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Red Dwarf Planets May Lack Needed Volatiles

We can identify a number of circumstellar disks, but most are too far away to provide internal detail, much less the kind of activity that seems to be showing up around the red dwarf AU Microscopii. For at 32 light years out in the southern constellation Microscopium, AU Microscopii is presenting us with an unusual kind of activity that may have repercussions for the question of life around red dwarf stars in general. As presented at the recent meeting of the American Astronomical Society, fast-moving blobs of material are eroding the disk.

The consequence: Icy materials and organics that might have developed in asteroids and comets may instead be pushed out of the disk, long before they could provide the infall of materials thought to have benefited planets like ours. “The Earth, we know, formed ‘dry,’ with a hot, molten surface, and accreted atmospheric water and other volatiles for hundreds of millions of years, being enriched by icy material from comets and asteroids transported from the outer solar system,” said co-investigator Glenn Schneider (Steward Observatory, Tucson, Arizona).

Image: These two NASA Hubble Space Telescope images, taken six years apart, show fast-moving blobs of material sweeping outwardly through a debris disk around the young, nearby red dwarf star AU Microscopii (AU Mic). The top image was taken in 2011; the bottom in 2017. Hubble’s Space Telescope Imaging Spectrograph (STIS) took the images in visible light. This comparison of the two images shows the six-year movement of one of the known blobs (marked by an arrow). Credit: NASA, ESA, J. Wisniewski (University of Oklahoma), C. Grady (Eureka Scientific), and G. Schneider (Steward Observatory).

Researchers estimate that the blob of material in the image above is moving at about 24,000 kilometers per hour. It would have moved more than 1.3 billion kilometers between 2011 and 2017, roughly the distance between the Earth and Saturn when the two are at their closest approach to one another. Continually pushing small particles containing water and other volatiles out of the system, such circumstellar materials could cause the AU Microscopii disk to dissipate in 1.5 million years. Each blob — and thus far the team has found six of them — is thought to mass four ten-millionths the mass of Earth.

The ejection speeds among the six identified blobs range between 14,500 kilometers per hour and 43,500 kilometers per hour, well beyond escape velocity for the star. Their current distance ranges from 1.5 billion kilometers from the star to more than 8.8 billion kilometers. AU Microscopii’s relative proximity makes it possible for Hubble to resolve substructure in at least one of the blobs, which may eventually make it possible to discover their origins.

Image: The box in the image at left highlights one blob of material extending above and below the disk. Hubble’s Space Telescope Imaging Spectrograph (STIS) took the picture in 2018, in visible light. The glare of the star, located at the center of the disk, has been blocked out by the STIS coronagraph so that astronomers can see more structure in the disk. The STIS close-up image at right reveals, for the first time, details in the blobby material, including a loop-like structure and a mushroom-shaped cap. Astronomers expect the train of blobs to clear out the disk within only 1.5 million years. The consequences are that any rocky planets could be left bone-dry and lifeless, because comets and asteroids will no longer be available to glaze the planets with water or organic compounds. Credit: NASA, ESA, J. Wisniewski (University of Oklahoma), C. Grady (Eureka Scientific), and G. Schneider (Steward Observatory).

We wind up with planets lacking the nearby volatiles to enrich them, giving us the prospect of dry, dusty worlds without life. We can add this to the other factors that challenge the emergence of life around red dwarf stars, such as possible tidal lock and the resulting climate issues, not to mention heavy ultraviolet flux from young stars that could strip away the atmosphere of planets in the habitable zone. AU Microscopii is itself 23 million years old, an infant in stellar terms. Bear in mind that red dwarfs are the most common type of stars in the galaxy.

“The fast dissipation of the disk is not something I would have expected,” says Carol Grady (Eureka Scientific, Oakland, California), a co-investigator on the Hubble observations. “Based on the observations of disks around more luminous stars, we had expected disks around fainter red dwarf stars to have a longer time span. In this system, the disk will be gone before the star is 25 million years old.”

The AU Microscopii data were gathered by the European Southern Observatory’s Very Large Telescope in Chile as well as the Hubble Space Telescope Imaging Spectrograph (STIS) by a team led by John Wisniewski (University of Oklahoma). The STIS visible light images, taken in 2010-2011, were followed up by near-infrared work at the the SPHERE (Spectro-Polarimetric High-contrast Exoplanet Research) mounted on the VLT. The work also draws on disk observations of AU Microscopii by the Hubble Advanced Camera for Surveys in 2004.

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{ 24 comments… add one }
  • Adam Wu January 16, 2019, 15:49

    Water could be added back to such a desiccated planet by cometary impacts after formation, however.

  • Alex Tolley January 16, 2019, 17:01

    “The Earth, we know, formed ‘dry,’ with a hot, molten surface, and accreted atmospheric water and other volatiles for hundreds of millions of years, being enriched by icy material from comets and asteroids transported from the outer solar system,”

    Do we know that? I thought that it was now reasonably established that most of the water on/in the Earth was from the accretion of volatile-rich material. Comets and other volatile-rich bodies added to the water, especially after the surface cooled. The surface may have been dry, but not the interior and outgassing would hydrate the surface as it cooled.

    To make the claim the authors’ do, they are assuming that AU Microscopii is typical of red dwarf stars. I don’t think that is warranted until there is confirming evidence from a sample of other red dwarfs.

    • stefan January 17, 2019, 17:45

      Assume the idea that Earth collided with a Mars sized object called Theia and created the moon. During that event any water that had been on Earth’s surface would have been lost. The water currently on Earth’s surface must have appeared later. We can see that the moon solidified and then afterward got bombarded. Comets delivered at least a large portion of the oceans’ mass.
      The Theia collision is standard. Accepted by most academic sources.

      • Alex Tolley January 20, 2019, 20:07

        Don’t be so sure about all that cometary water. If the bulk of Earth’s total water was acquired from volatile-rich bodies during formation, then that water is important to the overall water composition of Earth. The surface may have become molten during the bombardment, but that doesn’t mean that internal water could not have made up much of the oceanic water.

        https://news.agu.org/press-release/scientists-theorize-new-origin-story-for-earths-water/

  • Harold Shaw January 16, 2019, 17:34

    I may have missed something but is there a case made for fast moving blobs being a common feature in red dwarf protoplanetary disks?

  • jonW January 16, 2019, 17:55

    It is endlessly sobering, and not a little grim, that almost all the research presenting new understanding of planetary formation and evolution tends to lead *away* from the probability of things we believe to be favourable to the emergence of life.

    I mean I understand the biases involved (life “as we know it”, etc.) but still.

    • Bruce D. Mayfield January 17, 2019, 11:06

      The universe is what it is, not what people wish that it is.

      • Abelard Lindsey January 18, 2019, 14:30

        I like it!

        Its like Phillip K Dick’s “reality is what continues to exist when you stop believing in it”.

  • Geoffrey Hillend January 16, 2019, 19:34

    I would not go as far as saying the red dwarf planets are lacking volatiles. There still can be volatiles from the protostellar nebula or cloud. It does look like a lot of water in our oceans came from comets in the late heavy bombardment period based on their higher D/H ratio. There is also the atmospheric stripping of gases by the solar wind.

  • DJ Kaplan January 16, 2019, 22:26

    I’m not able to understand some of this, but it stands to reason that red dwarfs provide a much different environment than does our own sun, in regards to not only light spectra, but chemistry and probably a lot of other things as well.
    In studying red dwarfs, however, I think the thing to look at is “what they do have” rather than “what they don’t have”.

    • Bruce D. Mayfield January 17, 2019, 18:31

      One thing RD systems have is extremely (by our solar system’s standard) tight planetary spacing. This would result in easier exchange of materials though impacts. Also, some volatiles striped from inner planets might be captured by outer planets.

  • Michael Fidler January 16, 2019, 22:29

    So all the Trappist 1 planets are dried out Mercuries? That does not sound very logical since their densities are lower.
    https://photojournal.jpl.nasa.gov/jpeg/PIA22094.jpg

    1. What could be happening is the original nebula is high in metals.
    2. An edge on disc with inner planets projecting shadows into nebula.
    3. Star rotates fast, sunspots common and large flares common, could be projected into the planetary nebula disks.

    • Michael Fidler January 16, 2019, 22:34

      4. Flares, sunspots and CMEs could be ejecting material into nebula.

  • Michael Fidler January 16, 2019, 22:46

    5. Fast moving plasma blobs in the geomagnetic tail of planets as in Sol’s Jupiter.
    6. Magnetic re-connection taking place in the plasma of planetary nebula disks.
    7. Magnetohydrodynamic (MHD) events in the nebula’s disk.

  • wdk January 17, 2019, 0:50

    In some discussions we have worried whether likelihood of life would be threatened in the formation of “water worlds”. This indicates to me another violent roll of the dice. But if a world were several times more massive than the earth, I would suspect that its gravity well would make
    complete removal of volatiles more difficult and recovery more effective.
    So if this particular case were more frequent, I would bet that we would
    be finding relatively dessicated Neptunes eventually – martinis but not too dry.

    • Bruce D. Mayfield January 17, 2019, 13:29

      Right. But here also there would be a possible path toward a few exceptions to the rule that Red Dwarfs are out for water+land surface HZ planets; a super earth or mini neptune that can retain just enough of its water under a thick atmosphere. Sure, the atmo is going to get pealed away over time, but slowly enough for life to adapt, for a very long period perhaps.

  • Geoffrey Hillend January 17, 2019, 0:54

    I am leaning towards the idea that red dwarf planets lack volatiles. Spectroscopy should tell us that in the future.

  • ljk January 17, 2019, 10:35

    Just more evidence that we need to go to these star systems DIRECTLY to find out exactly what the stories are there. Either that or we get some serious telescopes into space. Or both.

  • Hal January 17, 2019, 10:48

    Wait
    >Each blob … is thought to mass four ten-millionths the mass of Earth.
    Earth is 6 x 10^24 kg Ten million is 10^7, divide by four gives 2 x 10^17 kg. That is around the mass of Saturnian shepherd moons, Pandora and Prometheus. That’s an entire solar system expelling one small asteroid…

  • Michael January 17, 2019, 12:09

    These blobs should condense into comets, could be a common source of interstellar objects.

  • andy January 17, 2019, 19:04

    I’m not particularly convinced by this conclusion, especially since the observations seem to suggest that volatile-rich planets do exist in close-in orbits around red dwarfs – TRAPPIST-1 and GJ 1214 spring to mind here. Whether planets around red dwarfs can remain in a “sweet spot” volatile abundance where there are enough volatiles for oceans but not so much that the continents are drowned or buried under a high-pressure ice mantle is another matter.

    Nevertheless I suspect that if habitable worlds do exist around red dwarfs, they are not going to be particularly good analogues of Earth even for planets with bulk densities that suggest an Earth-like composition. There might well be multiple “families” of terrestrial planets out there, with Solar System-like in situ formation being merely one possible route to producing rocky worlds. The formation histories of terrestrial planets around other stars may well be vastly different, involving substantial migration from beyond the ice line, possibly with some of their history spent as sub-Neptunes. This could have implications for their internal structure and the minerals making up the bulk of the planet, their geological evolution and suchlike.

  • Ashley Baldwin January 18, 2019, 11:37

    I agree with Michael re Trappist -1 being a practical example that bucks the trend found in this paper.

    Although the masses of most of the planets still have large error margins -that rule out accurate density calculations- this is not the case for Trappist-1 f . It has a low density consistent with a substantial ice ” volatile” fraction . Unlike relatively large M1 AU Microscopii , Trappist-1 is an old star – around 7.5 Gig yrs old. Now relatively quiescent ( 0.38 low energy optical flares /day) . Late M dwarfs like Trappist -1 are very much more active early in their life than later M 1 stars like AU Microscopii. They also have a much longer active pre main sequence period too , which combined should dessicate any protoplanetary diak and impact the formation of protoplanets. But the very existence of the seven Trappist 1 suggests otherwise with the low density of Trappist 1 f also clearly demonstrating the persistence of significant volatile fraction ( it isn’t even the most exterior planet) even after enduring stellar activity for so long . Theories have been formulated to support this state of affairs and currently posit that planets form out around the ice line of the star ( which would presumably be a significant fraction of an AU for an early pre main sequence star like AU Microscopii ) before subsequently migrating inwards . A migration ultimately stopped by mean motion resonanc in a chain of multiple planets.

    It may be that these protoplanets on show around AU Microscopii will be enitely evaporated to be replaced at a later date epoch by planets migrating in from further out. As with biological evolution of species , it is premature to draw conclusions from one cross sectional data set.

    This is exactly why we need a bespoke , multiple protoplanetary disk imaging telescope like EXCEDE.

  • Gary Wilson January 18, 2019, 14:48

    One star system does not a significant data set make. Far too many conclusions from far too little data. We don’t even know the mechanism for the expulsion of these “blobs” do we? And the masses involved are tiny, as Hal pointed out. If we can get data from 1,000 or better yet 10,000 red dwarf systems with planets we might just possibly be able to posit a general rule.

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