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Rogue Planet Discoveries Challenge Formation Models

As we begin the New Year, I want to be sure to catch up with the recent announcement of a discovery regarding ‘rogue’ planets, those interesting worlds that orbit no central star but wander through interstellar space alone (or, conceivably, with moons). Conceivably ejected from their host stars through gravitational interactions (more on this in a moment), such planets become interstellar targets in their own right, as given the numbers now being suggested, there may be rogue planets near the Solar System.

Image: Rogue planets are elusive cosmic objects that have masses comparable to those of the planets in our Solar System but do not orbit a star, instead roaming freely on their own. Not many were known until now, but a team of astronomers, using data from several European Southern Observatory (ESO) telescopes and other facilities, have just discovered at least 70 new rogue planets in our galaxy. This is the largest group of rogue planets ever discovered, an important step towards understanding the origins and features of these mysterious galactic nomads. Credit: /ESO/COSMIC-DANCE Team/CFHT/Coelum/Gaia/DPAC.

A brief digression on the word ‘interstellar’ in this context. I consider any mission outside the heliosphere to be interstellar, in that it takes the spacecraft into the interstellar medium. Our two Voyagers are in interstellar space – hence NASA’s monicker Voyager Interstellar Mission – even if not designed for it. The Sun’s gravitational influence extends much further, as the Oort Cloud attests, but the heliosphere marks a useful boundary, one that contains the solar wind within. The great goal, a mission from one star to another, is obviously the ultimate interstellar leap.

How many rogue planets may be passing through our galactic neighborhood? Without a star to illuminate them, they can and have been searched for via microlensing signatures. But the new work, in the hands of Hervé Bouy (Laboratoire d’Astrophysique de Bordeaux), uses data not just from the Very Large Telescope but other instruments in Chile including the adjacent VISTA (Visible and Infrared Survey Telescope for Astronomy), the VLT Survey Telescope and the MPG/ESO 2.2-metre telescope.

Tens of thousands of wide-field images went into the survey. The target: A star-forming region close to the Sun in the constellations Scorpius and Ophiuchus. The work takes advantage of the fact that planets that are young enough continue to glow brightly in the infrared, allowing us to go beyond microlensing methods to find them. About 70 potential rogue planets turn up in this survey. Núria Miret-Roig (Laboratoire d’Astrophysique de Bordeaux), first author of the paper on this work, comments:

“We measured the tiny motions, the colours and luminosities of tens of millions of sources in a large area of the sky. These measurements allowed us to securely identify the faintest objects in this region, the rogue planets…There could be several billions of these free-floating giant planets roaming freely in the Milky Way without a host star.”

Image: The locations of 115 potential FFPs [free-floating planets] in the direction of the Upper Scorpius and Ophiuchus constellations, highlighted with red circles. The exact number of rogue planets found by the team is between 70 and 170, depending on the age assumed for the study region. This image was created assuming an intermediate age, resulting in a number of planet candidates in between the two extremes of the study. Credit: ESO/N. Risinger (skysurvey.org).

But we need to pause on the issue of stellar age. What these measurements lack is the ability to determine the mass of the discovered objects. Without that, we have problems distinguishing between brown dwarf stars – above 13 Jupiter masses – and planets. What Miret-Roig’s team did was to rely on the brightness of the objects in order to set upper limits on their numbers. Brightness varies with age, so that in older regions, brighter objects are likely above 13 Jupiter masses, while in younger ones, they are assumed to be below that value. The value is uncertain enough in the region in question to yield between 70 and 170 rogue planets.

Planet formation is likewise an interesting question here. I mentioned ejection from planetary systems above, but these free-floating planets are young enough to call that scenario into question. In fact, other mechanisms are discussed in the literature. The paper notes the possibility of core-collapse (a version of star formation), with the variant that a stellar embryo might be ejected from a star-forming nursery before building up sufficient mass to become a star. We need to build up our data on rogue planets to discover the relative contribution of each method to the population.

This work uncovers more rogue planets by a factor of seven than core-collapse models predict, making it likely that other methods are at work:

This excess of FFPs [free-floating planets] with respect to a log-normal mass distribution is in good agreement with the results reported in σ Orionis [a multiple system that is a member of an open cluster in Orion]. Interestingly, our observational mass function also has an excess of low-mass brown dwarfs and FFPs with respect to simulations including both core-collapse and disc fragmentation . This suggests that some of the FFPs in our sample could have formed via fast core-accretion in discs rather than disc fragmentation. We also note that the continuity of the shape of the mass function at the brown dwarf/planetary mass transition suggests a continuity in the formation mechanisms at work for these two classes of objects.

So the formation of rogue planets is considerably more complicated than I made it appear in my opening paragraph. The authors believe that ejection from planetary systems is roughly comparable to core-collapse as a planet formation model. That would imply that dynamical instabilities in exoplanet systems (at least, those containing gas giants) produce ejections frequently within the first 10 million years after formation. Investigating these extremely faint objects further will require the capabilities of future instrumentation like the Extremely Large Telescope.

The paper is Miret-Roig et al, “A rich population of free-floating planets in the Upper Scorpius young stellar association,” published online at Nature Astronomy 22 December 2021 (abstract).


{ 34 comments… add one }
  • James Franklin January 4, 2022, 11:55

    Free Floating Planets, a subject I raised as a 12 year old with the late Patrick Moore back in 1979, Patrick dismissed the idea in a long conversation and I thought no more about it for years, until reports of them started popping up in research papers, I smiled and wondered how Patrick would have addressed the subject if we discussed it again.

    Personally, I wonder if many of the FFTs arise from multiple star systems, where stars orbits are unstable or perhaps highly eccentric, giving planets time to form before big brother comes in and throws them out of the club. I suspect that many are likely ejected by near collisions with Gas Giant planets, I doubt that single encounters with anything significantly less that a Jovian mass world would impart sufficient energy to eject a planet from the gravitational grip of the parent star, more likely throw them onto eccentric orbits or into a collision with another planet – gravitationally or physically.

    When it comes for formation, I have wondered for many years if we are missing the basics of the formation of bodies in space – Perhaps core collapse occurs after electrostatic forces bind sufficient mass together in a basic accretion process, where sufficient mass can be drawn in, a star forms, where it cannot, we end up with something less than a star. It is possible stars start off as “planet” cores that just keep accreting matter, the core gets more and more crushed and when enough hydrogen in present, the star ignites. However, this is not to say I do not accept core collapse – I just think that nature will use whatever method is has to given the local circumstances and environmental conditions.

    It is also highly likely that many FFTs are planets stripped from the outer edges of planetary systems by passing stars, we know that stars can get very close to each other, and if the planet has previously been thrown into an highly eccentric orbit by local encounters, then perhaps it can be ejected from the system, perhaps pulled along by the passing star but then “dropped” as it pulls away.

    For all we know, Triton could be an example of a captured exoplanet!!

  • Mike Serfas January 4, 2022, 13:52

    The preprint is at https://arxiv.org/abs/2112.11999 . What strikes me first about these rogue planets is that they are very large, on the brink of brown dwarf classification; definitely Jupiter-class. I assume they must have moons … do they have Io-like moons trapped in orbital resonances with long term tidal heating?

    Our Io is not highly rated as a tourist destination, but this is in part because Jupiter’s magnetic field strips away its atmosphere and water. Its volcanic eruptions and lava flows are over 1000 K, but there don’t seem to be protected niches for life. Is it reasonably conceivable there might yet be a rogue gas giant presently (or soon) within 0.2 light-years of the Sun, with a moderately tidally heated moon outside the range of its magnetic field, a moon that has a thick warm atmosphere and liquid water and a biosphere powered by “anti-solar” autotrophs that radiate infrared to the dark sky? Give us something to keep an eye out for. :)

  • Geoffrey Hillend January 4, 2022, 17:08

    I have to question the idea that the rouge exoplanets have to all be ejected from a solar system. First there are so many of them. The mathematical probability favors their location to be in situ. It boils down to whether or not a gas giant can form without a solid core. It might be different for 13 Jupiter masses and above or brown dwarfs.

    For example. The planets in a solar system must form from a flat protoplanetary disk because the centrifugal force and gravity of the star prevents gas from moving towards or away from the star, so it only collapses vertically. Consequently, the planets have to form from a flat ring of gas, dust and pebbles. The gas from the central star collapses from every direction towards the center. Several brown dwarfs, or failed stars might form this way from a large gas cloud, and some brown dwarfs have planets orbiting them.

  • Geoffrey Hillend January 4, 2022, 17:52

    From what I recall reading, the electrostatic forces at first prevent the dust from binding together in the protoplanetary dust cloud. Once there is enough dust the gravity overcomes the electrostatic forces and the dust cloud collapses to form a central star. Recall gravity dominates on the large scales and the electromagnetic forces dominate the small scales only short range, but not the large scale because there are an equal number of positive and electric charge which cancel each other out over long distance with the electromagnetic force, so gravity is strong on the large scales. Hawking, 2001.

    The electromagnetic field of the proto star is strong enough to keep the dust from moving horizontally, which helps with the collapses of the dust towards the center from all angles.

  • Henry Cordova January 4, 2022, 21:08

    MACHO Madness…?

    The Stellar Mass Function is not completely understood, but we do know that as the masses of stars drop, their numbers increase explosively; explaining the prevalence of old red dwarfs in the galaxy. But there is no reason why the SMF cannot continue down to planetary sized bodies–or even smaller! And since they are cold, dead and unchanging, they may very well last forever.

    Add to this the planets dynamically ejected from solar systems, and those surviving after their primaries go supernova, there may be an enormous number of these tiny, cold objects flying around the galaxy. Because of their small size, the proportion of galactic mass due to these creatures may be tiny in spite of their numbers. But they must still be a significant component of the Milky Way.

    Fascinating, Captain. Fascinating.

    • kzb January 6, 2022, 18:52

      The core collapse formation mechanism only works down to about 1 Jupiter mass. Also, notice there are fewer brown dwarfs than proper stars. The IMF does not continue to increase with lower mass, that is the current picture anyway.
      This study tends to confirm what I am saying, because there are 20 stars for every planetary mass object found.
      PMOs less massive than Jupiter are ejected planets, and they are likely more common, but they are not part of the stellar IMF.

      • Henry Cordova January 7, 2022, 17:10

        You’re probably right, but that old astronomer’s bogeyman still rears its ugly head. Maybe the reason for these observations is more to do with selection effect. Brown dwarfs are harder to see than red dwarfs, and rogue planets are harder yet to find. And all are hard to spot if they don’t orbit a brighter companion.

        There are 53 known systems (66 stars) within a 5pc radius of Sol, true, a very small sample. But 9 of them are brown dwarfs, 48 of them are red dwarfs. There is still room for debate.

        Source: 2022 RCAS Handbook

        Beyond that, the data is sparse and incomplete.

  • kzb January 4, 2022, 21:50

    The article says the FFP ratio to (stars + brown dwarfs) is only 0.045. In other words there are roughly 20 stars/brown dwarfs per free-floating planet. So it seems statistically unlikely that there are FFPs closer to us than Alpha Centauri.
    This is in the mass range of 4-13 Jupiter masses however. It seems to me that small planets are more likely to be ejected from a stellar system than massive ones, so perhaps smaller FFPs may have a much larger space density. What do others think?

  • Afterthought January 5, 2022, 1:47

    Stars too small to be stars, not “rogue” planets.

    • Henry Cordova January 5, 2022, 9:50

      Perhaps there is a continuum, of objects being formed from stars, to brown dwarfs, to planets, to “rogues”, all the way down to planetoids and comets. We just tend to perceive this gradient as distinct classes of objects because our detection methods are tuned to detect different types

  • Michael Fidler January 5, 2022, 7:43

    I would think that the same processes that form comets in deep space would also create planets. Stellar explosions from large to small supernovas cause formation of multiple star masses from giants to M dwarfs so it would also cause planetary mass to form in the wake of their huge explosions. The large stellar clouds in the galaxy arms could have many ways that core collapse without a large mass nearby. The intense radiation from new born O,B and A stars could also trigger a large number of low mass objects. Just as the vortices form in streams passing rocks interstellar dust clouds stream past stars and stars fly thru them creating eddies. Magnetic flux tubes may be very common through out the galaxies structure and can cause the collapse of gas, dust and molecular clouds to form planets in the depths of space. JWST may bring forth a huge amount of information on such objects and the Nancy Roman Telescope should give us a better picture of their numbers and structure in the galaxy.

    • Michael Fidler January 5, 2022, 18:22

      An early transition to magnetic supercriticality in star formation.


      Coherent interstellar magnetic field detected.

      Therefore, the transition from magnetic subcriticality to supercriticality—i.e., when the field can and cannot support the cloud against gravity, respectively—occurs in the envelope instead of the core, in contrast with the conventional picture.

      How the interstellar magnetic field dissipates to enable cloud collapse remains an unsolved problem in star formation. The main proposed solution has long been ambipolar diffusion—the decoupling of neutral particles from plasma—in cloud cores.

      The coherence of the magnetic field revealed by the HINSA Zeeman effect means that dissipation of the field occurs during the formation of the molecular envelope, possibly through a different mechanism than ambipolar diffusion.


    • Michael Fidler January 7, 2022, 3:18

      Superconducting Comets?

      Not as far fetched as you think! What would be the the effect on comets with large amounts of meteorites with lead, indium and tin embedded in their structure. The deep cold of deep space creating the perfect environment for superconductivity…

      What These Superconducting Space Rocks Tell Us About the Galaxy.


      Superconductivity found in meteorites.


      “In this paper, we report the presence of superconducting material in two meteorites. We further characterize these phases as alloys of lead, tin, and indium. These findings could impact our understanding of several astronomical environments. Superconducting particles in cold environments could affect planetary formation, shape and origin of magnetic fields, dynamo effects, motion of charged particles, and other processes.”

      “On the Threshold of a Dream…”

    • Michael Fidler January 8, 2022, 23:42

      Earth in a tunnel? Decades-old mystery unfolds.



      “Earth in a tunnel would mean a tunnel-like structure in our sky. We could see it, these astronomers said, if our eyes were tuned to the “light” of radio waves. The Van-Gogh-like lines here show the orientation of the tunnel-like structure’s magnetic field. Notice the bright star in the center of the image. That star is Vega in the constellation Lyra the Harp, which marks the solar apex, or direction of our sun’s motion through the Milky Way galaxy. Image via Dominion Radio Astrophysical Observatory/ Villa Elisa telescope/ ESA/ Planck Collaboration/ Stellarium/ J. West./ University of Toronto.

      A Unified Model for the Fan Region and the North Polar Spur: A bundle of filaments in the Local Galaxy.

      “We present a simple, unified model that can explain two of the brightest, large-scale, diffuse, polarized radio features in the sky, the North Polar Spur (NPS) and the Fan Region, along with several other prominent loops. We suggest that they are long, magnetized, and parallel filamentary structures that surround the Local arm and/or Local Bubble, in which the Sun is embedded. We show this model is consistent with the large number of observational studies on these regions, and is able to resolve an apparent contradiction in the literature that suggests the high latitude portion of the NPS is nearby, while lower latitude portions are more distant. Understanding the contributions of this local emission is critical to developing a complete model of the Galactic magnetic field. These very nearby structures also provide context to help understand similar non-thermal, filamentary structures that are increasingly being observed with modern radio telescopes.”


  • Spaceman January 5, 2022, 11:44

    Has anyone heard about the following research suggesting that rogue planets ranging from Jovian to terrestrial mass are quite common:

    Probing Extragalactic Planets Using Quasar Microlensing by Dai et al (2018)

    “Previously, planets have been detected only in the Milky Way galaxy. Here, we show that quasar microlensing provides a means to probe extragalactic planets in the lens galaxy, by studying the microlensing properties of emission close to the event horizon of the supermassive black hole of the background quasar, using the current generation telescopes. We show that a population of unbound planets between stars with masses ranging from Moon to Jupiter masses is needed to explain the frequent Fe Kα line energy shifts observed in the gravitationally lensed quasar RXJ 1131–1231 at a lens redshift of z = 0.295 or 3.8 billion lt-yr away. We constrain the planet mass-fraction to be larger than 0.0001 of the halo mass, which is equivalent to 2000 objects ranging from Moon to Jupiter mass per main-sequence star.”

    And the follow-up paper:

    Confirmation of Planet-Mass Objects in Extragalactic Systems by Dai et al (2019)

    “Quasar microlensing serves as a unique probe of discrete objects within galaxies and galaxy clusters. Recent advancement of the technique shows that it can constrain planet-scale objects beyond our native galaxy by studying their induced microlensing signatures, the energy shift of emission lines originated in the vicinity of the black hole of high redshift background quasars. We employ this technique to exert effective constraints on the planet-mass object distribution within two additional lens systems, Q J0158−4325 (zl=0.317) and SDSS J1004+4112 (zl=0.68) using Chandra observations of the two gravitationally-lensed quasars. The observed variations of the emission line peak energy can be explained as microlensing of the FeKα emission region induced by planet-mass microlenses. To corroborate this, we perform microlensing simulations to determine the probability of a caustic transiting the source region and compare this with the observed line shift rates. Our analysis yields constraints on the sub-stellar population, with masses ranging from Moon (10−8M⊙) to Jupiter (10−3M⊙) sized bodies, within these galaxy or cluster scale structures, with total mass fractions of ∼3×10−4 and ∼1×10−4 with respect to halo mass for Q J0158−4325 and SDSS J1004+4112, respectively. Our analysis suggests that unbound planet-mass objects are universal in galaxies, and we surmise the objects to be either free-floating planets or primordial black holes. We present the first-ever constraints on the sub-stellar mass distribution in the intra-cluster light of a galaxy cluster. Our analysis yields the most stringent limit for primordial black holes at the mass range.”

    • kzb January 6, 2022, 19:02

      The trouble is, there is a mismatch between what is found for external galaxies and our own galaxy. Few FFPs are found in our own galaxy by microlensing; there are no more FFPs than stars. Yet in external galaxies they find thousands per star. IMHO, this is because the bodies are actually AU-sized gas clouds and not solid planets.

  • kzb January 5, 2022, 12:10

    Afterthought: no, these objects of 4-13 Jupiter masses are classed as planets (the better term is Planetary Mass Object (PMO), because surely the word ‘planet’ implies it is in orbit around a star). Between 13 and 80 Jupiter masses they are classed as brown dwarfs.
    Obviously the objects under discussion are massive. Four times the mass of Jupiter as a minimum. It seems to me there must be a lot more lower-mass free-floating planets than are counted in this study.

  • Dave Moore January 5, 2022, 16:55

    Slightly tangental to this, I suggest that a study of hot Jupiters by the James Webb using infernometric nulling or central star occultation looking for outer planets in their systems.

    There are two theories of how hot Jupiters get were they are: The first is that they form further out in the planetary nebula and then spiral in to their present position; the second is that the are the result of planet-planet scattering with initially have highly elliptical orbits, but as time goes on, they are tidally circularized.

    If spiraling in is the main mechanism, then the outer planets of the system should have neat, circular orbits. If planet scattering is the main mechanism, then in not all cases will the other planet be ejected to be a rouge planet. In some cases, it will will finish up in an elliptical orbit that sometimes will go out quite far from the star, making it easy to spot. The outer system in this case should look disrupted with planets in elliptical orbits.

    This would clear up a point in our understanding of planetary formation.

  • Geoffrey Hillend January 5, 2022, 16:58

    I recall reading magnetic fields help remove the angular momentum from the proto stellar cloud during collapse so their are important for initiating star formation. I don’t know if that results in making it hard for the dust to move in the magnetic field of a star. Brown dwarves do have strong, magnetic fields.

  • Neil Stahl January 6, 2022, 11:53

    In my reading inner (at least) planet formation stops when the star ignites and blows the necessary materials away. Far from star-forming regions it would seem the planet formation would be way slower but could continue way, way longer. Maybe rogue planets are still being formed in some regions far, far away.
    Let us hope no rogue Jupiters wander into the Solar System. We are pretty settled and don’t need any new bullies stirring things up. We also don’t need our Oort cloud being stirred up but occasionally it probably is. This is an interesting and possibly more than theoretically important topic.
    Anyone care to write a story about traveling the galaxy on rogue planets which generate enough of their own heat to support life? And have enough of a variety of metals to support a civilization. And, of course, enough water.

    • Alex Tolley January 6, 2022, 13:57

      If rogues were common and wandered into the solar system at all, surely they would have disrupted our system over the last 4 bn years? Is there any indication that this has been the case based on the current distribution of planets and moons? If not, then these rogues should be rare and we can probably count them out as stepping stones to the nearer stars, however attractive “island-hopping” is for slow interstellar travel. We really don’t need a “Zyra” dropping by with any sort of frequency unless actively controlled by ETI technology for starfaring or galactic-directed panspermia. ;)

      • Michael Fidler January 7, 2022, 3:39

        Alex, you sound like the astronomer that said it was impossible to reach the moon. Neptune has a large moon orbiting backwards, Uranus is lopsided and may indicate a huge impactor, Saturn rings and an inflated atmosphere, Jupiter’s giant red spot. Venus being completely remelted 800 million years ago, the earth with 3/4th of its surface being regurgitated over and over again. No, nothing to see here!
        Catastrophism reins on this planet as I can attests to in the Typhoon Odette last month. The solar system and the galaxy are the same, we just have not been able to see in close enough detail to know better. As I have said before wait till we pass thru the next spiral arm of our galaxy…

        You’re going to need more then a hard hat!

        • Alex Tolley January 7, 2022, 9:33

          (…) the earth with 3/4th of its surface being regurgitated over and over again.

          Are you really trying to suggest that this was/is caused by an interstellar rogue planet? That is Velikovskyian in its wrongness.

          This really smacks of the same logic that IDers use to try to debunk evolution – “You mean you don’t have a complete explanation for feature X? The Bible explains it all.”

          We really don’t need rogue planets from interstellar space wandering into our system to explain every mystery. A planet formed in our own system may be just as likely to explain the Earth-Moon system. Invoking other sources then requires explaining why so many RPs are entering the system, which implies there are very many out there, yet without evidence of that. Occam’s Razor applies. If a RP is responsible for some phenomenon, then evidence needs to be found, with a cohesive amount of supporting evidence to make that theory the best one.

  • Peter Elia January 6, 2022, 13:37

    I’ve studied stellar interactions in dense environments like globular clusters, looking for things like blue stragglers (hot blue stars formed by the merger/collision/mass transfer of two smaller stars). One thing we’re learning in recent years is that these aren’t necessarily rare phenomena — it could be moderately common for stars to interact in these dense environments (as opposed to the simpler view that stars basically all form in place and evolve according to the IMF, not bothering one another). Something like 30% of all stars experiencing a close interaction in their lifetimes would not be surprising.

    I’d be curious to compare the number and distribution of FFPs in young open clusters vs. old globular clusters. Old GCs undergo something called *cluster* core collapse, where the heavy stars trade kinetic energy with the lighter ones (like a probe boosting off Jupiter), and you get a runaway effect where the heavy stars move to the center, deepen the gravity well, and thus bring in more heavy stars.

    So old clusters are much more dense, i.e. there are more interactions. If you could compare the number of FFPs between young and old clusters of similar mass, you might be able to suss out the different mechanisms at work.

  • Scott Guerin January 6, 2022, 14:45

    What a fascinating article. I am reminded of Piers Anthony’s 1970 novel Macroscope. In it, on page 295, the Macroscope, which is a kind of time traveling communication device sensitive to Macron particles (analogous to neutrinos in my mind), reveals that Earth and moon were captured fragments of a passing planet that fell inside the Roche limit and broke up.

  • Geoffrey Hillend January 6, 2022, 16:51

    I agree with the idea that brown dwarfs form from accretion especially when it orbits another Sun like star. What about binary brown dwarf systems? One could assume that since they are rare they most likely formed in a star system and were ejected which is possible. I can’t remove my doubt though that every binary brown dwarf system or isolated brown dwarf must form from accretion around a star system and was ejected. The JWST is optimized for infrared since it has gold mirrors, so it can easily image brown dwarfs and those even below five Jupiter masses. NASA.

    The proto stars magnetic field would still put a drag on the dust cloud and remove some angular momentum from the proto star. The accretion disk grabs most of the angular momentum from a star to form the planets.

  • Brian Altmeyer January 6, 2022, 17:09

    There’s a certain poetry to the fact that the word planet means “wanderer.”

    Studies like this are of course about distributions rather than absolute existence. Naturally there will be some ejected from star systems; some formed directly from clouds; some with complex histories every bit as strange as (or even stranger than) the star-bound planetary census we’ve seen.

    If direct collapse created some, could there be ultra-low-density planets too fragile to have survived a stellar environment? Little more than blobs of gas with only tiny snowflake cores? That would be fascinating, and perhaps very different from the fluidic monsters we call “gaseous” simply by comparison with the rock we live on.

    • kzb January 6, 2022, 18:57
      • Brian Altmeyer January 7, 2022, 0:56

        Fascinating concepts. Sounds like they’re describing very large versions of comets. Lots to digest.

        The inversion of the temperature gradient into a hollow hydrogen-helium “snow shell” is the kind of strange but intriguing idea I love to see, although it seems (intuitively) awkward in something as hot and dense as the Milky Way.

        Maybe masses ejected into intergalactic space early on?

        It’s great that people are thinking about what’s physically possible, and not just what’s considered likely to be common. So often in history we found that the assumed distributions are totally wrong.

        Devil’s advocate though: The universe from our perspective is so huge, cold, and old, that the space of the possible makes it easy to be loose with the facts. The paper could do more investigation of laboratory results for hydrogen and helium states at very low temperatures, rather than just noting that simple limits fail to rule out these structures.

        For instance, on a separate topic that isn’t in the paper, I see some of the attempts to explain Oumuamua as “hydrogen snow” as a bit hand-wavy, and this looks to be several stages of conjecture beyond that. Although it’s honest about that, it is kind of just picking locations in a phase diagram and saying “This could happen.”

        As simple as a phase diagram seems, the “weeds” of the reality is incredibly huge.

        It would be great to see some observational predictions that are highly specific to the idea.

        Thanks for the link.

        • kzb January 7, 2022, 13:24

          Yes I find their work fascinating also. The main interest is that these objects would explain quasar microlensing and also the “extreme scattering events” observed in radio astronomy. So there is definite evidence for their existence, and also being very common, thousands per star.

  • Geoffrey Hillend January 6, 2022, 19:22

    Gilese 229B is 50 Jupiter masses and a brown dwarf. The core temperature is 500,000 Kelvin at 170Gbar or gigabars. 170 billion bars of pressure. The Earth’s atmosphere is one bar pressure or 14.7 pounds per square inch. Jupiter’s core temperature is 17,000 K at 70Mbar or million bars. P. 198, the New Solar System. Beatty and Chaikin.

    I guess the gas and dust in a proto stars is mixed or not differentiated like a gas giant with a rocky core which had more time to form? At any rate the temperature goes up too fast for that with a temperature of 10 million kelvin needed for the fusion of hydrogen into helium with a fast collapse of gas and dust.

  • ljk January 19, 2022, 15:31

    Can life exist on worlds that have no parent star? Maybe…


  • ljk January 26, 2022, 14:30

    “The Music of the Spheres”: We think that we’ve found its equation

    January 24, 2022 by Nicola Scafetta and Michael J. Bank


  • ljk February 23, 2022, 17:38


    [Submitted on 12 Jan 2022]

    The Cosmic Hitchhikers Hypothesis: Extraterrestrial Civilizations Using Free-Floating Planets for Interstellar Colonization

    Irina K. Romanovskaya

    I propose the Cosmic Hitchhikers hypothesis as follows. Advanced extraterrestrial civilizations may use free-floating planets as interstellar transportation for space exploration and interstellar colonization.

    Large groups or populations of their biological species, post-biological species, and technologies may become Cosmic Hitchhikers when they ride free-floating planets to reach, explore and colonize planetary systems.

    To get an interstellar ride, Cosmic Hitchhikers may travel to free-floating planets passing close by their home worlds. Otherwise, they may use astronomical engineering to steer free-floating planets toward their home planetary systems.

    Cosmic Hitchhikers may also ride objects native to the outer regions of their planetary systems, which become free-floating planets when ejected by astronomical engineering or by their stars during the asymptotic giant branch evolution.

    During interstellar travel, Cosmic Hitchhikers may apply astronomical engineering to steer their free-floating planets toward the planetary systems of their choice.

    Whereas riding free-floating planets may not save travel time, it avoids the technical challenges of interstellar spacecraft transporting large populations.

    Each civilization of Cosmic Hitchhikers may colonize several planetary systems. Its colonies may grow into autonomous civilizations, changing the number of civilizations in the Galaxy.

    Over the last 4 billion years, Cosmic Hitchhikers or their artifacts riding free-floating planets might have passed by the Solar System. Therefore, their artifacts might exist in the Solar System or in our stellar neighborhood. SETI and SETA should include the search for Cosmic Hitchhikers and their artifacts.

    Keywords: SETI, SETA, free-floating planet, extraterrestrial civilization, interstellar travel, interstellar colonization, artifact, Cosmic Hitchhikers

    Comments: 20 pages. The legal name of the author is Irina Mullins. Irina Mullins, a Professor of Physics and Astronomy at Houston Community College, writes under her maiden name, Irina K. Romanovskaya

    Subjects: Popular Physics (physics.pop-ph)

    Cite as: arXiv:2202.03364 [physics.pop-ph]
    (or arXiv:2202.03364v1 [physics.pop-ph] for this version)


    Submission history

    From: Irina Mullins [view email]

    [v1] Wed, 12 Jan 2022 21:50:19 UTC (221 KB)


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