A mathematical model suggests there is an unusual region of space where objects can get pulled into the sun’s orbit – meaning we may have to redraw the boundary of the solar system.
The sun’s gravitational pull may be able to capture incoming objects from as far as 3.8 light years away, including interstellar comets and even rogue planets. That would make our solar system much bigger than previously thought.
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| Artistic illustration of a rogue planet MasPix/Alamy |
The extent of our sun’s gravitational influence into the galaxy is unclear, but it seems to stretch at least to the Oort cloud, a sphere of trillions of icy objects that surrounds the sun at a distance of more than 1 light year. This is sometimes regarded as the outer edge of the solar system.
Edward Belbruno at Yeshiva University in New York and James Green, a physicist and former chief scientist at NASA, investigated whether our sun might trap even more distant objects. Modelling our star and the surrounding space, they showed there was a region 3.81 light years away where this might be the case.
Their calculations identify places of gravitational balance, known as Lagrange points, between the sun and the centre of the galaxy. Similar points exist in our solar system, where the gravitational pull of Earth and the sun are balanced, for example, and they are used to keep spacecraft such as the James Webb Space Telescope in a fixed position.
For this outer solar region, there would be two entry points where the sun and galaxy’s gravitational spheres interact to make objects enter an unusual orbit around our sun.
Incoming objects would be on elliptical orbits that, over time, trace out a jagged path that looks like a Mandelbrot set – a fractal pattern with endless symmetrical swirls and shapes, says Belbruno. Objects would essentially remain in this orbit forever, unless they were knocked out by other gravitational influences such as the nearby red dwarf star Proxima Centauri, located just 4.2 light years from our sun.
Green says the region is scientifically interesting because it could trap certain objects, such as rogue planets – worlds unbound to a star that drift through space – or interstellar comets and asteroids like ‘Oumuamua, which entered our solar system in 2017. “We should be looking at the Lagrange points of our sun for rogue planets that may go through there,” says Green, noting such worlds could “disrupt everything in the inner part of the solar system” if they were nudged in our direction.
Calculating the orbit of an object under the gravitational pull of both the sun and the centre of the galaxy is an example of a three-body problem – the astronomical riddle that inspired the sci-fi trilogy by Cixin Liu and its recent TV adaptation.
Simon Portegies Zwart at Leiden University in the Netherlands says the fractal shape of the orbits derived in the paper is “really novel” mathematical work. “They determine orbits in chaotic, restricted three-body environments are fractal,” he says. “I’ve never seen that before.” He isn’t so sure about the region’s ability to capture planets, though. “Since it didn’t capture a star, it probably wouldn’t capture a planet,” he says.
Nonetheless, Green says the region would be worth exploring with upcoming surveys, such as the Vera Rubin Observatory set to switch on in Chile next year, to look for any possible objects that might have been captured there. If confirmed to exist, it might give a true edge of our sun’s influence. “I would feel comfortable in saying 3.81 light years is the boundary of our solar system,” says Belbruno.
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