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Detection of Extragalactic Planets?

I was pleased to be a guest on David Livingston’s The Space Show last week. David’s questions are always well chosen, as were those of the listeners who participated in the show, and we spoke broadly about the interstellar effort and what it will take to eventually get human technologies to the stars. The show is now available in David’s archives.

I suspect that if David and I had spoken a couple of days later, the topic would have gotten around to gravitational microlensing, and specifically, the news about planets in other galaxies. On the surface, the story seems sensational. In our own galaxy, we can use radial velocity and transit studies on stars, but here our working distances are constrained by our method. The original Kepler field of view in Cygnus, Lyra and Draco, for example, contained stars ranging from 600 to 3000 light years out — get beyond 3000 light years and transits are not detectable.

Image: The Sun is about 25,000 light years from the center of the galaxy, about half the distance from the center to the edge. The blue cone shows the region of the Milky Way that Kepler explored for planets. Kepler looked along a spiral arm of our galaxy. The distance to most of the stars for which Earth-size planets can be detected by Kepler is from 600 to 3,000 light years. Less than 1% of these stars in the region are closer than 600 light years. Stars farther than 3,000 light years are too faint for Kepler to observe the transits needed to detect Earth-size planets. Credit: NASA/JPL-Caltech/R. Hurt (SSC).

Gravitational microlensing, in which a star moves in front of a more distant star so that light from the background object is distorted by the foreground star’s gravitational field, can turn up distant planets within our own galaxy. In fact, it’s quite a useful tool because it is not limited by line of sight — no planet needs to transit — and is not dependent on the planet’s distance from its star.

Microlensing has allowed us to find planets thousands of light years away, near the center of the Milky Way. We see the pattern of the microlensing event temporarily disrupted by a spike of brightness as the planet around the closer star causes its own gravitational disruption.

But how do we hope to find planets billions of light years away? At the University of Oklahoma, Xinyu Dai and Eduardo Guerras have tackled the question using data from the Chandra X-Ray Observatory, working with microlensing models calculated at the university’s Supercomputing Center for Education and Research. Their work revolved around the microlensing properties of a supermassive black hole at the center of quasar RX J1131–1231. The background quasar, about 6 billion light years away, is what is being lensed by the foreground galaxy, which is 3.8 billion light years out.

We are dealing with what is known as a quasar-galaxy strong lensing system, one in which a background quasar is being gravitationally lensed by a foreground galaxy. The result is that multiple images of the quasar form, as seen in the image below. The light from the background quasar crosses different locations in the foreground galaxy, and is lensed as well by nearby stars in the lens galaxy, an effect called quasar microlensing. The latter is a useful tool, for astronomers have used it to study accretion disks around supermassive black holes at the center of quasars. It can also provide information about the lensing galaxy itself.

Image: The gravitational lens RX J1131-1231 galaxy with the lens galaxy at the center and four lensed background quasar images. Credit: University of Oklahoma.

Let’s turn to the paper, where the relevance of this to extragalactic planets emerges:

As we probe smaller and smaller emission regions of the accretion disk close to the event horizon of the SMBH, the gravitational fields of planets in the lensing galaxy start to contribute to the overall gravitational lensing effect, providing us with an opportunity to probe planets in extragalactic galaxies…

The authors analyze the Einstein ring created by lensing effects to show that emissions close to the Schwarzschild radius of the central supermassive black hole of the source quasar will be affected by planets in the lensing galaxy. Thus we have a way of identifying a population of planets in another galaxy, though not planets orbiting a central star. For the paper goes on:

We have shown that quasar microlensing can probe planets, especially the unbound ones, in extragalactic galaxies, by studying the microlensing behavior of emission very close to the inner most stable circular orbit of the super-massive black hole of the source quasar. For bound planets, they contribute little to the overall magnification pattern in this study.

These planets would be, in other words, so-called ‘rogue’ planets not associated with any star, for bound planets of the kind we are used to studying with radial velocity and transit methods would be below the threshold of detection — they would not change the magnification patterns being observed.

The numbers on these rogue planets are impressive: Roughly 2000 objects per main sequence star in sizes ranging between the Moon and Jupiter — 200 of these per main sequence star would be in the Mars to Jupiter range.

These numbers, Dai and Guerras note, are consistent with theoretical studies showing a large population of unbound planets in our own galaxy. Stanford’s Louis Strigari and colleagues, for example, have found that there may be up to 105 compact objects per main sequence star in the Milky Way, using evidence from microlensing as well as direct imaging. Our galaxy may, in other words, be well populated with such objects, most of these relatively small but some larger than Jupiter. See Island Hopping to the Stars for more on Strigari’s work.

What to make of all this? Before this work, the only evidence for an extragalactic planet was the microlensing event PA-99-N2, detected in 1999, and consistent with a star in the disk of M31, the Andromeda galaxy, lensing a background red giant. A planet of 6 times Jupiter’s mass is one explanation for the lensing profile, but there is no way to confirm the possible planet.

Now we have not the detection of individual planets but a hypothesis that lensing data of a galaxy 3.8 billion light years away can be explained by the presence of a population of unbound planets and other compact objects much smaller than planets. The idea that there would be planets in other galaxies is hardly unusual, given their numbers in our own Milky Way. But it’s an exciting thought that we can now begin to study extragalactic exoplanets, even if we’re extremely early in the process and there is much to be learned. As the paper notes:

It is possible that a population of distant but bound planets (Sumi et al. 2011) can contribute to a significant fraction of the planet population, which we defer to future investigations. Because of the much larger Einstein ring size for extragalactic microlensing, we expect that two models, the unbound and the distant but bound planets, can be better distinguished in the extragalactic regime.

The paper is Xinyu Dai & Eduardo Guerras, “Probing Planets in Extragalactic Galaxies Using Quasar Microlensing,” accepted at Astrophysical Journal Letters (preprint). And if you’re interested in the PA-99-N2 event, one source is Ingrosso et al., “Detection of Exoplanets in M31 with Pixel-Lensing: The Event Pa-99-N2 Case,” Twelfth Marcel Grossmann Meeting: on Recent Developments in Theoretical and Experimental General Relativity. p. 2191 (preprint).


{ 27 comments… add one }
  • kzb February 5, 2018, 14:44

    Can anyone comment on the relation of this work to similar microlensing studies interpreted as gas clouds? Also these clouds could be the origin of Extreme Scattering Events in the radio. See:

  • Alex Tolley February 5, 2018, 15:17

    If there are 1E5 bodies per star, that means that there could be more than a few rocky/gaseous planets between us and Proxima. We don’t seem to have detected them, but is that because they would be too cold to emit radiation above background? Could they be detected by other methods?

    • andy February 5, 2018, 17:29

      I’d be careful about extrapolating the number density of objects in this case. The lens galaxy is a giant elliptical, so the population of objects may well be different to what we find in the local neighbourhood.

      • Paul Gilster February 5, 2018, 17:35

        A major point, that’s for sure. The lensing galaxy is quite a bit different from our home spiral.

        • J. Jason Wentworth February 7, 2018, 5:41

          Barred spiral, you mean. (After seeing pictures of barred spirals, which I find pretty, I had long thought, “It would be great if we lived in one,” and a few years ago, sure enough, my fancy was reported to be fact…). What causes central bars is, as far as I know, still not known. This also, rather indirectly, raises a question:

          How might the numbers of Nomads (planets “unaffiliated” with stars) “per star” in galaxies differ between spiral, barred spiral, elliptical, and irregular galaxies? In addition:

          The answer is not purely academic (as Alex also alluded to above), because if it could be shown that Nomads lie between our Sun and our nearest stellar neighbors, interstellar probe missions–even relatively slow ones–would be easier to justify politically, psychologically, and emotionally (having true *worlds*–not only comet nuclei and/or interstellar asteroids–along the way would greatly attenuate what might be called, colloquially, the “Are we there yet?” syndrome…)

    • J. Jason Wentworth February 7, 2018, 6:27

      One method that might work has been suggested for detecting comet nuclei way out yonder in the Oort Cloud (if this would work for comets, it should work Nomads, which are far bigger):

      Oort Cloud comets briefly occult stars, which could be detected from Earth-based telescopes (it might be confused with atmosphere-caused twinkling, but using space-based telescopes would eliminate this “generator of false positives.”). One problem–not insurmountable–is that Oort Cloud comets, being slow-moving (about 300 km/h) and spread thinly and randomly all over the celestial sphere, occult stars randomly, all over the sky. Another is that an even more distant Nomad might, due to the effects of distance, angular motion, and its size, mimic the occultation characteristics of the closer, Oort Cloud comets, but there may be a way around these problems:

      A space-based negative (convex) mirror, in the shape of a third or so or a sphere, could reflect half of the sky all at once, which could be viewed continuously by one or more cameras placed around its rim, to detect random occultations. (I once saw a picture of such an arrangement–not made for tracking such occultations, but just for regular viewing–that a “Sky & Telescope” reader made for viewing the sky from horizon to horizon. It used a polished, old-style Volkswagen “partial sphere” chrome-plated hubcap.) As well:

      It may be possible to discriminate between relatively nearby Oort Cloud comets and distant Nomads, using stars (or even galaxies) themselves. In some areas of the sky, objects passing in front of far distant objects (closely-spaced open star clusters, and/or even globular clusters, and perhaps also luminous nebulae and/or the band of the Milky Way itself) might betray their proper motions. If so, perhaps they could be eliminated as Oort Cloud comets in this way, and measuring their trigonometric parallax with pictures taken six months apart (many should occult *some* star or other object six months after the first picture is taken) should determine their true distance.

  • andy February 5, 2018, 16:20

    I’m reminded of the 1996 claim by R.E. Schild based on time delays in the lensed images of the Twin Quasar Q0957+561 for a large population of planets in the lens galaxy. As far as I can tell, the variability in the images is now attributed to intrinsic variations of the quasar itself.

  • James Stilwell February 5, 2018, 16:42

    You may have put this one on the back burner but others have not…

    The Gravitational Lens and Communications
    by PAUL GILSTER on NOVEMBER 6, 2009
    If we can get the right kind of equipment to the Sun’s gravitational focus, remarkable astronomical observations should follow. We’ve looked at the possibilities of using this tremendous natural lens to get close-up images of nearby exoplanets and other targets, but in a paper delivered at the International Astronautical Congress in Daejeon, South Korea in October, Claudio Maccone took the lensing mission a step further. For in addition to imaging, we can also use the lens for communications.

    The communications problem is thorny, and when I talked to JPL’s James Lesh about it in terms of a Centauri probe, he told me that a laser-based design he had worked up would require a three-meter telescope slightly larger than Hubble to serve as the transmitting aperture. Laser communications in such a setup are workable, but getting a payload-starved probe to incorporate a system this large would only add to our propulsion frustrations. The gravitational lens, on the other hand, could serve up a far more practical solution.

    The comments are truely astounding.

  • Gary Wilson February 5, 2018, 16:46

    Interesting article but clearly nearby extrasolar planets should be the main priority. If we want to understand what types of planets are out there (and hopefully someday explore some of them) that must be the focus. It seems logical that early exploration (such as Kepler) had to focus on a region of space in which star systems were vastly variable distances apart, but now we need a very precise catalogue of the types of planets in our immediate neighborhood.

    • Paul Gilster February 5, 2018, 16:53

      Upcoming missions like TESS and CHEOPS will play a big role as we build the catalog of nearby planets.

    • andy February 5, 2018, 17:45

      I fundamentally disagree with this attitude. We’re not going beyond our solar system for a very long time if at all. There’s not much practical difference between nearby and distant exoplanets, and if anything distant planets might tell us more about the dependence of the planet formation process on environment (this study probes a giant elliptical rather than a barred spiral) that wouldn’t be so easy to determine by studying the local neighbourhood. Exoplanetary science is advancing far faster than prospects for actual interstellar exploration, there’s no reason to panic that we won’t have a decent idea of what’s going on in nearby planetary systems by the time we get to actually sending anything there.

      • Ron S. February 5, 2018, 20:12

        I would go even further. This work provides useful statistical data on planet formation in the distant past due to the immense distance of the lens galaxy.

  • Robin Datta February 6, 2018, 0:52

    A series of stepping stones and a one-way progress in the long view (multiple millennia for a step) might be a possibility if we don’t do ourselves in in the very short term as in some explanations for Fermi’s paradox.

  • xcalibur February 6, 2018, 2:41

    Over the centuries, we’ve learned that the rest of the universe does not revolve around us. The earth is a planet, within a solar system, within a galaxy, amongst many other galaxies. The detection of exoplanets in other galaxies is further evidence against the rare earth hypothesis and the presumption of human exceptionalism.

    • J. Jason Wentworth February 7, 2018, 7:11

      Humans are by definition exceptional, because in this universe none are to be found anywhere but on Earth (the same is true of all other living things here). Regarding “civilizational exception” (are we the only creatures who “do science,” build radio telescopes, explore space, and dispatch interstellar spacecraft [the Pioneers, Voyagers, and New Horizons are such, though very rudimentary and mostly symbolic as starcraft]?), biology doesn’t exactly give us encouragement for expecting to have neighbors we can communicate with:

      Of all of the millions of species that have arisen on Earth, exactly *one* developed–or even needed to develop–tools and a way of living and thinking that could lead to science and technology, which were neither necessary or inevitable (non-spacefaring cultures far outnumber space-capable ones even today). Eve a casual look at nature shows that it is perfectly possible to prosper–and propagate one’s kind–with little or no intelligence at all, and even possessing the requisite intelligence is useless for developing a technological civilization if one’s body and/or environment won’t permit it, as is the case with octopi, dolphins, and whales. In this vein:

      Arthur C. Clarke’s short story “Second Dawn” involves highly intelligent, rather unicorn-like telepathic beings who can do abstract mathematics and design technological devices. But having hooves rather than hands, they can progress no further until one of them befriends a relatively unintelligent, sub-human-like race on their world, after which both races progress swiftly by working symbiotically. The “Eerie Silence” that has met SETI attempts so far (Paul Davies’ book of that title examines these and other disappointing and disquieting facts) doesn’t engender optimism, either. While I certainly support SETI (and also Bracewell probes [which–when they become feasible–can also investigate stars and exoplanets, whether or not life is present]), I won’t be surprised if the alien equivalent of bacteria is the highest form of life on most worlds that possess life.

      • xcalibur February 8, 2018, 9:18

        These are valid points. However, we have only one example to work from, the earth. There is simply not enough knowledge of the wider universe, and conditions on exoplanets, to make definite statements on the commonality of life, sentience, or civilization. While the lack of evidence is discouraging, it shouldn’t be — there are many plausible explanations for why they may be out there, but undetected. To give a few examples, we may have simply missed their signals, they may be on the opposite side of the galactic core, or ETI may have built civilizations eons ago which are now extinct.

        Personally, I think commonality and complexity of life are in inverse proportion. Meaning, microbes should be very common, intelligent civilizations should be rare, with non-sentient plants/animals in the middle. This is the pattern seen on earth, and it is the most logical extrapolation.

        The earth is a significant for its rich biosphere. We are significant as an intelligent and civilized species. But what I mean by exceptionalism is that this does not necessarily make us superior to the rest of the universe. It is entirely possible that there are other civilizations and/or sentient species out there. Our expanding knowledge of exoplanets only makes this more plausible.

  • kzb February 6, 2018, 6:58

    andy -R E Schild did indeed explain the microlensing by free floating planets, just as this paper does. But as you point out, in 2009 this was apparently refuted. So are things going round in circles, or is there fresh evidence in this paper that the fluctuations are definitely NOT due to intrinsic fluctuations in the source?

    • andy February 7, 2018, 13:21

      As far as I could tell, the paper doesn’t even reference the Schild claims. I wonder if his activities as the editor of the Journal of Cosmology have anything to do with it. The results do seem to be fairly similar claims, and it would be interesting to see if this analysis of RX J1131-1231 offers any new insights into the observations of the Twin Quasar and vice-versa.

  • Harry R Ray February 6, 2018, 10:55

    A completely UNDISCUSSED consequence of all this: MACHOs(maybe miniMACHOs is a better term)may, after all, comprise a SIGNIFIGANT FRACTION of ALL of the dark matter in the universe!!!! I am assuming that observations of objects signifigantly LESS massive than the moon are NOT possible with this method due to sensitivity constraints, but: astrophysicists should be able to EXTRAPOLATE the BULK DENSITY of these objects as well with a Baysian analysis of the trends in the mass ratios of the existing objects to calculate what this fraction is!!!!

    • Ron S. February 6, 2018, 14:26

      Observational studies have been done for occultations and lensing events (depending on whether the foreground object is small or larger than the background object) within our own galaxy. There are nowhere near enough of these bodies (mass & number) to be a viable explanation for dark matter (I don’t have references to the papers at hand). This is why WIMPs remain under consideration despite the tenuousness of theoretical backing and weak experimental results to date.

      • Michael C. Fidler February 6, 2018, 19:30

        The rest of the MACHO mass are Cloaked Dyson Spheres (CDS) so no need for any of those puny WIMPs. ;-}

    • kzb February 7, 2018, 8:09

      As Ron S says below the number of MACHOS needed seems to be ruled out by microlensing studies of our own galaxy halo.

      HOWEVER, I am trying to get people to look at an alternative idea, and that is the objects observed in the current paper are not solid planets but compact gas clouds. I’ve little knowledge of microlensing theory, but such clouds would be rejected by local microlensing searches, but presumably satisfy the conditions in very distant galaxies like in the paper under discussion.
      See the material here:

  • ljk February 6, 2018, 15:05
  • Spaceman February 7, 2018, 15:35

    Some, including Scott Gaudi and David Bennett, are skeptical of the claims being made here for extragalatic planets. Since follow-up observations in microlensing are hard to come by, are there any ways this planet interpretation could be strengthened or weakened? I would love to see the results of a follow-up study. The numbers of unbound planets seem totally reasonable in light of what we know about planet formation.

  • Andrew Palfreyman February 8, 2018, 19:27

    Space Polynesia: remote-controlled fusion powered beamers lasting centuries installed at outposts at ever-increasing distances.
    And why not, indeed?

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