On the coincidence between the close passage of HD7977 and the Pliocene-Pleistocene transition

Abstract

The Oort Cloud's dynamical evolution is significantly influenced by both the galactic tide and stellar flybys. This study investigates the particular case of HD7977's close encounter 2.47 Myr ago, which likely repopulated the Inner Oort Cloud and potentially triggered a significant comet shower on Earth. Our results demonstrate that the shower's intensity strongly depends on HD7977's impact parameter ($b$), with possible flyby distances ranging from 2,300 AU to $\sim$ 13,000 AU. For the closest approach ($b \sim 2,300$ AU), the terrestrial impact probability of 1 km comets increases by an order of magnitude compared to the steady state, slightly exceeding the asteroid impact probability at this size scale. We propose an analytical method to compute the probability of comet showers impacting Earth, which saves considerable computation time compared to N-body simulations. We identify a threshold diameter $D_0 = 2.25$ km for which yields $P = 1$ in our model, with $D_0$ following a logarithmic dependence on $b$. These findings suggest that HD7977's flyby may have caused an enhanced comet flux during the Pliocene-Pleistocene transition, which could plausibly be related to the environmental changes at this era.

Figures (4)

• Figure 1: Panel (a)-(c) is the snapshots of the REBOUND simulation. Colors represent the angular momentum value of each comet. Those with blue colors have a possibility of entering the inner solar system. The pentagram symbol denotes the Sun, the gray trajectory represents the flyby path of HD7977, and the purple trace illustrates the orbit of the selected comet. The insets inside each plot show the magnified orbit evolutionary of a selected representative comet. Panel (d) is the distribution of angular momentum change of Inner Oort cloud comets.
• Figure 2: (a) Distribution of $log_{10}(q)$ before and after the passage of HD7977 at different $b$, where $q$ represents the perihelion distance of comet orbits and $b$ represents the closest distance of HD7977 to the Sun. (b) Rate of comets with diameter $\geq 1~\text{km}$ colliding the Earth during the 3.5 Myr timescale. Year 0 represents the present (2025 A.D.). There was a peak of comet shower shortly after the passage of the HD7977 (-2.47 Myr). The transparent shaded region indicates the uncertainty in the results, which is relatively small.
• Figure 3: (a) Collision probability ($P$) with Earth as a function of Oort Cloud comet diameter ($D$). The critical diameter $D_0$ corresponding to a unit collision probability ($P=1$) is determined as 2.25 km. For $D=1$ km, the derived probability $P=5.08$ suggests an estimated $\sim 5$ potential Earth-colliding comets within the modeled parameter space. (b) Threshold diameter $D_0$ versus impact parameter $b$. The relationship exhibits a logarithmic dependence, yielding $b\approx7,000$ AU at $D_0=1$ km.
• Figure 4: (a) Schematic of orbital parameters, where $\vec{r}$ is the position vector of a comet, $\vec{v}$ is the velocity vector, $\alpha_0$ is the angle between $\vec{r}$ and the x-axis, $\theta$ is the angle between $\vec{v}$ and the x-axis, $\phi$ is the orientaion of the ellipse orbit and axis 1 and axis 2 are two rotation axes. (b) Comparison between impulse approximation theory and simulation, using $|\Delta v_x|$ as a metric.