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The next time you think your collecting habit is getting out of control, you can console yourself by knowing that the sun is a collector too.
But instead of hoarding comic books (guilty), baseball cards, old sneakers, or Pokémon cards, the sun snatches up rogue planets as they pass by, and it’s not selective. The sun is capable of snatching up Jupiter-sized gas giants that get too close, as well as smaller planets; our star then traps them at the edge of the solar system.
As any collector will tell you, one of the most important parts of the collecting hobby is trading, and the solar system does just that.
It has the ability to replace its valuable rogue planets with those captured by the neighboring star system Alpha Centauri. And new research suggests that runaway worlds captured by the solar system could orbit its outer reaches for billions of years before approaching the sun, potentially wreaking havoc in the inner solar system.
Relating to: Rogue planets could originate from ‘warped Tatooine’ binary star systems
Scientists have suspected for some time that the Solar System might capture passing objects such as comets and asteroids and collect them beyond the Oort Cloud, which is believed to be a spherical shell of trillions of icy objects located on the outer edges of the Solar System.
The new research shows that this “capture zone” 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 study also reveals that our planetary system can draw in objects much larger than comets or asteroids.
“We found that when objects are captured into our solar system, the distance at which they can be captured is much greater than previously thought,” study author and Yeshiva University mathematics professor Edward A. Belbruno told Space.com. “We found objects that can be captured from about 3.81 light-years away, which is close to the next star system, the Alpha Centauri system.”
While our solar system cannot capture an object with a mass close to or greater than the Sun’s, because it would be affected by the Sun’s gravity, it can capture stray planets with a mass up to a thousand times that of Jupiter.
“It’s a very interesting result,” Belbruno continued.
Adopting a cosmic orphan
Rogue planets are worlds that have been ejected from their planetary systems. This can happen when a passing star interferes with the gravitational stability of a planetary system, or simply through natural turbulence in a young star system.
It is estimated that the Milky Way is filled with a large 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 ejected from their home systems and are wandering around the Milky Way as cosmic orphans.
While much research has focused on how rogue planets might be ejected, fewer studies have focused on the possibility that these celestial orphans could find new homes.
The duo of scientists behind this research, including former NASA expert James Green, discovered something surprising about this trapping of passing rogue planets. two Green realized that there are gravitationally stable points, or “Lagrange points,” where entrapment can occur.
“You have two entire regions with two little openings through which things can enter the solar system,” Belbruno said. “One points toward the Galactic Center; the other points away from it.”
The mathematics professor, who has a passion for celestial mechanics, explained that when rogue planets enter the solar system through these openings, they will first begin moving very slowly around our sun, staying at a distance of about 3.81 light-years for about 100 million years. After that time, they will begin to spiral inward, a process that can take billions of years.
Belbruno explained that one interesting factor he and Green found about this migration is that the newly added object to the solar system orbits in a pattern called a “fractal curve.” This is a mathematical curve with a shape that repeats the same pattern of irregularities as it grows. A famous example of this is the Mandelbrot set. “You get really cool-looking curves, especially as the object gets closer to the capture position,” Belbruno explained.
I gotta catch them all!
While the Sun isn’t picky about the mass of planets it captures, it does have a criterion for its collection of orphan planets.
For a rogue planet to be captured, Belbruno explained, 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 considering that runaway planet-mass objects have been found moving faster than 1.2 mph, that’s a cosmic crawl. million miles per hour (1.9 million kilometers per hour).
The slow speeds of these captured rogue planets mean that there could be a veritable reservoir of such objects orbiting the Sun (at distances of up to 3.81 light-years) for billions of years.
But what happens if a captured rogue planet enters the solar system? That impact would depend on the size and mass of that planet, Belbruno said.
“Let’s say that Earth was the size of Jupiter and drifted into the solar system,” he said. “A Jupiter-sized object coming into our solar system would cause the orbits to shift quite significantly, in a sense. It would have immediate effects on the dynamics of how the Earth moves around the sun, and it would certainly affect life on this planet.
“It is not inconceivable that some planets in the solar system could also spiral out of control.”
Of course, the rogue planet may never make it to the inner solar system. As noted above, orbiting the Sun at a distance of 3.81 light-years would bring a captured rogue planet very close to Alpha Centauri, which could capture its own rogues and keep them at bay.
This means that the two star systems can swap outlaws they’ve captured, like kids trading baseball cards.
“Our solar system’s gravity is tipped out close to that of Alpha Centauri, which means that Alpha Centauri’s capture region is very close to that of our sun,” Belbruno said. “The two systems completely overlap in terms of gravitational influence, and that means it would be very, very natural for things to go back and forth.”
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Scientists’ work is currently based on mathematical modeling, and Belbruno notes that it would be extremely difficult to detect a planet beyond the Oort cloud because rogue planets emit so little light.
One factor that has helped the search is that Belbruno and Green identified two capture points, Lagrange points that are a natural place to begin the hunt for adopted orphan planets. Such a search would be beyond the capability of telescope technology at present, but the James Webb Space Telescope (JWST) can detect the thermal emissions of such a world in infrared light.
“There could be a lot of objects out there in eclipse patterns, kind of spiraling around the sun,” Belbruno concluded. “If they get in, they can’t get out — even if they entered the solar system 4 billion years ago. It’s certainly possible that there are stray planets out there that are already caught up.
“We just don’t know.”
A preprint of the work can be viewed on the arXiv article repository.