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Wanderers Between the Galaxies

The idea of planets outside their normal settings is unsettling. It implies that beyond the stars all around us there may be worlds without suns, dark planets presumably pushed there by gravitational instabilities in their home systems. We’ve looked at such ‘nomad’ worlds before, noting that ice overlaying a frozen ocean might trap enough geothermal heat to create life-sustaining conditions. Estimates on how many such planets might exist vary widely, but in one recent paper Louis Strigari (Stanford University) has calculated that 105 objects larger than Pluto may exist for every main sequence star (see Island-Hopping to the Stars for more on Strigari’s work on free-floating planets). Survey missions like Gaia may help us find some of these.

If a wandering world between the stars is a chilling prospect, what about stars that wander between the galaxies? We have plenty of evidence for their existence. Strip away the emissions from our own Solar System and the Milky Way itself and what you get is the Cosmic InfraRed Background (CIRB), with near-infrared and optical wavelength starlight being redshifted into the infrared, while some of the light is absorbed by dust and re-emitted in the far-infrared. Edward Wright (UCLA) offers up a concise take on the CIRB with photos and a useful diagram.

Early satellites working in the infrared like the Infrared Astronomy Satellite (IRAS) began the modern era of CIRB studies, now being pursued by instruments like the Spitzer Space Telescope. Nature has just published new work from Asantha Cooray (UC-Irvine) and team that uses data from Spitzer to study the infrared background glow, a phenomenon that Cooray says has remained mysterious despite recent studies. Not that it hasn’t spawned theories: Alexander Kashlinsky (NASA GSFC) has argued that the blotchy patterns the infrared glow makes on Spitzer images is coming from the first stars and galaxies. Cooray and company disagree.

Homing in on a section of sky covering an arc equivalent to 50 full Moons — the Boötes field — in a survey that admittedly lacked the sensitivity of Kashlinsky’s data, the Cooray team used their larger-scale study to conclude that the infrared glow is too bright to come from the first galaxies. Instead, they see the light as coming from stars beyond the halos of galaxies, some of them in the spaces between entire galactic clusters. Says Cooray: “A light bulb went off when reading some research papers predicting the existence of diffuse stars. They could explain what we are seeing with Spitzer.”

Image: New research from scientists using NASA’s Spitzer Space Telescope suggests that a mysterious infrared glow across our whole sky is coming from stray stars torn from galaxies. When galaxies grow, they merge and become gravitationally tangled in a violent process that results in streams of stars being ripped away from the galaxies. Such streams, called tidal tails, can be seen in this artist’s concept. Scientists say that Spitzer is picking up the collective glow of stars such as these, which linger in the spaces between galaxies. This artwork is adapted, in part, from galaxy images obtained from the NASA/ESA Hubble Space Telescope. Credit: NASA/JPL-Caltech.

The James Webb Space Telescope should help us take this work to the next level, revealing the objects that make up the background infrared light. If Cooray’s team is right, the presence of these lone stars could be accounted for by galactic collisions whose gravitational interactions would propel such stars into the deep. This scenario — and the more gradual absorption of dwarf galaxies by larger neighbors — may thus explain our intergalactic wanderers. Could such stars have intact planetary systems despite the forces applied to them in these interactions?

On such questions hangs not only interesting science but wildly speculative venues for science fiction. The paper is Cooray et al., “Near-infrared background anisotropies from diffuse intrahalo light of galaxies,” published in Nature 494 (2012), pp. 514-516 (abstract / preprint).


Comments on this entry are closed.

  • Tom Mazanec October 25, 2012, 10:37

    Our sun may one day be such a nomad star. In several billion years, M31 and the MIlky Way will collide. There is, I understand, a small but significant chance that the sun will be swept up in a tidal tail in that collision.

  • JoeP October 25, 2012, 12:55

    With all the recent articles on this fine blog regarding the difficulties of traveling to our nearest neighbor star system, a “mere” 4LY away, it is sobering to imagine how isolated we would feel as part of one of these possible intergalactic solar systems.

    It makes our problems a bit easier to tolerate! Perspective :)

  • kzb October 25, 2012, 13:52

    I found it difficult to understand. In particular, what fraction of the stars in the universe are they saying are between galaxies?

  • FrankH October 25, 2012, 15:21

    Ned Wright is one of the smartest people I’ve met, and he has an excellent page on Cosmology: http://www.astro.ucla.edu/~wright/intro.html

    I think a swarm of unaccounted for stars, neutrinos and other known particles are a better explanation for the missing mass in the Universe than the mystical, magical Dark Matter. Throw in a form of gravity modifications (MOND, TeVeS or similar) for low accelerations and the missing mass problem would probably be solved. If it works without MOND I’m fine with that too – I just think we’re better off without needing to conjure magical unicorns like dark matter to explain away observations.

    For a SF treatment of the idea of wandering worlds and stars, Iain M. Bank’s “Against a Dark Background” is set in a planetary system 1M LY from the nearest star.

  • Kappy October 25, 2012, 15:31

    Let’s say hypothetically that these ejected stars held onto accretion disks or planetary bodies. If a habitable planet orbited one of these stars, what would they see in the night sky? Would it be mostly empty except for a few points of light as distant galaxies. Would they have vast areas of darkness with other areas where nearby galaxies were resolvable by the naked eye (if they had eyes similar to ours). Would intelligent creatures ever look to space or would they believe they were alone?


  • ljk October 25, 2012, 15:47

    Here is an article on this subject from 2009, where they conservatively estimate that 0.05 percent of stars are what they call “tramps”:


    Here is one possible view of how the sky might look for an intergalactic planet:


  • Kenneth Harmon October 25, 2012, 17:12

    Up until recently I thought that it was widely believed by Scientists that entire Intergalactic Solar Systems were virtually impossible. There might be lone planers or stars wandering in Intergalactic Space, but not entire fully formed Solar Systems. Now it seems as if this is being rethought as some of those Tidal Tails may consist of fully formed Solar Systems and perhaps even Binary Stars with Multiple Planets. How likely does this appear to be now and is there any definitve work in this area since I thought that the Physics simply did not support it? However, if it is true it would make for a great story.

  • Malcolm RF October 25, 2012, 17:17

    Iain M Banks (Spoiler Alert! but probably already too late) wrote a neat novel, “Against a Dark Background”, about a civilization existing in a planetary system so distant from any galaxy that they appeared only as faint nebulae.This isn’t evident until the end of the book but it works well as a metaphor for our own isolated existence on a single planet.

  • Phil October 25, 2012, 18:29

    JoeP read my mind. That would be true isolation that would surely deeply affect the mindset of any technological civilisation that came to understand that reality.

    No stars in the night sky, no constellations, maybe if youre lucky a dim, dim distant spiral glowing softly like some kind of beautiful, but cruel consolation prize.

    Closer to home, Im still holding out for discoveries closer than A Cen further down the Y spectral ladder in my lifetime.

  • James Jason Wentworth October 26, 2012, 0:01

    If the inhabitants of any planets around such intergalactic stars are even remotely like humans, they may have (or may still) worship their suns as deities.

  • coolstar October 26, 2012, 0:35

    I didn’t know about the Iain Banks story, but I’m pretty indifferent to his work anyway. Such planets almost certainly exist, by the way. How about the opposite problem; planets around stars in globular clusters. we now know there will be comparatively few because of the low metallicity but imagine the sky! There’s at least one sf story based on this idea, but I can’t for the life of me remember it….It may have been by Poul Anderson?
    For a realistic idea of the night sky there, given by professionals in the field, check out http://www.iac.es/gabinete/iacnoticias/winter98/xplaneta.htm.

  • NS October 26, 2012, 4:04

    Such a civilization could study distant galaxies in the same ways we do, although without nearby stars to observe (besides their own sun) it might take them a lot longer to make sense of what they see. With other stellar systems out of reach in any foreseeable future their space programs would presumably be entirely focused on exploring their own system. If it was similar to ours they’d still have plenty to do.

  • andy October 26, 2012, 7:31

    Yes this did remind me of Against a Dark Background.

    As for that possible view from an intergalactic planet, the background galaxies are FAR too bright.

  • ljk October 26, 2012, 15:13

    Kenneth Harmon October 25, 2012 at 17:12:

    “Up until recently I thought that it was widely believed by Scientists that entire Intergalactic Solar Systems were virtually impossible. There might be lone planers or stars wandering in Intergalactic Space, but not entire fully formed Solar Systems.”

    Keep in mind that until recently, most astronomers thought that multiple star systems would be unable to retain any planets. And before that, they did not imagine that giant exoworlds could exist so close to their suns, circling them completely in mere days.

    Then we go back to not accepting that other galaxies could exist besides or beyond the Milky Way until the 1920s, and onward into the past where the idea of Earth circling the Sun and not being the Center of Everything was just crazy, to say nothing of heretical.

    Next up: Alien life and intergalactic solar systems.

  • ljk October 26, 2012, 15:15

    25 October 2012

    ** Contact information appears below. **

    Text & Images:



    Yale astronomers have caught the Milky Way having a snack.

    Using the Sloan Digital Sky Survey, researchers have discovered a band, or stream, of stars believed to be the remnant of an ancient star cluster slowly being ingested by the Milky Way, Earth’s home galaxy.

    “The Milky Way is constantly gobbling up small galaxies and star clusters,” said Ana Bonaca, a Yale graduate student and lead author of a study forthcoming in Astrophysical Journal Letters. “The more powerful gravity of our Milky Way pulls these objects apart and their stars then become part of the Milky Way itself.”

    Researchers have previously found evidence of the Milky Way eating up dwarf galaxies. Bonaca argues that the newly found stellar stream is the remnant of a star cluster rather than of a larger galaxy, because the stream is very narrow.

    “Our discovery is more of a light snack than a big meal for the Milky Way,” says Marla Geha, associate professor of astronomy at Yale and a co-author of the study. “Studying this digestion process in detail is important because it gives us new insight into how all galaxies form and evolve.”

    The new band of stars, or stellar stream, it is the first of its kind found in the southern galactic sky, a region that has been hard to examine due to a relative lack of deep-sky imaging there. Deeper imaging enables astronomers to detect fainter stars.

    Named the Triangulum stream, the newly discovered stellar stream could also help astronomers reconstruct how the Milky Way’s mass is distributed, further revealing its dynamic structure.

    Galaxies are believed to form hierarchically through the merger of smaller galaxies and still smaller star clusters. Stellar streams form as they are ripped apart by the gravitational force of galaxies. This process may be the primary way galaxies such as the Milky Way grow in mass, the researchers say.

    Triangulum was found by searching a region recently surveyed by the Sloan Digital Sky Survey III (SDSS-III), an international collaboration that is mapping the sky through wide-field photometry.

    Bonaca, Geha and co-author Nitya Kallivayalil, a Yale postdoctoral fellow, relied specifically on the survey’s Data Release 8, which included information about vast new areas of the southern galactic sky.


    Eric Gershon
    +1 203-415-3108

    The study is available now on the arXiv preprint server at:


    Research support was provided by the National Science Foundation and the Alfred P. Sloan Foundation.

  • GaryChurch October 26, 2012, 15:58

    I found the discovery of planets accelerated to millions of miles an hour by different mechanisms to be amazing. I wonder just how fast we could get a hollow sphere several miles in diameter (a Bernal Sphere) going by gravity assists in our solar system? Just using some h-bombs for every assist, what kind of velocity could be built up for a star voyage?
    Of course, slowing down once you get there is a problem- probably what most of the H-bombs carried on board would be used for.

  • stephen October 26, 2012, 17:13

    If .05 percent of stars are between galaxies, 200 million are from our galaxy alone.
    2e19 intergalactic stars in the universe.

    Taking another approach…taking the galaxy as a cylinder, using the formula for the surface area, the area exposed to intergalactic space is something like 30 billion square lightyears. (That’s very approximate.)

    Some of the stars might be sent out into intergalactic space *on purpose.*

  • Kappy October 26, 2012, 17:17

    @NS :

    Would they have a space program? Would they have invented the telescope? I guess they probably would have invented the telescope if they had other planets in their system or were interested in their sun at some point. It would follow then that they would probably want to send probes or visit said planets.


  • ljk October 26, 2012, 21:40

    Perhaps it would take an entire solar system to support an ETI society long enough to make it from one galaxy to another?

  • Rob Henry October 27, 2012, 19:55

    I am a little confused here. There is no way to prevent stars from being steadily ejected from every galactic core (except if we postulate some sort of intelligent control of stellar motions). I suspect that is where ljk gets his 0.05% figure, and stars expelled this way would tend to be stripped of their (outer) planets and gain very high relative motions.

    Thus I think that the above article must refer to the huge number of stars that would have to be expelled if two equally sized galaxies coalesce. In this second case the new galactic entity would start with a very high internal kinetic energy in proportion to its gravitational binding energy, and so, there is no need to do a detailed analysis. Such a case dictates that a very high proportion of stars in this new merger must be rapidly expelled, and under conditions that are much milder than in the former cases. If such mergers were rampantly common, we would have a completely different picture with, perhaps,10% of stars being intergalactic, with most having retained the majority of their original planets.

  • jkittle October 28, 2012, 23:04

    Given reasonable speed for these rogue star systems, what is the chance they would dive into another galaxy before the star ( assuming it is a yellow sun like ours) swells to a red giant? and one they get there, might they be captured. Is the universe old enough for this process to have seeded life from one galaxy to the next? this might be important to dispersal of life it the initiation of life is rare but its dispersal around and among star systems in a normal galaxy is common.

  • James Jason Wentworth October 29, 2012, 1:23

    Might some of the small galaxies (especially irregular galaxies) be groups of stars that were ejected from larger galaxies for various reasons and, having similar velocities like stars in clusters, eventually “formed their own new herds,” so to speak?

  • kzb October 29, 2012, 8:29

    ljk and Rob Henry: it may be the case that 0.05% was the estimate in 2009, but my understanding is that the work being discussed could raise this estimate substantially. But I find the language of the paper ambiguous.

    Also, these press releases like to “big up” galactic cannabalism and mergers. But I think the current estimate of MW stars from “cannabilism” is no more than 4%. Plus there are numerous papers arguing that the MW has not undergone a major event in many billions of years.