Lone Planets “More Common Than Stars”

by Jason Major on May 18, 2011

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Artist's concept of a free-floating Jupiter-sized orphan planet. NASA / JPL-Caltech

We happen to live in a solar system where everything seems to be tucked neatly in place. Sun, planets, moons, asteroids, comets… all turning and traveling through space in relatively neat and orderly fashions. But that may not always be the case; sometimes planets can get kicked out of their solar systems entirely, banished to roam interstellar space without a sun of their own. And these “orphan planets” may be much more numerous than once thought.

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.

“Although free-floating planets have been predicted, they finally have been detected, holding major implications for planetary formation and evolution models.”

– Mario Perez, exoplanet program scientist at NASA Headquarters in Washington.

From the NASA release:

Previous observations spotted a handful of free-floating, planet-like objects within star-forming clusters, with masses three times that of Jupiter. But scientists suspect the gaseous bodies form more like stars than planets. These small, dim orbs, called brown dwarfs, grow from collapsing balls of gas and dust, but lack the mass to ignite their nuclear fuel and shine with starlight. It is thought the smallest brown dwarfs are approximately the size of large planets.

On the other hand, it is likely that some planets are ejected from their early, turbulent solar systems, due to close gravitational encounters with other planets or stars. Without a star to circle, these planets would move through the galaxy as our sun and other stars do, in stable orbits around the galaxy’s center. The discovery of 10 free-floating Jupiters supports the ejection scenario, though it’s possible both mechanisms are at play.

“If free-floating planets formed like stars, then we would have expected to see only one or two of them in our survey instead of 10. Our results suggest that planetary systems often become unstable, with planets being kicked out from their places of birth.”

– David Bennett, a NASA and National Science Foundation-funded co-author of the study from the University of Notre Dame.

The study wasn’t able to resolve planets smaller than Saturn but it’s believed there are likely many more smaller, Earth-sized worlds than large Jupiter-sized ones.

Read the full NASA news release here.

The study, led by Takahiro Sumi from Osaka University in Japan, appears in the May 19 issue of the journal Nature.

About 

A graphic designer in Rhode Island, Jason writes about space exploration on his blog Lights In The Dark, Discovery News, and, of course, here on Universe Today. Ad astra!

David Jones May 18, 2011 at 7:52 PM

what about planets surrounding a star that goes supernova? do they get obliterated or ejected? if they get ejected, could be another source of wandering planets

Anonymous May 18, 2011 at 8:44 PM

I would guess they’d be obliterated. Maybe if there is significant mass loss from the star before it explodes the planet could migrate out to bigger and bigger orbits and eventually detach altogether.

Anonymous May 19, 2011 at 11:37 AM

This may be of interest:

http://www.universetoday.com/85167/what-triggers-a-type-ia-supernova-chandra-finds-new-evidence/

That discusses a second star in a binary system which would have a better chance of surviving of course but a gas giant may well survive. Supernovae are often asymmetric and the exploding star may be ejected out of the system leaving the planets behind to wander off on their own.

Anonymous May 19, 2011 at 2:45 PM

My earlier comment goes along with this; that there is undoubtedly much more “ordinary” mass that we don’t see “yet” that contributes to the CDM thereby lessening the calculated non-baryonic contribution.

Anonymous May 19, 2011 at 11:39 PM

There is certainly more but I believe the photon to baryon ratio can be determined fairly well from the ratio of primordial elements so we already have a reasonable idea of the total.

Anonymous May 18, 2011 at 8:22 PM

Unfortunately 2012 hoaxers are already running with this as proof that the end is near.

J. Major May 18, 2011 at 11:47 PM

It doesn’t take much, does it?

Torbjörn Larsson May 18, 2011 at 8:37 PM

Wow. Many of these loners may have biospheres too, ice covered oceans running on crustal heat.

Link FAIL: NASA release link returns address “//www.universetoday.com/85781/lone-planets-more-common-than-stars/The%20study,%20led%20by%20Takahiro%20Sumi%20from%20Osaka%20University%20in%20Japan,%20appears%20in%20the%20May%2019%20issue%20of%20the%20journal%20Nature.” *

& text: “You 404’d it. Gnarly, dude.”

To paraphrase:

‘Twas brillig, and the linky toves
Did gyre and gimble in the wabe;
All gnarly were the borogoves,
And the mome raths outgrabe.

* Ah, found the NASA news release

J. Major May 18, 2011 at 10:13 PM

Fixed the release link. Thanks for the alert.

Anonymous May 18, 2011 at 9:16 PM

What would the speed of those objects be relative to the Sun?
Can we determine a maximum speed?

Most stars including the sun probably moves in the same general direction around the milky way, so estimate that these objects would have a speed a bit higher than escape velocity of their previous star.

Martin Henderson May 18, 2011 at 11:24 PM

The escape velocity would have to be achieved, but many situations can cause this to be achieved. For example a small rocky world careens around a massive jupiter in a near miss, being flung towards the parent star, wrapping around in a near miss, giving it enough momentum to produce a highly elliptical orbit, which that massive jupiter can knock it out of pulling it from the sun. It’s speed could also be lower then the parent star, if a large planet on the outskirts of a planetary disk tugs/speeds up a smaller planet in its wake, flinging it out of the solar system. There are also a few scenarios where the planet can be flung in the “wrong direction”, such as orbiting 180º around a planet before escaping. Orbital movements are stable regardless of direction.

About maximum speed, theoretically there is no limit. The fastest known star moves at 715,214 m/s (0.002c) (compare this to the fastest spacecraft speed to date of 4,000 m/s). Any gravational interaction can accelerate an object in orders of magnitude above terminal velocity. A star can accelerate an object to a maximum of the orbital speed at the tightest possible orbit (remembering that the closer an object is, the faster it needs to go to orbit, or inverted the closer an object is, the faster it will be accelerated). However a tight binary pair can rotate incredibly fast, to a limit of when the sheer speed destroys either of them. Given the theoretical existence of binary black holes, there is no rotation limit.

Potatoswatter May 19, 2011 at 4:02 AM

Voyager 2 is leaving our system at 15.5 km/s. Of course spacecraft deeper in gravity wells go faster, but that’s apples and oranges. I don’t know how much more it will slow down, but pretty sure the ultimate speed is more than 4 km/s.

See also http://www.universetoday.com/84307/fastest-spacecraft/ .

Albert May 19, 2011 at 8:20 AM

Star S2 ( http://en.wikipedia.org/wiki/S2_%28star%29 ) can reach speed of 5.000 km/s – (source: tv show Through the Wormhole with Morgan Freeman).

Anonymous May 20, 2011 at 8:39 PM

I assume that the fastest know star is not closebye and nor moving in our direction?

Anonymous May 18, 2011 at 9:16 PM

What would the speed of those objects be relative to the Sun?
Can we determine a maximum speed?

Most stars including the sun probably moves in the same general direction around the milky way, so estimate that these objects would have a speed a bit higher than escape velocity of their previous star.

Anonymous May 18, 2011 at 10:02 PM

Any thoughts as to what they contribute to the missing galactic mass (CDM)?

Christoph May 18, 2011 at 10:24 PM

Not enough of them (If they don’t find MANY more, that is)

Potatoswatter May 18, 2011 at 10:49 PM

It would seem that they measured the density along the line of sight to the galactic center, and then extrapolated to the rest of the galaxy. So, the extrapolation process means this prediction says little about the galactic halo where the dark matter is.

I’m not an astronomer, but I gather that planets are disqualified for other reasons, such as insufficient time over the universe’s lifespan to form that much heavy matter.

Christoph May 18, 2011 at 11:08 PM

Well, that points in the same direction: not enough of them.
But now I’ve got a question: How much metall do you need to form a planet?

Potatoswatter May 19, 2011 at 12:48 AM

I have no idea how much metal. But CDM is also an explanation of spiral galaxy structure, and if memory serves the other kind of gravitational lensing has revealed that spiral structure predated the existence of much metal at all.

Also, my reply was intended for wjwbudro, I don’t recall seeing your comment before at all…

Torbjörn Larsson May 19, 2011 at 12:12 PM

Good point, in LCDM the DM mass should have been fairly constant over time as I understand it. Planets make bad candidates for missing mass.

Martin Henderson May 18, 2011 at 11:30 PM

Even if there was many more than expected, this would be a tiny drop in the bucket of galactic mass. Remember that a star is huge, 99% of the matter in our solar system is in Sol. Assuming this is not atypical, this means that for all the visible matter in stars, only a tiny fraction can be planets. Currently, it is held that there is more “missing” matter then visible.

Potatoswatter May 18, 2011 at 10:54 PM

If we know the local density of rogue planets, and assume their velocities are similar to nearby stars, can we calculate how often a planet in our system is randomly perturbed and by how much?

Justin Hartberger May 19, 2011 at 1:20 PM

If you are meaning how often a rogue planet off alone orbiting the galaxy effects planets in our solar system – almost assuredly never. We’d be far more likely to effected by a nearby star. Think of it this way…for us to notice the effect of gravity from any passing object, it has to get close enough to overpower the gravity from the Sun to at least some degree. The closer to the Sun an object is, the more massive/closer that object would have to be. The areas of the solar system where the known planets reside is but a fraction of the Sun’s gravitationally dominated space. (the Oort Cloud is hypothesized to extend out to 50,000 AU, which is almost a light year).

Take a good-sized trampoline, put someone who weighs 400+ lbs in the middle, and then drop a marble on the edge somewhere. Observe how much of a dent the marble makes. You can even use more massive objects, but keep in mind how close they are getting to the weight of your ‘star’. Now imagine that trampoline was the size of a small town and just think about how big and how close one of those little marbles would have to get to even be noticeable. Not trying to be patronizing, but essentially the likelihood of having to worry about something like that is so negligible (even astronomically speaking) that it might as well just be listed as Zero.

Anonymous May 19, 2011 at 6:33 AM

Hey this almost comes back to the comment I made a couple of weeks ago in regards to the planet that was found roating counter to the orbital direction of the other planets in that system. My comment at the time was “Could this planet originally be from outside the system it now inhabits and have been captured by the stars gravity and enter its orbit, albeit in the wrong direction?”.
I can’t remember who answered me but the answer was that the probability was minute but I would have to assume that at least it was possible……..well at least after reading this I’d have to say that. Certainly not impossible.

Torbjörn Larsson May 19, 2011 at 12:17 PM

Not impossible no, and it would up the frequency of capture attempts. But it wouldn’t tip the balance that a retrograde planet most likely is a disturbed original planet, since those are at hand (naively more attempts of disturbance).

Justin Hartberger May 19, 2011 at 1:26 PM

It would be similar to my reply above to Potato. The unlikeliness by comparison to it being a disturbed native planet’s orbit is not even close. Taking that bet would be similar to a footrace between an olympic runner and average joe with a broken leg…sure Joe could win…technically…

Anonymous May 19, 2011 at 5:40 PM

Depends on what you mean with capture. You can’t capture a rough object without some mechanism that reduces its speed below escape velocity of the big star/planet that wants to capture it. In order to slow down, something else must speed up.

Anonymous May 19, 2011 at 1:09 PM

The Bullet Cluster illustrates the nature of /CDM, where the luminous ordinary matter in the galaxy collision interacts by various forces to stick together, while the DM from the two galaxies continues on their original trajectories. Ordinary matter of most any kind would not behave this way.

These lone wolf planets are a very small percentage of a galactic mass. The article here indicates the detection of 10 of these planets by microlensing in a stellar formation region. This does not suggest an overwhelming amount of matter is locked up in these planets.

These planets are most likely very cold jovian planets. Interstellar space is about 30deg-C and that is really pretty cold. I doubt these planets have anything we would call life. There were speculations about life in the atmosphere of Jupiter, floating balloon-like organisms and fliers and so forth. Yet I suspect these ideas amount to wishful speculation. In fact I think these ideas about life in the sub-ice layer regions of Europa and Ganymede might also amount to much the same.

LC

Justin Hartberger May 19, 2011 at 1:39 PM

I agree completely on the life statements. I’d place my bets on Titan/Mars/Ceres for some sort of life long before anything else in the solar system. Dawn needs to hurry up with the dead rock with a flat bottom and move on to our little dwarf H2O buddy.

Also the whole subsurface oceans thing…it would require an effort like what they’ve been doing at Lake Vostok and I don’t see that being all that feasible for a space probe project currently.

Anonymous May 19, 2011 at 4:54 PM

What better place to probe than a methane(?) lake on Titan and the elimination/reduction of the complexities of a “hard” surface landing and the land mobility mechanics make it all the more appealing.

Anonymous May 19, 2011 at 1:15 PM

I had a strong suspicion that there are more rogue planets than stars. In stars, the lower the mass, the more common the star is. Going by this, I figure there are more Brown dwarfs then M-stars, and more Jovian-type orphan/rogue planets than brown dwarfs. I suppose there might be even more smaller planets, including rocky or icy worlds. No doubt the number of comet and asteroid-like objects is even greater.

It’s crowded out there.

The question remains – how many formed in solar systems and ejected, and how many formed independently through gas collapse, not even reaching brown dwarf scale.

Also, how will these be defined (orphaned planets, rogue planets, planemos, etc.)

Anonymous May 19, 2011 at 4:41 PM

I posted this earlier and you reaffirmed my thinking;
that there is undoubtedly much more “ordinary” mass that we don’t see “yet” that contribute to the CDM thereby lessening the calculated non-baryonic contribution.

Michael Dwyer May 20, 2011 at 8:18 AM

Even if there’s 10 times more free floating planets than stars they would still make up a small fraction of a percent of the galaxy’s mass. The difference would fall well within the error bars for our current estimates of our galaxy’s mass.

Michael Dwyer May 20, 2011 at 8:18 AM

Even if there’s 10 times more free floating planets than stars they would still make up a small fraction of a percent of the galaxy’s mass. The difference would fall well within the error bars for our current estimates of our galaxy’s mass.

Torbjörn Larsson May 20, 2011 at 4:01 PM

Even if there’s 10 times more free floating comments than articles they would still make up a small fraction of a percent of the site’s text mass. The difference would fall well within the error bars for our current estimates of our site’s mass. =D

[Not really though.]

Apparently Disqus doesn’t detect duplicates.]

Uncle_Fred May 22, 2011 at 1:44 AM

Disqus might not, but I do.
Comment removed.

Matthew May 19, 2011 at 2:37 PM

Hm… gives some food for thought about the concepts of “Planet X” and “Nemesis” Theories that have been floated around by scientists and other folks.

J. Major May 20, 2011 at 3:11 AM

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