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The next time you’re worried that your collecting mania is getting a little out of hand, you can comfort yourself with the thought that the sun is a bit of a collector, too.
But instead of hoarding comic books (guilty as charged), baseball cards, vintage sneakers, or Pokémon cards, the sun is snapping up passing, out-of-control planets—and it’s not picky. The sun is capable of scooping up small planets and Jupiter-sized gas giants that stray too close, and our star then keeps them at the edge of the solar system.
As any collector will tell you, one of the most important parts of any collecting hobby is trading. And the solar system does that too.
It’s capable of swapping its beloved rogue planets for those snatched up by its neighboring star, Alpha Centauri. And new research suggests that runaway worlds absorbed by the solar system could orbit its outer edges for billions of years before edging closer to the sun, potentially wreaking havoc in the inner solar system.
Related: Rogue planets may originate from ‘twisted Tatooine’ binary star systems
Scientists have long suspected that the solar system can capture and collect passing objects, such as comets and asteroids, outside the Oort Cloud, a spherical shell of trillions of icy bodies thought to be at the outer edges of the solar system.
The new research shows that this “capture region” extends far beyond the Oort Cloud, which is about two light-years from Earth (about 126,000 times the distance between Earth and the sun). The work also reveals that our planetary system is capable of pulling in bodies much larger than comets or asteroids.
“We found that when objects are captured in our solar system, the distance at which they can be captured is much greater than what was previously thought,” Edward A. Belbruno, study author and professor of mathematics at Yeshiva University, told Space.com. “We found that objects that can be captured are from about 3.81 light-years away, which is close to the next galaxy over, the Alpha Centauri system.”
While our solar system cannot capture an object with a mass equal to or greater than the Sun, as this would affect the Sun’s gravity, it can capture runaway planets with a mass equal to that of Jupiter, which is a thousand times less massive than the Sun.
“It’s a very interesting result,” Belbruno continued.
Adopting a Cosmic Orphan
Rogue planets are worlds that have been ejected from their home planetary systems. This can happen when a passing star disrupts the gravitational stability of a planetary system or simply through natural turmoil in a young star system.
The Milky Way is estimated to be populated by a huge number of free-floating rogue planets. Our galaxy may contain as many as a quadrillion (10 followed by 14 zeros) rogue planets that have been expelled from their home systems to roam the Milky Way as cosmic beings.
While much research is devoted to the question of how to expel renegade planets, less attention is paid to the possibility that these celestial beings can find a new home.
The duo of scientists behind this research, including ex-NASA expert James Green, discovered something surprising about this catch of passing rogue planets. There are actually two Green realized that there were gravitationally stable points, or “Lagrange points,” where entanglement could occur.
“You have two whole regions, little openings, where things can enter the solar system at those two Lagrange points,” Belbruno said. “One points toward the Galactic Center; one points away from it.”
The math professor, who has a passion for celestial mechanics, explained that when renegade planets enter the solar system through these openings, they will first move very slowly around our sun, keeping a distance of about 3.81 light-years for about 100 million years. After this period, they will begin to spiral inward, a process that can take billions of years.
Belbruno explained that one interesting factor he and Green found regarding this migration is that the new addition to the solar system will rotate in a pattern called a “fractal curve.” This is a mathematical curve with a shape that repeats the same pattern of irregularity as it is enlarged. A famous example of this is the Mandelbrot set. “You get these really cool curves, especially as the object gets closer and closer to its capture location,” Belbruno explained.
I gotta catch ’em all!
While the Sun isn’t picky about the mass of the planets it captures, it does have one criterion for its orphan planet collections.
Belbruno explained that for a rogue planet to be captured, it would have to be traveling at a relatively slow speed of a few hundred miles per hour. That’s extremely fast by Earth standards, but it’s a cosmic crawl, considering runaway planetary objects have been detected traveling at over 1.2 million miles per hour (1.9 million kilometers per hour).
The low velocities of these captured, runaway planets mean that there could be a veritable supply of such objects orbiting the Sun for billions of years (at distances up to 3.81 light-years).
And what would happen if a captured renegade planet found its way into the solar system? Well, Belbruno said that this effect would depend on the size and mass of that planet.
“Suppose that world was the size of Jupiter and it came into the solar system,” he said. “A Jupiter-sized object coming into our solar system would cause the orbits to shift dramatically in some sense. It would have immediate effects on the dynamics of the Earth’s motion around the sun, and that would absolutely affect life on this planet.
“It is not inconceivable that this will cause a number of planets in the solar system to drift.”
Of course, the rogue planet might never make it to the inner solar system. As noted above, a captured rogue planet orbiting the sun at a distance of 3.81 light years would come very close to Alpha Centauri, which could capture its own villains and keep them at bay.
This means that the two galaxies could exchange their captured villains, like children exchanging baseball cards.
“The gravity of our solar system is close to that of Alpha Centauri, so this would mean that the capture zone of Alpha Centauri would be very close to that of our own sun,” Belbruno said. “The two systems completely overlap in terms of gravitational effect, and this means that objects going back and forth would be very, very natural.”
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The scientists’ work is currently based on mathematical models. Belbruno points out that it would be extremely difficult to spot a star beyond the Oort Cloud, because star planets emit little light.
One helpful factor in the search is the fact that Belbruno and Green have designated the two capture sites, with these Lagrange points a natural place to begin hunting for adopted orphan planets. Such a search may be beyond the reach of telescope technology at present, although the James Webb Space Telescope (JWST) could potentially detect thermal emissions from such a world in infrared light.
“There could be a lot of objects in fixed patterns, sort of orbiting the sun,” Belbruno concluded. “If they come in, they can’t get out — even if they came into the solar system 4 billion years ago. It’s certainly possible that there are already captured, runaway planets.
“We just don’t know.”
A preprint of the study can be viewed on the paper repository arXiv.