When this happens, the influence may seem negligible at first. A small tug. A slight variation in a distant orbit. But under the right conditions, a single passing star could alter the configuration of multiple planets<\/a>. The consequences, though improbable in any short-term frame, are significant across cosmic time.<\/p>\n New research now quantifies that risk. And it finds the solar system is more susceptible to outside disruption than long believed.<\/p>\n Flyby Stars Introduce New Instability to Earth\u2019s Orbit<\/p>\n A study conducted by Nathan Kaib of the Planetary Science Institute and Sean Raymond of the University of Bordeaux has introduced a critical update to our understanding of long-term solar system stability<\/a><\/strong>. Their simulations, run over a five-billion-year period, incorporate the gravitational influence of passing stars<\/strong>, also known as field stars<\/strong>, that move through the galaxy and occasionally near the solar system.<\/p>\n In contrast to earlier models that treated the solar system as an isolated system, Kaib and Raymond show that stellar flybys, while rare, can produce meaningful gravitational effects. Their analysis, published on the arXiv preprint server<\/a>, finds that Earth faces a 0.2 percent chance<\/strong> of either being ejected from the solar system or experiencing a planetary collision<\/strong> over that period. Mars, too, has a 0.3 percent probability<\/strong> of being lost due to similar mechanisms.<\/p>\n These findings are based on thousands of simulations. Researchers modelled a wide range of flyby scenarios using varied stellar masses, approach distances, and relative velocities. Results show that even a single strong encounter can trigger a series of orbital disturbances, beginning with the innermost planets and propagating outward.<\/p>\n Gravitational Cascades and the Role of Mercury<\/p>\n The simulations frequently identify Mercury<\/strong> as the initial point of instability. Already subject to orbital shifts due to Jupiter\u2019s<\/strong> gravity, Mercury becomes far more unstable under the influence of a passing star. In many simulations, the planet\u2019s orbit grows increasingly elliptical until it either collides with the Sun or with Venus. That disruption, in turn, can destabilise other inner planets.<\/p>\n In the most severe scenarios, Venus<\/strong> or Mars<\/strong> is nudged into a path that interferes with Earth\u2019s orbit. This can lead to a direct collision or, more often, an encounter that sends Earth toward Jupiter<\/strong>, whose powerful gravitational field could then eject the planet entirely.<\/p>\n The overall likelihood of such a chain reaction is low but significant. The study notes that Mercury\u2019s instability increases by 50 to 80 percent<\/strong> when field stars are considered.<\/p>\n \u201cIt\u2019s a little scary how vulnerable we may be to planetary chaos,\u201d said Renu Malhotra, a planetary scientist at the University of Arizona, in remarks to Science News<\/a>. She was not involved in the research.<\/p>\n Not All Stars Pose a Threat, but Some Come Close<\/p>\n The gravitational influence of a flyby depends on proximity and speed. Stars passing within 100 astronomical units (AU)<\/strong> of the Sun, or roughly 100 times the distance from Earth to the Sun, are the most disruptive. Those moving at less than 10 kilometres per second<\/strong> relative to the solar system are especially significant, as their slower pace allows more time for gravitational effects to take hold.<\/p>\n Kaib and Raymond estimate that there is a 5 percent chance<\/strong> of such a close encounter occurring within the next five billion years. Though still unlikely, this is substantially higher than previous estimates, which often excluded these stellar interactions entirely.<\/p>\n Stellar flybys are not purely theoretical. In a 2015 paper in The Astrophysical Journal Letters, researchers documented the close passage of Scholz\u2019s Star<\/a> about 70,000 years ago. That star likely came within 0.8 light-years of the Sun and passed through the outer Oort Cloud<\/strong>, potentially disturbing comets or icy bodies.<\/p>\n![\"An](\"https:\/\/www.europesays.com\/us\/wp-content\/uploads\/2026\/01\/052825_kc_earth_inline.jpg\")
Once thought to be safe, poor Pluto (true-color image from the New Horizons spacecraft shown) faces a 4 percent risk of ejection by or collision with a giant planet during the next 5 billion years due to gravitational mischief from passing stars. Credit: NASA\/Johns Hopkins University Applied Physics Laboratory\/Southwest Research Institute<\/strong><\/p>\n