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Exoplanets: The Interplanetary Dust Factor

I usually get up while it’s still dark and take a walk. The idea is to shake the night’s dreams out of my head, listen to the birds waking up and pull in a lot of fresh air, all conducive to thinking about what I want to write that day. Last fall I kept noticing the glow before morning twilight that marked the zodiacal cloud, faint enough to be lost in moonlight and challenging to see when competing with city lights. But catch the right conditions and its diffuse glow is apparent, as in the photograph below, a striking example of zodiacal light’s effect.

Image: Sometimes mistaken for light pollution, zodiacal light is sunlight that is reflected by zodiacal dust. It is most visible several hours after sunset on dark, cloudless nights surrounding the spring and fall equinoxes, when the Earth’s equator is aligned with the plane of the solar system. Credit: Malcol.

What we’re seeing, especially at times when the ecliptic is at its largest angle to the horizon, hence autumn and spring, is the light of the Sun reflecting off dust in the Solar System. Most of the material in this interplanetary dust cloud is concentrated along the plane of the system, and as you would expect, similar dust clouds are to be found around other stars. The Spitzer Space Telescope, for instance, has found evidence for a strong dust cloud around the star HD 69830, presumably the result of collisions within the system. No planet has yet been found there.

We often talk about interstellar gas and dust as serious issues for spacecraft moving at a substantial percentage of the speed of light. But what about interplanetary dust? The complications it poses involve how we see exoplanets, particularly those in the habitable zone.

Imagine zodiacal light perhaps a thousand times brighter than our own, enough to outshine the Milky Way. The challenge such light presents to astronomers are potentially serious but not well quantified, which is one reason why researchers using the Large Binocular Telescope Interferometer (LBTI) on Arizona’s Mt. Graham has been at work in a program called HOSTS — the Hunt for Observable Signatures of Terrestrial Systems. The team’s paper in the Astrophysical Journal gives us a look at the survey’s early results.

“There is dust in our own solar system,” says Philip Hinz, the lead for the HOSTS Survey team and associate professor of astronomy at the University of Arizona. “We want to characterize stars that are similar to our own solar system, because that’s our best guess as to what other planetary systems might have life.”

Image: This artist’s concept illustrates what the night sky might look like from a hypothetical alien planet in a star system with an asteroid belt 25 times as massive as the one in our own solar system (alien system above, ours below. Credit: NASA/JPL-Caltech.

Steve Ertel (University of Arizona) is lead author of the paper, which delves into the question of how much dust within a stellar system can affect our ability to see planets within it. All of this goes into planning for future space telescopes, with the HOSTS survey examining the issue for 30 nearby stars. What we learn from the paper is that exozodiacal dust in the stars surveyed is typically less than 15 times the amount found in our own Solar System’s habitable zone.

But planets with larger dust volumes become seriously problematic. Epsilon Eridani, long of interest because of its proximity to the Sun (10.5 light years) is one of these. Says Ertel:

“It is very nearby. It’s a star very similar to our sun. It would be a very nice target to look at, but we figured out that it would not be a good idea. You would not be able to see an Earth-like planet around it.”

Even so, Epsilon Eridani offers us a useful study in planet formation, albeit one with serious challenges for observers. This is from the paper, referring to a previously studied dust clump in the system, which could indicate:

…local dust production in the known asteroid belt and potential shepherding by a planet interior to the belt which could also be creating the clump. There is a long history of planet claims for εEri, but radial velocity detection is complicated by stellar activity induced jitter. The existence of the planet claimed by Hatzes et al. (2000) and Benedict et al. (2006) has been debated in the literature (Anglada-Escudé & Butler 2012; Howard & Fulton 2016), it is possible that a planet of period 6.8 – 7.3 yr and mass 0.6 – 1.55 MJup does orbit the star. Attempts to infer the presence of outer planets based on the ring structure are problematic due to the uncertain nature of the intrinsic disk morphology.

This is interesting stuff, although as the paper is at pains to note, we are very early in the study of dust distribution at this level. The issue can tell us something about the possibility of planets within a star system. Given the standard model — that dust is formed during asteroid collisions and spirals inward so that it is distributed throughout the entire system — the survey turned up at least one surprising result. We’ve known for some time that the star Vega has a large belt of cold dust in about the same relation to Vega as the Kuiper Belt is to our system. There is also a disk of hot dust very close to the star. From the paper:

A most puzzling result is our non-detection of warm dust around Vega, for which massive asteroid belt and Kuiper belt analogs have been detected in the mIR to fIR and a large amount of hot dust has been detected in the nIR. This raises the question of what mechanism clears the region between ∼0.5 AU and ∼5 AU from the star of dust.

We have as yet to detect planets around Vega, but the lack of habitable zone dust may be telling us about a massive planet whose gravitational influence could be clearing this area or, as Ertel notes, several Earth-mass planets. Several other stars in the survey showed, unlike Vega, no dust belts close to the star or far from it, but large amounts of warm dust in the habitable zone. A massive asteroid belt producing numerous collisions could be the culprit in such cases.

Overall, the HOSTS survey to this point has been able to make four new detections of habitable zone dust among its 30 stars; among these, three are the first to be found around Sun-like stars, and two occur around stars without any previous detections of circumstellar dust. The paper notes that the survey’s sensitivity is five to ten times better than previous results. Future exo-imaging attempts will be well served by extending the survey to a larger sample of stars, so we’ll know how the quantity of dust in a given system affects our ability to see HZ planets.

The paper is Ertel et al., “The HOSTS survey – Exozodiacal dust measurements for 30 stars,” Astrophysical Journal Vol. 155, No. 5 (17 April 2018). Abstract / preprint.


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  • Thomas W. Hair April 27, 2018, 21:05

    Hi Paul, I too like to get up when it’s dark sometimes…maybe it’s an old man thing, but I live in an especially dark, quiet area of southwest Florida. I even have an analog sextant that I use once in a blue moon during astronomical or nautical twilight along the Caloosahatchee to shoot a star or maybe Venus. The digital one I used on my P-3 patrol plane was much easier, and Navy folk still learn the old trade because if a major conflict were to erupt GPS might be one of the first things to go down.

    • Paul Gilster April 27, 2018, 21:38

      When I was getting my commercial ticket, my flight instructor used to occasionally shut down the instrument panel at night when we were doing longer flights and force me to figure out how to navigate back home by the stars. We didn’t do it enough that I got very proficient, but I loved the connection with a far older skill.

    • ljk April 28, 2018, 18:13

      I wonder how much longer before a truly dark night is a thing of the past:


      If our descendants cannot see the stars, how will they know to reach for them?

      • James Stilwell April 29, 2018, 11:37

        Good point…Issac Asimov wrote a famous book featuring a world city called Trantor…the city covers the whole earth, every square foot with human habitation…except for the few acres the Emperor retains for his inner courtyard…Asimov never mentions that the stars might not be visible at night…his mistake and a large one if true and assuming that my memory is remembering correctly. Maybe Google will know…science fiction is supposed to be well ahead of the curve of future history…

        • ljk April 30, 2018, 9:23

          I am sure Asimov and many of his contemporaries took such things as seeing the stars for granted. I recall one character who lived on Trantor saying he had not been outside in years and so no real reason to venture there.

          Asimov also said he himself preferred being indoors largely to write but also just in general. That trend has only increased since his time thanks to our gadgets and the Internet.

          Even now NASA often glorifies light pollution by showing views of Earth at night from the ISS and declaring how pretty it looks. To me that is like viewing massive forest fires from space and saying Oh how lovely when it is anything but for the planet, animal, and human life on the ground in those regions.


          I highly recommend this book by Paul Bogard titled The End of Night: Searching for Natural Darkness in an Age of Artificial Light…


          • Michael C. Fidler May 1, 2018, 11:40

            One of the best places to see the night sky are the National and State parks throughout the U.S. Each Spring we go back from Bohol to Oregon and take our 5th wheel out of storage. We camp for the summer at Detroit Lake State park in Oregon, as my wife and I are interpretive host for the park. We due a program each weekend evening with telescopes about the heavens, moon and planets and sometimes even what is talked about here. The nice part is the dark skies and people be able to see the Milky Way and relate to nature.

      • J. Jason Wentworth April 29, 2018, 23:22

        Thank you–I’m going to spread this link around! I remember the pre-mercury vapor, pre-sodium vapor era of street lighting in Miami, when our neighborhood had only two quite dim (by comparison, as well as intrinsically) incandescent street lights within a block of our house; they were the old-fashioned kind with the fluted, white-enameled metal reflectors that blocked light from traveling upward, and:

        They provided ample illumination of the street and surroundings, yet we could easily see the Milky Way and even fifth- or sixth-magnitude stars (the sky was peppered with them). In 1978, the incandescent street lights were replaced with sodium vapor ones, whose plastic light diffusers spread dazzlingly bright yellow-orange light upward as well as to the sides and downward. From then until Hurricane Andrew knocked out all power for awhile in 1992, we never saw the Milky Way or dim stars again. Also:

        Even the white mercury vapor lights, which use the same light diffusers as the sodium vapor ones (they were used elsewhere in town before sodium vapor ones replaced them; the mercury vapor ones are still available as home security lights), don’t wash out the night sky like the sodium vapor lamps. The human eye is more sensitive to the sodium vapor lamps’ yellow light (the “firefly-colored” yellow-green chemical lightsticks emit light that the human eye is most sensitive to, and such illuminating LEDs are now available). In addition:

        For human health, plant and animal welfare (the problems of excessive night lighting mentioned in the video need to be widely publicized!), saving energy (and thus money), and to enable continued optical astronomical work, all current outdoor lighting systems should be phased out in favor of the lower-intensity, illumination spot-targeted lighting systems covered in the video; plus, buildings that don’t need to be brightly lit up shouldn’t be (dimmer, spot-targeted [and eye response wavelength-targeted], but still crime-deterring lighting systems could be used instead–yellow-green LED fixtures could illuminate roads, walkways, and buildings without “turning night into day” and producing skyglow and light intrusion). As well:

        The old-style streetlight fixtures and lamp posts are stylish-looking (we recently had “new-old” [‘retro’-style] ones of both types–with LED light bulbs–installed here in downtown Fairbanks, along with low-intensity yellow LED vertical sidewalk lighting columns), and they all provide night-time safety without banishing the night and its charms.

        • ljk April 30, 2018, 9:27

          I recall reading about Floridians who survived Hurricane Andrew in 1992 being amazed and even scared about seeing the full night sky above them unblemished by light pollution.

          Many people truly do not appreciate or realize what we are doing to ourselves and the rest of life on Earth by cutting off the stars in this manner.

          • J. Jason Wentworth May 1, 2018, 3:24

            I didn’t hear any reports about my fellow south Floridians being scared by what they saw at night (amazed, yes–several local radio stations and at least one TV station remained operational), but I would not be surprised if some, or even many, people found the Milky Way troubling, because here is a sure sign that many people live lives that are too distant from nature and her rhythms:

            In replies #14, #15, and #16 on the “CosmoQuest” science and space forum ^here^ (see: http://forum.cosmoquest.org/showthread.php?21322-Daylight-lunar-observation ), forum members discussed a lunar phobia that they never conceived of people having (although one of the affected people was only 8 years old at the time, so I’ll cut her some slack—although I never experienced this phobia at that age, or at any age). What is the phobia? It consists of becoming disconcerted, and even frightened, upon seeing the Moon in the *daytime* sky! (One of the sufferers, an adult man, feared that it was crashing to Earth!)

            The closest thing to this that I ever observed involved my mother, and it wasn’t a matter of fear, but of confusion (which my father intentionally generated, for the sake of humor). Once when we were walking out the door of the local B & T (Bell & Tiger) grocery store in Hiawassee, Georgia, my father—noticing a crescent Moon in the blue afternoon sky—said calmly, “I see an Earth satellite.” My mother, who hadn’t been looking at the sky, suddenly looked up and asked, “Where?”, expecting to see an artificial satellite (some, like the old Echo 1 & 2 balloon satellites and the ISS, were/are visible in the daytime). He pointed at the Moon and said—just as calmly—“There.” :-) Also:

            Even daytime astronomical observations of the Moon, while they are less frequently conducted than nighttime ones, are not without merit (meteorite lunar impacts during daytime meteor showers can be detected). For the amateur astronomer who is learning his or her way around the Moon with binoculars or a telescope, and for the “pleasure viewer” who uses an instrument—or just her or his unaided eyes—to look at the Moon just for fun, our natural satellite provides lovely daytime views (see: http://www.google.com/search?q=daytime+lunar+astronomical+observations&tbm=isch&tbo=u&source=univ&sa=X&ved=0ahUKEwj7p-m17vbUAhUR42MKHdv4DRYQsAQIRQ&biw=1440&bih=794&dpr=1 ).

            • ljk May 1, 2018, 11:21

              I read about this in an issue of Astronomy magazine in the early 1990s (do not ask me what issue, although I think it was a letter to the magazine).

              One fellow was outside at night guarding what was left of his home and property after Hurricane Andrew came through (I recall a weather satellite image of the hurricane just before it hit Florida: The darn thing was as big as the whole state!). With no artificial lights and not much to do, he was sitting in his lawn chair looking up at the night sky, which was filled with more stars than he likely ever saw before in person. The Milky Way arched overhead, also bigger and brighter than had been seen before. He reported not only a profound sense of awe and wonder, but also some fear at seeing so many stars at once in the sky.

              I once met a person who had witnessed a moonbow, that big ring around the Moon which happens just before it either rains or snows. They had never seen this phenomenon before and had no idea what it was. He also expressed a measure of fear as to what it could mean. No wonder our ancestors flipped out about anything in the sky that didn’t match their daily experiences and limited educations.

              As for daylight sightings of the Moon, I personally delight in them and love studying our celestial neighbor up close in this state when possible. The Moon is more subdued during this time, which makes looking at it a bit easier.

              • J. Jason Wentworth May 1, 2018, 22:23

                Pascal–who was terrified by the vast spaces *between* the stars–would likely have laughed at that fellow’s fear of seeing so many stars at once. We found Hurricane Andrew’s size in the satellite photos disconcerting, because a miss looked impossible, and:

                I’m not surprised at the “moonbowphobe’s” reaction. When a luminous sounding rocket barium release “fireball” suddenly appeared in our evening sky in northern Georgia in the late 1970s or early 1980s, an excited rumor that “the moon blew up!” spread at school the next day (I pointed out that if that *had* happened, it would have been all over the world news…). Interestingly, the blue Moon (and blue Sun–fine particles from large Canadian forest fires were the cause) that was seen in Canada and the UK in 1950 didn’t trigger fear, as Patrick Moore reported, even though the Moon was an electric blue. Also:

                Aesthetically, I’ve always appreciated the daytime Moon’s more subdued contrasts (bluish when high in the sky, reddish or orange-ish near sunset and sunrise). Even observational work can be done then, including seeing TLP if they occur at those times.

                • ljk May 2, 2018, 11:45

                  It is nice to see TLPs finally being taken seriously by the astronomical community.

                  This is in contrast to such books as Epic Moon, where the authors went out of their way to trash not only most of the observational claims but even certain ones who made their claims, in particular the Soviet astronomer who thought he captured a volcanic eruption on the Moon in 1959:


                  • J. Jason Wentworth May 5, 2018, 11:01

                    They’re just showing their ignorance–Audouin Dollfus (who discovered Saturn’s moon Janus) clearly observed a TLP from the Pic du Midi Observatory in the early 1990s, USAF selenographers (Greenacre and Barr) watched several colorful luminous ones in 1963 from the Lowell Observatory, Kozyrev’s 1959 TLP-in-progress spectrograms at the Crimea Observatory clearly indicated C2 emission in Alphonsus (and the central peak was obscured by a reddish cloud that Patrick Moore also saw), and the Apollo 11 crew (in lunar orbit) and Earthside obsevers simultaneously saw a luminous TLP in Aristarchus–other than that, I have no opinion about TLP. :-)

                    • ljk May 7, 2018, 11:20

                      The authors of Epic Moon are certainly not ignorant, but they do seem to be coming from the Moon is a Dead Rock school and wanted to keep it that way to remain credible.

                      Just as some SETI folks used to go out of their way to discredit any and all UFO reports and the possibilities of interstellar travel just to keep SETI’s “head” above the fringe waters – I was witness to a Harvard lecture in 2000 where a panel of prominent SETI folks dismissed the possibility of starships with a single slide graph, from a light projector no less – saying the Moon has geological activity was just as big a professional cultural taboo. No doubt TLPs felt quite similar to UFOs for them.

                      This was also just before all that water ice was found under the lunar surface. Ironically an astronomer in the 20th Century said the Moon was largely made of ice. Naturally he was dismissed, but he did have a following – from Nazi Germany!

  • J. Jason Wentworth April 28, 2018, 17:01

    This article is incomplete (or maybe it’s Part 1 of a two-part one?). :-) The HD69830 system should also have a very obvious gegenschein (“counter-glow,” seen opposite the Sun in the night sky: http://www.google.com/search?q=gegenschein&tbm=isch&tbo=u&source=univ&sa=X&ved=0ahUKEwj1weTP6N3aAhVO7WMKHeIdDp8QsAQISg&biw=1440&bih=794 ), which is very faint in our solar system, and:

    Such dusty, light-scattering stellar systems–including that of Epsilon Eridani–simultaneously present a problem and an opportunity. The presence of such large amounts of “interexoplanetary” dust, which makes Earth-based observations unfruitful, makes interstellar probes–especially the stellar- and/or exoplanetary-orbiting variety (although fly-through probes would be useful)–more attractive for investigating such systems. Also:

    The dust lane would (as was recognized in early Soviet ion-drive starprobe studies, in the early 1970s) enable such probes to locate the plane in which the target star’s equator (and the orbits of its planets) lies, long before the probes come close, which would enable minimum-energy maneuvers to enter the system in that plane. An ordinary occulting disc-equipped coronagraph/camera combination would enable detection of the zodiacal light dust plane at great distances, even for (relatively) “dust-poor” stellar systems like ours.

    • Zachary April 29, 2018, 22:11

      I greatly enjoyed the use of concluding colons throughout. It struck me as interesting and unique.

    • ljk April 30, 2018, 8:58

      Quoting J. Jason Wentworth on April 28, 2018, 17:01 (interesting time there :^)):

      “The dust lane would (as was recognized in early Soviet ion-drive starprobe studies, in the early 1970s)…”

      Are these studies available online somewhere? How do they compare to similar later studies by other nations? Thank you.

      • J. Jason Wentworth May 1, 2018, 4:36

        The Soviet ion-drive interstellar probe papers may be online (I hadn’t looked until just now [I’ve had numerous duties and unwelcome “domestic distractions” lately]). I just Googled “International Astronautical Congress Baku, Azerbaijan 1973,” which was where the papers were presented, and these links came up (see: http://www.google.com/search?source=hp&ei=3hboWoG2F4eQjwP6v5vIDA&q=International+Astronautical+Congress+Baku%2C+Azerbaijan+1973&oq=International+Astronautical+Congress+Baku%2C+Azerbaijan+1973&gs_l=psy-ab.12…59897.118332.0.121574.…1.1.64.psy-ab..3.50.5618…0j38j0i131k1j0i3k1j0i22i30k1j0i22i10i30k1j33i160k1j33i21k1j33i22i29i30k1j33i10k1j0i7i30k1.0.pDmD-Ugm0Cg ), and:

        These papers and probes are mentioned in the book, “Is Anyone Out There?” by Jack Stoneley with Anthony T. Lawton (Warner Paperback Library Edition, 1974 [Amazon link: http://www.amazon.com/Anyone-Out-There-Jack-STONELEY/dp/0446765740/ref=sr_1_1?s=books&ie=UTF8&qid=1522566430&sr=1-1&keywords=Is+Anyone+Out+There%3F+by+Jack+Stoneley ]); the book says (among many other things) that:

        One such paper was presented to the Baku conference by Dr. Zakirov of the Institute of Applied Mathematics at the USSR Academy of Sciences. Its conclusions were all endorsed by Professors M. Y. Marov and V. A. Egorov, also of the academy–their conclusions were:

        1. We already have the means to launch probes with ion-drive engines that will reach the nearest stars.

        2. Gravitational disturbances from the Sun, the Galaxy, and the target star will not affect the major part of the flight, but must be allowed for during launching and parking in the orbit of a star or planet.

        3. It is now possible to accurately program the thrust power during the journey.

        4. Flight time [please see the following note] would be about four hundred years to Barnard’s Star (six light-years away) and six hundred years to a star approximately twelve light-years away. [NOTE: Elsewhere in the book it is mentioned that ion-drive probe velocities of 5% of the speed of light are possible.]

        5. The greatest navigational error expected is about ten percent, which could easily be detected and corrected.

        6. A civilization more advanced than Earth’s could migrate to its nearest stars by ion-drive propulsion in three to five percent of the lifetime of that civilization (shades of Greg Matloff’s interstellar Worldship studies…). Also:

        The book includes a list of such a probe’s approach, braking, orbit insertion, planet survey, and (if intelligent local signals are detected) contact procedures. One of the early ones involves detecting the target star’s zodiacal light dust band during the early approach, in order for the probe to enter circumstellar orbit in the habitable (liquid water) zone, in the star’s equatorial plane where the planets would orbit.

        • ljk May 2, 2018, 10:44

          Thank you for the very detailed reply and pointers. I always worry about such work being lost to time, especially in the age of the Internet when it is far easier to look for what is online rather than actually have to visit a physical library and sift through dusty shelves. The situation gets even worse if the papers were written in, gasp, foreign lands in different languages.

          • J. Jason Wentworth May 5, 2018, 11:10

            You’re welcome. Yes, I’ve even had young ‘uns insist that I was making something up, because the books that I referenced weren’t available as Googlebooks scans (or even listed online); “since they weren’t on the internet, they didn’t exist.” :-) “Is Anyone Out There?” apparently only appeared (in the U.S., at least–its authors [including Anthony Lawton, a BIS member] were British) as a Warner Paperback Library book. It’s well worth reading, as it covers many now-obscure interstellar spaceflight-related topics (Amazon.com http://www.amazon.com and AbeBooks.com http://www.abebooks.com booksellers have it available).

        • ljk May 2, 2018, 11:39

          A related question: I remember reading in an article about Project Pluto, the insane nuclear-powered cruise missile carrying nuclear warheads Cold War plan:


          That in a side bar to the main article above, the project director, Ted Merkle, was quoted as saying “he wanted to explore the cosmos in a near-light-speed, ion-propelled rocket.”


          So did Merkle have an actual plan for such an interstellar drive in mind? Judging by his brief bio in the side bar, Merkle does not strike me as someone who would have thrown out such a statement without some real and serious science and engineering behind it.

          • J. Jason Wentworth May 5, 2018, 11:52

            Judging by the aerospace writer Walter B. Hendrickson, Jr’s matter-of-fact statement (and brief description, in his 1975 book, “Manned Spaceships to Mars and Venus: How They Work”) that Eugen Sänger and Irene Sänger-Bredt had designed a *fusion*–not matter/anti-matter–^photon rocket^ (essentially a laser rocket) capable of 1 g acceleration to reach almost the speed of light in about a year (or Mars or Venus in < 12 hours), I would not be surprised if Ted Merkle was deadly serious about the near-c ion-drive rocket, and:

            Thank you–I'd read both of those articles many years ago (in "Air & Space Smithsonian," in a dentist's waiting room); I remembered the Pluto history, and Ted Merkle's detached tracking of his liver cancer's advance, but not his statement about the near-lightspeed ion-drive ship. Also:

            A nuclear ramjet *is* a good idea–for rapid point-to-point travel on gas giant and ice giant planets (at the level at which balloon-borne refineries and habitats would float), because no accident could contaminate anything; the debris would just sink into the planets' cores. Ordinary aircraft would take days to traverse Jupiter or even Neptune, but a Pluto-derived nuclear ramjet aircraft could cut the distances down to size.

          • J. Jason Wentworth May 6, 2018, 7:15

            Speaking of Ted Merkle’s near-c ion-drive starship, I came across the following, similar concept:

            Reading through the book “Bound for the Stars,” I came across a familiar proposal (co-created by our own Greg Matloff), called LINAC. Although this was a ramjet, I got to wondering—since the Soviets said, as early as the 1973 CETI Conference, that 0.05 c ion drive starprobes were possible *then* (they’d be about the size of Saturn rockets, but could be built with then-current technology)—could *particle accelerator* technology be utilized to create electric thrusters with relativistic exhaust velocities? Sure enough, it can (see: http://starsdestination.blogspot.com/2013/02/accelerators-storage-rings.html )! Moreover, such engines can run on…water (or more precisely, a plasma made up of ionized atomic and/or molecular “fragments” of water molecules, such as H+, O-, and perhaps hydroxyl [HO+] ions). Jim O’Dell’s extensively thought-out propulsion idea would make an informative Centauri Dreams article…

    • J. Jason Wentworth May 9, 2018, 4:58

      The authors of “Epic Moon” do look ignorant–perhaps willfully so, to avoid ridicule, as you mentioned in connection with SETI–regarding TLP, though. A mechanism that causes some of them was even discovered by the later Apollo missions (the ones that carried extensive survey instruments in the SIM bays of their Service Modules). TLP most often occur along the edges of the maria (and sometimes in the floors of certain craters), and:

      Underground uranium deposits produce radon gas, which collects in pockets until it erupts, levitating lunar dust. By scanning for the relative abundances of two isotopes of radon produced by uranium (one of which decays before it can filter out above the surface) as the Apollo CSMs orbited overhead, they were able to determine the depths of the deposits. Yes, Patrick Moore had a lot of fun at the expense of Hanns Hörbiger’s WEL (“World Ice Theory”), according to which all celestial objects–even the stars–except for the Earth were made of ice! :-)

  • Michael April 29, 2018, 2:02

    I would have thought that the planet’s night time sky would be lit up with shooting stars, it would be an amazing sight.

    • J. Jason Wentworth April 29, 2018, 23:47

      That depends on the dust particle sizes (and on the density of the exoplanet’s atmosphere–while low atmospheric density usually implies low gravity, this isn’t a hard-and-fast rule; Venus and Earth have similar gravitational field strengths [Venus’ surface gravity is ~90% of ours], yet out atmosphere is much thinner *and* shallower than that of Venus), and:

      Below a certain dust particle size, such micrometeoroids are too small to burn up and produce brilliant meteors. They are so lightweight that they are quickly slowed down by the aerodynamic drag, so that they never burn, but just drift down to the ground (after meteor showers–during which the larger particles produced bright, highly-luminous meteor streaks across the sky–micrometeorites have been found on flower petals). Also:

      If those dusty exoplanetary systems contain a similar gradation of particle sizes (I’d guess that they probably do), their planets should have tremendous shooting star displays. If the dust is very fine, like the particles of Jupiter’s gossamer ring (its particles are about the size of the particles in cigarette smoke), their meteor showers would be paradoxically paltry, but a lot of micrometeorites would “dust up” the ground below; either way, those planets would have impressive zodiacal light and gegenschein illumination in the sky (both are sort of “long-playing meteors”).

    • ljk April 30, 2018, 8:55

      If you would like to have your very own meteorite dust, just go up onto the roof of your house….


      • J. Jason Wentworth May 1, 2018, 4:45

        *Nods* Rain gutters also collect meteoric dust for us; they have been examined for such “incident” dust for decades (small but usable samples are extracted from the Earth-originating dust and dirt).

  • Michael C. Fidler April 30, 2018, 9:47

    The exterrestrials that must have cleared out the goldilocks zone of dust around Vega to put the wormhole in for the movie Contact! :-)

    • ljk April 30, 2018, 11:11

      The ETI in Contact had set up their giant galactic listening/transmitting station to orbit the star Vega in the diametrically opposite pattern from the dust plane. I think any cosmic wormholes would be impervious to such things, or easily deflected if you can actually build one.

      As to why Vega was chosen for Contact in both the novel (1985) and film (1997) versions, see here:

      http://www.coseti.org/klaescnt.htm#The Star Vega

      To quote from the above essay section:

      The main reason Vega was likely chosen as the star where the ETI transmitted from was due to the discovery made by the Infrared Astronomical Satellite (IRAS) in 1983 from Earth orbit. IRAS detected a debris disk around Vega, the first ever witnessed. Astronomers theorized that our Sol system and others formed from the condensation of interstellar dust and gas called protoplanetary disks. The center of the disk collapsed into a fusion-powered star. Debris knots in the outer parts of the disk turned into the planets, moons, and minor bodies.

      It was thought that the debris seen circling Vega was evidence of a solar system in the making, short-lived as it may be on the celestial scale of stellar events. This meant that if Vega had such a disk, perhaps then other more stable stars also formed planetary systems in this manner, which in turn could have organisms evolving upon them. While it was not the same as finding either actual extrasolar planets or life, it was an exciting step in that direction.

      Other protoplanetary disk discoveries from the IRAS data soon followed, but Vega was remembered as being the first. This major astronomical find of the early 1980s is probably what inspired Sagan to make Vega the place to locate the ETI’s interstellar listening post for the novel, which was published in 1985.

      In the film, when the Message is first picked up by Ellie’s team at the VLA and its galactic source determined (they got its right ascension and declination correct, by the way), the staff debates whether the signal could actually come from Vega. The star was rightly considered to be too new and surrounded by debris. They speculate that perhaps any ETIs in that system are just visiting and possibly using “laser blasters and photon torpedoes” to keep the gas, dust, and rocks from causing a fatal impact with their starship. In the novel, the ETI satellite orbited Vega so as to avoid the debris disk.

      There was one thing that Sagan and the other astronomers of the 1980s did not know about Vega: The debris disk might actually be just the star’s equatorial region bulging out from Vega’s very rapid rotation rate. In 1994, Canadian astronomers discovered that Vega was rotating once on its axis every eleven hours, instead of five days as previously thought. For comparison, our sun rotates along its equator once every twenty-five days. This caused its poles to flatten and its middle to extend outward. They also determined that one of Vega’s poles was pointing towards Earth, not its equator. So while there may be many stars with early planetary systems throughout the Milky Way galaxy, Vega no longer appears to be among them. Of course, with the pace of new discoveries in astronomy these days, this too could change in the near future.

      • J. Jason Wentworth May 1, 2018, 4:53

        So Vega, with its very rapid rotation, is actually an oblate ellipsoid, and may be having “starstuff” flying off its equator (as Pleione in the Pleiades is depicted in “Cosmos”). If it were closer to us than Sirius, Vega might be the prime “Exotic Astrophysics” target star in the stellar targets list in “The Starflight Handbook” (I’m sure it will be visited by starprobes, just later, as more distant Mercury was visited after closer Venus and Mars were).

  • ljk September 20, 2018, 16:13


    A deep search for planets in the inner 15 au around Vega

    Tiffany Meshkat, Ricky Nilsson, Jonathan Aguilar, Gautam Vasisht, Rebecca Oppenheimer, Kate Y. L. Su, Eric Cady, Thomas Lockhart, Christopher Matthews, Richard Dekany, Jarron Leisenring, Marie Ygouf, Dimitri Mawet, Laurent Pueyo, Charles Beichman

    (Submitted on 18 Sep 2018)

    We present the results of a deep high-contrast imaging search for planets around Vega. Vega is an ideal target for high-contrast imaging because it is bright, nearby, and young with a face-on two-belt debris disk which may be shaped by unseen planets. We obtained J- and H-band data on Vega with the coronagraphic integral-field spectrograph Project 1640 (P1640) at Palomar Observatory. Two nights of data were obtained in 2016, in poor seeing conditions, and two additional nights in more favorable conditions in 2017. In total, we obtained 5.5 hours of integration time on Vega in moderate to good seeing conditions (<1.5").

    We did not detect any low mass companions in this system. Our data present the most sensitive contrast limits around Vega at very small separations (2-15 au) thus far, allowing us to place new constraints on the companions which may be sculpting the Vega system. In addition to new constraints, as the deepest data obtained with P1640, these observations form the final legacy of the now decommissioned instrument.

    Comments: Accepted for publication in AJ

    Subjects: Earth and Planetary Astrophysics (astro-ph.EP)

    Cite as: arXiv:1809.06941 [astro-ph.EP]
    (or arXiv:1809.06941v1 [astro-ph.EP] for this version)

    Submission history

    From: Tiffany Meshkat [view email]

    [v1] Tue, 18 Sep 2018 21:27:35 GMT (226kb,D)