Some stars capture rogue planets

April 17, 2012, Harvard-Smithsonian Center for Astrophysics

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)
( -- New research suggests that billions of stars in our galaxy have captured rogue planets that once roamed interstellar space. The nomad worlds, which were kicked out of the star systems in which they formed, occasionally find a new home with a different sun. 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 just like baseball teams trade players," said Hagai Perets of the Harvard-Smithsonian Center for Astrophysics.

The study, co-authored by Perets and Thijs Kouwenhoven of Peking University, China, will appear in the April 20th issue of The .

To reach their conclusion, Perets and Kouwenhoven simulated young 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.

They studied young star clusters because capture is more likely when stars and free-floating planets are crowded together in a small space. Over time, the clusters disperse due to close interactions between their , so any planet-star encounters have to happen early in the cluster's history.

Rogue planets are a natural consequence of . systems often contain multiple planets. If two planets interact, one can be ejected and become an interstellar traveler. If it later encounters a different star moving in the same direction at the same speed, it can hitch a ride.

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.

Astronomers haven't detected any clear-cut cases of captured planets yet. Imposters can be difficult to rule out. Gravitational interactions within a planetary system can throw a planet into a wide, tilted orbit that mimics the signature of a captured world.

Finding a planet in a distant orbit around a low-mass star would be a good sign of capture, because the star's disk wouldn't have had enough material to form the planet so far out.

The best evidence to date in support of planetary capture 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."

Could our solar system harbor an alien world far beyond Pluto? Astronomers have looked, and haven't found anything yet.

"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."

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5 / 5 (3) Apr 17, 2012
How would that happen if there isn't already another planet around the star?
It is just not possible in a two-body system. No matter how the star is moving, in its reference frame the orbit of the planet will be completely symmetric around the periapsis. So if the planet comes from infinity, it will also escape to infinity.

The only way such a "capture" can happen is if the planet interacts with another planet that is already there, giving it some of its momentum and altering its orbit as well. Or if not another planet, then at least gas clouds or something, though that seems unlikely if the final resulting orbit is so far from the star.

What am I missing here?
3 / 5 (6) Apr 17, 2012
What am I missing here?

Not much.

The article gives an over-simplified explanation.

You are correct that some sort of additional interactions would usually be required, such as by passing through a dust cloud or colliding with comets or something to slow relative momentum (by picking up additional mass, etc.

A very close encounter, such as passing through the star's atmosphere, might allow the planet to be slowed through friction and mass exchange, and then go into a very long hyperbolic orbit, where it might have a chance to collide with more dust, asteroids or comets for additional mass exchanges, thus perhaps obtaining a relatively stable orbit...

The description of supposedly matching the star's velocity is BS.

If two objects in space are initially traveling parallel at the same velocity they will eventually collide due to gravity between them.

To be captured, the planet would need a perpendicular or skew path vs the star at a precise angle and velocity.
3.9 / 5 (7) Apr 17, 2012
This is occurring in a dense star cluster, so the third object that allows capture could be another star.
5 / 5 (3) Apr 17, 2012
Oh, and also, the assumption of symmetry assumes point masses, which would not be true if close passes induced tidal waves, or if the star was not spherically symmetrical due to rotation (or we could even through in relativistic frame dragging...).
2.7 / 5 (6) Apr 17, 2012
In astronomical terms, when they talk about "stable" orbits, they usually mean something that doesn't decay over time scales of 100 million to a billion or more years.

Anyway, the keyhole for a successful capture is going to be so tiny it's insane, perhaps as big as a few kilometers or tens of kilometers, or as small as a few meters wide...

Also, the planet can't just match linear velocity, it must obtain an exactly perfect angular momentum in the Star's reference frame to avoid a catastrophic collision with the star, or avoid simply being re-ejected immediately.
5 / 5 (4) Apr 17, 2012
Also remember that the environment in which the captures are expected to take place is a cluster with plenty of gas--and lots of new stars often as close to each other as Pluto is to the sun. A pair of temporarily close by stars can cause a planet to be captured, and even create a situation where a planet is captured by one star for half an orbit, then gets taken away by the second star.
1 / 5 (5) Apr 17, 2012
...just like atoms share electrons...
As above, so below...
1.7 / 5 (6) Apr 18, 2012
The more massive a star, the more likely it is to snag a planet drifting by.

I don't know who Hagai Perets is, but this is just bad science. The increased gravity of a larger star serves only to speed up the interaction, not increase the likelihood of capture.

I suppose in a crowded cluster a distant planet at aphelion might be pulled away by a passing star and shepherded into orbit by other mass/friction in the system, but, as cluster stars are moving relative to each other, an "ejected planet" from one star in the cluster is likely to have escape velocity for the whole cluster. And, it's certainly unlikely to encounter a star moving in just the exact trajectory to capture it, even temporarily (the relative velocities would have to be so low it could take forever (so to speak) for them to encounter each other).

How could any "scientist" even think the mass/weight of a star makes a difference? Maybe they think heavy items fall faster than lighter items in a vacuum too?
not rated yet Apr 18, 2012
> How would that happen if there isn't already another
> planet around the star?
> It is just not possible in a two-body system.

The article says "capture is more likely when stars and
free-floating planets are crowded together in a small
space" (paragraph 5) which implies more than just the
single star and planet interacting.
5 / 5 (4) Apr 18, 2012
...just like atoms share electrons...
As above, so below...

Electrons "orbit" around a nucleus in a probability cloud, not having definite paths or even locations. They "have a tendency to exist" within regions surrounding nuclei.
If you can find a star system where the planets wink in and out of existence and occupy probabilistic clouds, more power to you. Until then, though, don't infer too much from analogies other than a means to communicate quickly at the expense of accuracy.
not rated yet Apr 18, 2012
If the star and 'rogue' in question were in a nebula (i.e. highly conductive environment), you'd want to consider that differing charges would play a dominant factor in attraction there. That planet would try to stick to the magnetosphere of the star system in question like a sock sticking to your sweater in a clothes dryer.
not rated yet Apr 18, 2012
Aside from the capture issue discussed above, there was a story April 12 ( that reports on a study of exoplanet orbits which indicates that planetary systems are "predominantly aligned," i.e., in a disk, similar to our solar system. Obviously, captured planets wouldn't be "predominantly aligned" with native planets, so it would be interesting to know if the proportion of stars with non-aligned planets from the April 12 study correlates with the 3-6% capture rate of this study.
1.4 / 5 (11) Apr 18, 2012
Let's take a step back and check the reality here:

The nebular theory of planet formation predicts some simple facts:
1. Planets will orbit in the same direction as the star is revolving.
2. Planets will line up with the ecliptic.
3. Planets will be relatively close to their star otherwise it would take impossibly long to form.
4. The angular momentum of the system will be located largely within the star.
5. Hopefully, the planets will spin about an axis perpendicular to their orbits.

But: the actual observations named in the article all contradict these predictions:
1. Some planets are FAR, FAR from their stars.
2. Some planets can orbit in the opposite direction to the star's rotation.
3. SOme planets do NOT line up with the ecliptic.
4. Some planets roll along on their sides.
5. Some planets have retrograde rotation.

So to tie up all these kinds of anomalies, computer simulations are performed to show how it could happen naturalistically. Once that is done, it then becomes truth.
1 / 5 (3) Apr 19, 2012
But: the actual observations named in the article all contradict these predictions:
You need to read the article more carefully.

From the article:

Astronomers haven't detected any clear-cut cases of captured planets yet.
1 / 5 (4) Apr 19, 2012
kevinrtrs is again showing obvious bias towards determinism when the reality is actually significantly probabilistic
So to tie up all these kinds of anomalies, computer simulations are performed to show how it could happen naturalistically. Once that is done, it then becomes truth.

Beg pardon " then becomes truth" er no - thats one hell of a leap of faith !

Planet and star formation is mostly chaotic with periods and locations throughout the formation disc and beyond which temporarily appear subject to non recursive math(s) (ie non chaotic) but, overall have tremendous opportunity for collisions (chaotic) as our own solar system dynamics have also shown.

Eg: Recall comets plunging into Jupiter, had they instead hit the moons the outcome would have been astounding and the spin of other planets in our solar system - even our own Earth and Mars show tail end evidence of large collisions Eg Mars is misshaped and our Moon has anomalies in relation to the Earth's geology.
not rated yet Apr 22, 2012
Two large super-planets orbiting one another sounds like a low mass binary to me; not a capture. Since most star systems are binary, the "low mass" explanation seems more likely.

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