PLANET-SWAP shock: Stars grabbed dirtballs from other clusters
Galaxy was young then, explain boffins
Space boffins have suggested that billions of stars in our galaxy have captured roaming rogue planets as they tootle through the cosmos.
Artist's impression of a rogue planet tootling through the universe.
Credit: Christine Pulliam (CfA)
The rogue planets, which can be ejected from their own star's system by interaction with a fellow planet during the early stages of a system's formation, float freely out in space. If they pass a different star later on, which is moving in the same direction and at the same speed, they can hitch a ride and end up trillions of miles away.
All this planet-swapping took place long ago in young star clusters. Boffins simulated these clusters and found if the number of rogue planets equaled the number of stars, there was a three to six per cent chance the stars would grab a planet over time.
The clusters then disperse, pulling once-neighbouring planets away from each other to end up hundreds or thousands of times farther from each other than the Earth is from the Sun.
Picking out which planets in the systems we can see are actually rogue planets isn't easy though. These worlds are likely to have an orbit that's tilted in comparison to planets native to the star they end up with, and might even orbit backwards.
But native planets can also end up in tilted orbits just because of the ordinary effects of gravity within a planetary system.
The best evidence to support the rogue planet capture theory was the discover in 2006 by the European Southern Observatory, which spotted two worlds orbiting each other without a star.
"The rogue double-planet system is the closest thing we have to a 'smoking gun' right now," said author Hagai Perets, from the Harvard-Smithsonian Centre for Astrophysics. "To get more proof, we'll have to build up statistics by studying a lot of planetary systems."
As for a rogue planet in our own solar system, astronomers haven't spotted one - yet.
"We can rule out large planets," Perets said. "But there's a non-zero chance that a small world might lurk on the fringes of our solar system."
Hagai Perets' study, co-authored by Thijs Kouwenhoven from Peking University in China, will be published in the next issue of the Astrophysical Journal. ®
Re: Artist's impression
Look, if you want to go take a photograph of something which you know won't be illuminated, you bring a flash. Or two. Or half a dozen. In this case it's probably a dozen thermonuclear devices. And if it's a long journey there, a couple of days extra setting up the lighting so that you'll have the best image possible will be inconsequential.
This is a bit of a mish-mash article
I think that what has happened here is that the results from a very specific initial condition have been badly projected in to a general condition.
Just some of the things that bother me about this article:
"...end up trillions of miles away." - one light year is 6.8 trillion miles. However, one light year isn't very far away at all ("Space [the HHGTTG says] is big. Really big. [...] you may think it's a long way down the road to the chemist's but that's just peanuts to space..." D.N.A.). Having said that though, stars in a cluster can be less than one light year apart.
"The clusters then disperse..." - Why should a cluster disperse? The globular clusters around most galaxies appear to be some of the oldest parts of those galaxies, at least judging by the fact that they seem to contain the oldest stars, and they haven't dispersed despite being close to a mucking great gravity well and all the perturbations that implies. Extra-galactic clusters will hardly be perturbed by anything and should be even more stable. One of the notable things about clusters seems to be their stability.
"...pulling once-neighbouring planets away from each other to end up hundreds or thousands of times farther from each other than the Earth is from the Sun." - The Earth is 1 AU from Sol whilst Neptune is ~30 AU from Sol. Eris varies between ~40 to ~100 AU from Sol and the Oort cloud is reckoned to extend out to about ~50,000 AU - roughly one light year, so once again, hundreds or even thousands of times further away than Earth isn't really very far at all.
"If they pass a different star later on, which is moving in the same direction and at the same speed..." - A couple of other folk have already commented about this but to clarify, if the planet and the star are moving in the same direction and at the same speed, and they're close enough for one to capture the other, then they must have come from the same place, at the same time.
Sure, planets can be ejected from their parent systems but they have to pick up a lot of speed to do so and this means it's difficult for another star to capture them.
However, capture can occur if the ejected planet has previously been through sufficient close encounters with enough other stars to slow it and change its path such that it ends up on a relatively slow intersecting course with another star. But this won't be a 'same direction, same speed" path.
Re: Artist's impression
Nor me - astro or otherwise, but it never seems to stop me pontificating...
It appears also to have clouds, and frozen polar caps...
It must be near a star, or some source of heat...