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Rogue Planets Can Find Homes Around Other Stars

In this artist's conception, a captured world drifts at the outer edge of a distant star system, so far from its Sun-like host that the star's disk is barely resolvable at upper right. New research shows that one in 20 stars within our galaxy might have captured a free-floating planet. Credit: Christine Pulliam (CfA)

As crazy as it sounds, free-floating rogue planets have been predicted to exist for quite some time and just last year, in May 2011, several orphan worlds were finally detected. Then, earlier this year, astronomers estimated that there could be 100,000 times more rogue planets in the Milky Way than stars. Now, the latest research suggests that sometimes, these rogue, nomadic worlds can find a new home by being captured into orbit around other stars. Scientists say this finding could explain the existence of some planets that orbit surprisingly far from their stars, and even the existence of a double-planet system.

“Stars trade planets just like baseball teams trade players,” said Hagai Perets of the Harvard-Smithsonian Center for Astrophysics.

Astronomers now understand that rogue planets are a natural consequence of both star and planetary formation. Newborn star systems often contain multiple planets, and if two planets interact, one can be ejected in a form of planetary billiards, kicked out of the star system to become an interstellar traveler.

But later, if a rogue planet encounters a different star moving in the same direction at the same speed, be captured into orbit around that star, say Perets and Thijs Kouwenhoven of Peking University, China, the authors of a new paper in The Astrophysical Journal.

A captured planet tends to end up hundreds or thousands of times farther from its star than Earth is from the Sun. It’s also likely to have a, orbit that’s tilted relative to any native planets, and may even revolve around its star backward.

Perets and Kouwenhoven simulated young star clusters containing free-floating planets. They found that if the number of rogue planets equaled the number of stars, then 3 to 6 percent of the stars would grab a planet over time. The more massive a star, the more likely it is to snag a planet drifting by.

While there haven’t actually been planets found yet that are definitely a ‘captured’ world, the best bet would perhaps be a planet in a distant orbit around a low-mass star. The star’s disk wouldn’t contain enough material to form a planet that distant, Perets and Kouwenhoven said.

The best evidence of a captured planet comes from the European Southern Observatory, which announced in 2006 the discovery of two planets (weighing 14 and 7 times Jupiter) orbiting each other without a star.

“The rogue double-planet system is the closest thing we have to a ‘smoking gun’ right now,” said Perets. “To get more proof, we’ll have to build up statistics by studying a lot of planetary systems.”

As for our own solar system, there’s no evidence at this time that our Sun could have captured an alien world, which would lie far beyond Pluto.

“There’s no evidence that the Sun captured a planet,” said Perets. “We can rule out large planets. But there’s a non-zero chance that a small world might lurk on the fringes of our solar system.”

Read the team’s paper.

Source: CfA

About 

Nancy Atkinson is Universe Today's Senior Editor. She also works with Astronomy Cast, and is a NASA/JPL Solar System Ambassador.

Comments on this entry are closed.

  • Christopher Rose April 17, 2012, 8:11 PM

    At one point I thought I had read that stellar capture was one of several competing theories regarding the cause of Sedna’s unusual orbit.

    • David Krauss April 18, 2012, 1:28 AM

      No doubt. But we don’t yet know if it’s actually unusual…

  • leandro luna-diaz April 17, 2012, 10:46 PM

    dumb question, but here I go.. Could this be the missing matter thats being attributed to dark matter? like brown dwarfs, rogue planets etc???

    • .. .__. April 17, 2012, 11:21 PM

      Also being an interested layman, this question doesn’t sound that dumb to me.

    • Peristroika April 18, 2012, 12:50 AM

      I’m not going to do the math right now for you. It’s pretty basic. But suffice to say that the mass of many many planets is required to equal even one star. They don’t compare at all. So yes, this identifies some mass WHERE we did not suspect but since the mass of known stars basically includes their respective systems and these planets came from these systems, the fact that they are rogue doesn’t change the understood mass of the galaxy.

      • squidgeny April 18, 2012, 12:21 PM

        I don’t think the calculations of a star’s mass includes its respective system. Until 20 years ago we didn’t even know there were other systems – and certainly we don’t yet know the statistical distributions of mass locked up in systems for various types of star. I’d imagine the calculations of a star’s (and galaxy’s) mass ignore possible planets – unless those planets are emitting light.

        But anyway, yeah, there’s no way rogue planets could account for all the missing dark matter…

        • Torbjörn Larsson April 18, 2012, 7:48 PM

          If our system is anything to go by, a mature planetary system has as much mass in smaller bodies as in gas giants. Which is many orders of magnitude less than the systems’s star/stars.

      • Jon Souter April 18, 2012, 10:50 PM

        @Peristroika

        Granted – but we have very little data about the quantity/mass of planets, asteroids, black holes, gas and other as yet undetected barionic matter in the universe.

        There’s likely a lot more in our own solar system than we can account for, simply because we don’t yet have the technology to survey it.

        When you factor in the amount of barionic matter that could be lurking in interstellar space between us and our other galactic neighbours – and the sea of quantum particles that supposedly fizzle in and out of existence in the ‘vacuum’ of space – I think it’s fair to say we’re playing with a less than complete picture of reality.

    • Jonathon Moss April 18, 2012, 3:50 AM

      Also, dark matter is not simply the stars, planets and nebulae that are made of normal (baryonic) matter — protons, neutrons and electrons — but it’s actually something entirely different. Exactly what it is, is not clear, but it certainly isn’t the same stuff as we’re made of.

      • squidgeny April 18, 2012, 12:14 PM

        That’s not entirely true – dark matter is subdivided into baryonic (which these planets might be considered) and non-baryonic.

    • lcrowell April 18, 2012, 3:31 PM

      This would increase the contribution by ordinary matter to what we call dark matter a little bit. Dark energy is 74% of mass-energy of the universe, 22% is dark energy and the remaining 4% is luminous or ordinary matter. This is based on memory, but modulo some jiggle factor it is about this. Now we can break out the luminous stuff. That 4% is 3.5% gas and the remainining .5% is in stars. Now these data concerning rouge planets would suggest that if there are 100,000 times more planets than stars that the amount of luminous matter in the universe might jump from 4% to 4.5% and dark matter drops from 22% to 21.5%.

      Clearly, this is not enough to overthrow the concept of dark matter. The current thinking about the rather extraordinary number of planets simply adjusts the luminous or ordinary matter component a little bit bigger. In a related note the masses of neutrinos probably contributes as much mass as is contained in stars. So the amount of ordinary matter would then increase from 4.5% to 5%, and dark matter (which means the unknown matter source of gravity) drops to 21%.

      LC

      • Jon Souter April 19, 2012, 2:41 PM

        But aren’t these estimates based on formulas related to gravity – a force we still don’t fully understand and which itself is at the heart of the debate of why we need the – as yet – undetected dark matter & dark enetgy?

        I’m not bashing anybody – I’m just interested in how theories are layered together and whether the flaws in one can propagate to others in the stack.

  • Deejay Nunti April 17, 2012, 11:04 PM

    “Researchers in a joint Japan-New Zealand study surveyed microlensing events near the central part of our galaxy during 2006 and 2007 and identified up to 10 Jupiter-sized orphan worlds between 10,000 and 20,000 light-years away. Based on the number of planets identified and the area studied they estimate that there could literally be hundreds of billions of these lone planets roaming our galaxy….literally twice as many planets as there are stars.” —-quote from earlier UT post. And now “astronomers estimated that there could be 100,000 times more rogue planets in the Milky Way than stars.” . So which one is it ? 2 or 100.000 ?

    • squidgeny April 18, 2012, 12:25 PM

      The article you quote is from May 2011, and references a study that does indeed suggest “twice as many”.

      But the article referenced at the start of this article is dated February 2012, and is about a different study, which suggests 100,000 times as many.

  • bfmorris April 18, 2012, 3:00 AM

    I see potential implications for transpermia.

    • Torbjörn Larsson April 18, 2012, 7:55 PM

      Indeed, it is the first realistically useful transpermia vehicle reported. Slowly cooling ice moons may be even better.

      Unfortunately most of the captured planets ends up very far from the star and won’t seed the original system. But it will happen, with a small efficiency, assuming ejecta from impacts can take spores from the biosphere planet to a nearby planet.

      Of course, the likelihood that such an event will make a difference is even worse. The impact must reach a seedable planet before itself evolves life, and that time window is very small in comparison to star lifetimes.

      • bfmorris April 20, 2012, 1:02 AM

        “The impact must reach a seedable planet before itself evolves life, and that time window is very small in comparison to star lifetimes.”

        Before itself evolves life?
        Are you saying that because something has verifiably happened (ie here in our out of the way system) one time, it must happen again? Can something happen just one time, or does any given something happen multiple times?

  • bugzzz April 18, 2012, 3:10 AM

    I’m fascinated by rogue planets. Maybe because it’s such a new “discovery.” Their existence seems obvious now, but it’s as if nobody really thought about it until recently. I love when that happens. More cosmic surprises still left. Actually probably quite a few.

    • bfmorris April 18, 2012, 5:23 AM

      I wonder if tidal forces would have caused the destruction of features, if there were any, on the surface of these now frozen ghost planets.

    • bfmorris April 18, 2012, 5:23 AM

      I wonder if tidal forces would have caused the destruction of features, if there were any, on the surface of these now frozen ghost planets.

    • fleinkantarell April 18, 2012, 5:39 AM

      Asimov mentioned a rogue planet in “Foundation and Empire” in 1952. The Empire had their millitary hedquarter on one. It is a very old idea. I’m thrilled every time sci-fi is proved right :)

    • Torbjörn Larsson April 18, 2012, 8:01 PM

      From behind this curtain I pull … Flash Gordon of 1934. Mongo was a rogue planet passing Earth.

  • Simon Bacaltos April 18, 2012, 4:17 AM

    Those two new movies that came out Melancholia and the Another Earth could be a reality.

  • ren00r April 18, 2012, 8:46 AM

    The idea behind rogue planets is, that they are outcast from star systems, or they condense from interstellar matter, or both? Because in first case it would imply that every star create thousands of planets and I couldn’t imagine how outcast planet (which must exceed escape velocity of its parent star) could be captured in relatively slow, wide orbit. The second case is much more acceptable for me.

    • squidgeny April 18, 2012, 12:35 PM

      A star wouldn’t need to create thousands of planets in order for some to get ejected – all it needs is a few planets, perhaps some massive like Jupiter, making close passes by each other (or getting caught in resonance orbits). It’s thought that our own planets didn’t form in their current locations but went through a number of ‘migrations’ toward and away from the Sun, during which Jupiter may have kicked out a planet or two (and even put Uranus in its current position).

      Also, the existence of rogue planets doesn’t imply that they all get captured (and as you rightly imagine, they couldn’t be captured very easily if going that fast). I’d imagine such captures to be very, very rare.

      As for rogue planets condensing out of interstellar material…. at best that would form a Jupiter (a gassy giant with no internal fusion). And even that I’m not sure about. No chance of it ever forming a rocky planet though.

      • ren00r April 18, 2012, 3:10 PM

        I know that there’s no chance of rocky planet condensing out of interstellar material, because composition would be wrong. Although you misunderstood me in the other point. I’m not a native speaker, so I make lot of logical shortcuts while writing.
        So, when article states that number of planets is 2-100000 times higher than number of stars and you imply that condensation of planet-sized objects is rare,where would such a higher number of planets originate from?I deduced that each star must create AND eject 2-100000 planets on average.It’s ridiculous idea for me also,but each rogue planet must be created somewhere,they just don’t pop out of nowhere.

      • ren00r April 18, 2012, 3:10 PM

        I know that there’s no chance of rocky planet condensing out of interstellar material, because composition would be wrong. Although you misunderstood me in the other point. I’m not a native speaker, so I make lot of logical shortcuts while writing.
        So, when article states that number of planets is 2-100000 times higher than number of stars and you imply that condensation of planet-sized objects is rare,where would such a higher number of planets originate from?I deduced that each star must create AND eject 2-100000 planets on average.It’s ridiculous idea for me also,but each rogue planet must be created somewhere,they just don’t pop out of nowhere.

        • Torbjörn Larsson April 18, 2012, 8:26 PM

          But those numbers aren’t comparable, IIRC.

          The lower figure is the average number of rogue planets as observed with gravitational lensing surveys. The upper figure is the maximum number of rogue planets before they start to strain gravitational surveys. Somewhere in between would be the limit of rogues before the absence of observed such in recorded history would be peculiar, I believe from a rough estimate.

          In our case, the prediction that our system ejected a 5th gas giant is recent, comes from work on the very successful Nice model that predicts very many features of our system, and is compatible with the above results. If our system hadn’t ejected a planet it would have been deviating from the norm.

          But it seems our system, as so many times before, is an individual as all other systems observed but a very mundane individual. We don’t even have the record number of potential planets anymore.

        • squidgeny April 19, 2012, 12:03 PM

          Yes, I see what you mean now – those rogue planets would have to have come from highly populated systems. Perhaps that will reduce the upper limit of 100,000x.

          It’s worth noting though that our own system, in addition to 8 planets, also has many more objects (moons and dwarf planets) that would be considered rogue planets if we found them wandering in space. So it’s not beyond reason that a system could have “thousands of planets”.

  • Piotr Mali?ski April 18, 2012, 2:15 PM

    Someone stated that there may be a planet far away disturbing comets from the Oort cloud… it could be a rogue planet then.

  • Thomas Houck April 18, 2012, 4:42 PM

    Please read ‘When Worlds Collide’ and ‘After Worlds Collide’ written by Philip Wylie and Edwin Balmer in 1933 and you will clearly see that rogue planets are nothing new!

    • Grimbold April 18, 2012, 10:07 PM

      “A pail full of air” by Fritz Leiber is based on a similar idea.

  • Thomas Houck April 18, 2012, 4:46 PM

    Perhaps our system has ejected one or two rouge planets. You can imagine how chaotic it was here when things first started to jell what with a Mars sized object slamming into a proto-Earth….

  • Olaf2 April 18, 2012, 5:20 PM

    I am wondering how a star can capture a planet.
    If something is coming from outside the solar system then it will have an escape velocity (relative to the Sun). Somehow this speed mus be reduced to orbital speed.

    I can relate that a rough planet gets a gravity assist by Jupiter to slow down. Or in case of a double star that one of the stars give the gravity assist.

    • squidgeny April 19, 2012, 12:00 PM

      If a rogue planet was on a spectacularly unlikely trajectory, it might perform an aerocapture manoeuvre with the new star :)

  • Aqua4U April 19, 2012, 4:41 PM

    Velikovski revisited? His claim that Saturn might possibly/actually have been a captured rogue planet now seems a little bit less ‘far fetched’? Would that there were a way to prove this theory about Saturn one way or another would certainly make some headlines!

    Of Coors, he also claimed that Venus and Mars have shifted orbitally too. Like separating chaff from wheat, science has proved him wrong about so many things… so don’t take my comment too seriously!

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