for a period of roughly 10,000 years. Image Credit: ESO VPHAS / Wikimedia Commons CC BY-SA 4.0.

Nathan Kaib (Planetary Science Institute) and Sean Raymond (Laboratoire d’Astrophysique de Bordeaux, CNRS) have gone to work on the question of passing stars as it relates to the Solar System’s evolution. Study the changes that have occurred in Earth’s climate over the millennia and it appears that fluctuations in the eccentricity of Earth’s orbit are involved. Kaib points to a specific episode that is illuminated by the analysis in this paper:

“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 centigrade. 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.”

So we’re digging into the history of our planet’s orbit, along with that of the other planets. The authors’ research shows that stellar encounters are not uncommon. A star passes within 50,000 AU on average every million years, and within 10,000 AU every 20 million years. To study the matter, Kaib and Raymond use a hybrid integrator – a set of specialized numerical methods – called MERCURY to simulate these encounters, under conditions described in the paper. The computer runs show that orbital changes to Earth do indeed result, or are at least accelerated, by perturbations from other stars.

Complicating these calculations is the fact that orbital evolution is chaotic, so that beyond timescales on the order of 100 million years, it is impossible to do more than characterize it statistically. The authors point out that even the long-term stability of the Solar System is not guaranteed, as over the Sun’s lifetime there is a 1 percent chance that Mercury will be lost by collision with either the Sun or Venus.

Earth’s past or future orbital evolution can only be confidently predicted inside a time horizon much shorter than Earth’s age, as small uncertainties in current planetary orbits eventually lead to dramatically divergent behavior. The time horizon set by the internal chaos among the Sun’s eight planets is ∼70 Myr, but additional strong chaos resulting from encounters between large asteroids shortens the horizon by another ∼10 Myr (Laskar et al. 2011b). However, inside this time horizon, backward integration of the Sun’s planets has been used to predict the detailed past orbital evolution of the Earth (Laskar et al. 2004).

Such perturbations make it difficult to pin down Earth’s orbital evolution the farther back in time we go, and it appears as well that perturbations from asteroid interactions are less significant than stellar encounters in degrading our ability to predict orbital changes. That’s useful information, because most simulations of the long-term evolution of the planetary orbits have modeled the Solar System in isolation. Changes in Earth’s orbit can also be the result of interactions with the giant planets, but a passing star can affect their orbits, which in turn impacts Earth’s orbital trajectory. Indeed, adding the giant planets to the simulations shows what a major factor these worlds are on Earth’s orbit once they themselves are perturbed by the passing star. Kaib and Raymond call them “a dynamical link that ultimately allows the Milky Way’s stars to influence the long-term evolution of Earth’s orbit.”

The paper turns to a specific encounter, that with the star HD 7977, which is a G-class star in Cassiopeia now some 250 light years away. 2.8 million years ago, this star moved past the Solar System at about 27 kilometers per second, its trajectory taking it somewhere in the neighborhood of 13,200 AU from the Sun, although the authors point out the wide cone of uncertainty about the distance, which may have been as little as 3900 AU. The ‘impulse gradient’ that shows the level of perturbation to the planets was over an order of magnitude higher than the norm during this time.

Working this into their models, the authors find that the median value of 13,200 AU would have had little effect on the Solar System’s long term chaotic evolution, but a passage at 3900 AU would have affected the eccentricity of Earth’s orbit. This is in addition, of course, to whatever effect such a close passage would have on Oort Cloud objects. A passage at 3900 AU would be one of the ten most powerful encounters experienced in the history of our system given the star’s above average mass and an encounter velocity that is below the average.

Image: Illustration of the uncertainty of Earth’s orbit 56 million years ago due to a potential past passage of the Sun-like star HD7977 2.8 million years ago. Each point’s distance from the center corresponds to the degree of ellipticity of Earth’s orbit, and the angle corresponds to the direction pointing to Earth’s perihelion, or closest approach distance to the Sun. 100 different simulations (each with a unique color) are sampled every 1,000 years for 600,000 years to construct this figure. Every simulation is consistent with the modern Solar System’s conditions, and the differences in orbital predictions are primarily due to orbital chaos and the past encounter with HD 7977. Credit: N. Kaib/PSI.

What’s notable here is that the authors have demonstrated that stellar encounters can be significant drivers for system evolution, an area not as widely studied as the internal dynamics of the Solar System. Here’s their conclusion:

…stellar encounters significantly accelerate the chaotic diffusion of Earth’s orbit and the time back to which numerical simulations can confidently predict Earth’s orbital evolution is ∼10% shorter than previously thought. Second, this chaotic divergence that stellar passages impart on Earth’s orbit results from their perturbations to the giant planets’ orbits, and these perturbations roughly scale with the velocity impulse gradients of stellar encounters. Third, the known encounter with HD 7977 2.8 Myr ago has the potential to unlock new sequences of Earth’s past orbital evolution beyond 50 Myr ago that have not been considered or generated in previous modeling efforts. Although it takes tens of Myr for the effects of stellar passages to significantly manifest themselves, the long-term orbital evolution of the Earth and the rest of the planets is linked to these stars.

We seem to have overestimated our ability to describe Earth’s orbital state in earlier eras given what we’re learning about the disruptive effects of stellar encounters. These close brushes with other stars can potentially cause changes in eccentricity that have been overlooked. Factoring stellar encounters into future models is going to be complicated, and necessary.

The paper is Kaib & Raymond, “Passing Stars as an Important Driver of Paleoclimate and the Solar System’s Orbital Evolution,” Astrophysical Journal Letters 962 (14 February 2024) L28 (full text). The Hansen and Zuckerman paper I mentioned above is “Minimal conditions for survival of technological civilizations in the face of stellar evolution,” Astronomical Journal Col. 161, No. 3 (25 February 2021) 145 (full text). See also Bobylev & Bajkova, “Search for Close Stellar Encounters with the Solar System Based on Data from the Gaia DR3 Catalogue,” Astronomical Letters Vol. 48 (13 February 2023), 542-549 (abstract).