Chaotic streams and risks for Earth due to passing stars

In about 1.3 million years a passing star will cross the outermost region of our solar system, i.e. the Oort cloud. This stellar fly-by will certainly cause chaos in this region due to gravitational perturbation. In addition, it will trigger cometary streams towards the Sun and might thus increase the risk of collisions of comets with planet Earth. All these processes will be studied in detail in the framework of this FWF- project with the aim to get estimates for the probability of our home planet experiencing stronger bombardments in the future. The science of this project comprises the following studies. STELLAR FLY-BY Observations of the European space mission GAIA have shown that the K-type star Gliese 710 is approaching our solar system and will enter the Oort cloud at a distance to the Sun between 600 billion and 1800 billion kilometers. The Oort cloud is known as reservoir of billions of comets. Thus, it is quite obvious that a star which enters this reservoir of comets will trigger cometary streams towards the Sun and enhances the probability of impacts of small bodies with planet Earth. However, GAIA did not observe low-mass stars (M-type stars), which are the majority in the solar neighborhood (90 %). To estimate the threat of these stars when potentially passing the solar system, we extend our investigation of stellar encounters to M-type stars and to the even smaller Brown Dwarfs in order to provide a more extensive overview about the risk of impacts on planet Earth due to stellar fly-bys. CHAOS IN THE OUTER REGION OF OUR PLANETARY SYSTEM Chaos originated by a stellar encounter in the outer region of the solar system affects mainly comets of the Oort cloud and of the Kuiper belt. Our numerical simulations will study to what degree do stellar fly-bys and triggered cometary streams enhance chaos in the motion of planetesimals in the solar system and how does this increase the risk of collisions of small bodies with planet Earth. We also analyze the influence of the giant planets (Jupiter, Saturn, Uranus and Neptune) and pay special attention to the big Kuiper belt objects (e.g. Sedna or Eris) discovered so far and the still undetected Planet 9. These numerical studies will help to get estimates for the degree of chaos in the outer solar system caused by a stellar fly-by. COLLISIONS OF COMETS AND ASTEROIDS Cometary streams entering the main belt of asteroids between Mars and Jupiter might lead to collisions between comets and asteroids. Such interactions will be studied in detail in the framework of this project where we combine dynamical evolution and realistic collision simulations for the first time. In this context we will verify in which way the so-far unknown small asteroids which will be discovered by the Large Synoptic Survey Teleskop LSST (also known as Rubin Observatory) in the near future influence our collision study for planet Earth.

Since the stars in our galaxy (the Milky Way) orbit the galactic center on different trajectories, close encounters between stars can occur . Our Sun will have a stellar "visitor" in 1.29 million years. The star Gliese 710 is still ~62 light-years away from us and can only be observed with telescopes in the constellation Serpens. In 1.29 million years, it will be only 10 500 astronomical units (au) or 1.57 trillion km away from the Sun. The planets of the solar system will not be perturbed. According to our calculations, Gliese 710 would have to approach the Sun by up to 80 AU to destabilize the orbit of Neptune (at ~30 AU). Nevertheless, Gliese 710 will disturb the solar system as it will pass through the Oort Cloud. The Oort Cloud is known to be the reservoir of long-period comets (orbital periods > 200 years) and extends beyond the Kuiper Belt to 100 000 AU (~15 billion km). Based on various studies, it can be assumed that there are up to 1 trillion comets in this area, orbiting the Sun on different orbits (mostly very eccentric and in some cases hyperbolic). For the duration of the fly-by (approx. 64,000 years), Gliese 710 will perturb millions of comets along its orbit, causing them to either move toward the Sun and thus toward Earth or escape into interstellar space. This project studied the fly-by scenario of Gliese 710 for the first time using N-body simulations. Considering only the sphere of influence of Gliese 710 (with a radius of 8152 AU) along its orbit and assuming that there is one comet every 16 AU in this area, so that a total of 220 million objects are studied. Due to the fly-by of Gliese 710 approximately half of these comets are ejected into the interstellar space and 66 600 comets are directed toward the inner solar system. Of these, 4 900 comets will enter the observable area (within Jupiter's orbit), 20 of which will have their closest position to the Sun near Earth's orbit. However, these orbits are more or less inclined to Earth's orbit, making possible collisions with the terrestrial planets rather unlikely. Furthermore, over millions of years, long-period comets will repeatedly enter the inner solar system, but due to their different orbits they appear as individual phenomena. Finally, it should be noted that the orbits of the Sun and Gliese 710 will change slightly after their encounter in 1.29 million years. The shift in the Sun's orbit will be <50 au (i.e., the distance between the Sun and Pluto).