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TRAPPIST-1: Of Flux and Tides

Seven planets of roughly Earth-size make TRAPPIST-1 a continuing speculative delight, as witness the colorful art it generates below. And with three of the planets arguably in the star’s habitable zone, this diminutive star attracts the attention of astrobiologists anxious to examine the possible parameters under which they orbit. One thing that is only now receiving attention is the question of planet-to-planet tidal effects, as opposed to the star’s tidal effects on its planets.

Image: An artist’s impression of the perpetual sunrise that might greet visitors on the surface of planet TRAPPIST-1f. If the planet is tidally locked, the “terminator region” dividing the night side and day side of the planet could be a place where life might take hold, even if the day side is bombarded by energetic protons. In this image, TRAPPIST-1e can be seen as a crescent in the upper left of the image, d is the middle crescent, and c is a bright dot next to the star. Credit: NASA/JPL-Caltech.

In our Solar System, we’ve become familiar with the idea that tidal deformation can cause interior heating, a fact that could well support both Europa at Jupiter and Enceladus at Saturn with energy needed to retain temperatures suitable for life below their icy surfaces. The effects are extreme at Io (though hardly life-inducing!) and also noteworthy on Neptune’s large moon Triton. Here again TRAPPIST-1 stands out, because we know of no other system where planets, not moons, are so tightly wound that they can raise significant tides on each other.

Consider TRAPPIST-1g, the sixth planet in the system, which according to a study performed by Hamish Hay and Isamu Matsuyama (Lunar and Planetary Laboratory, University of Arizona) experiences the mixed effects of tidal heating from the central star and the other planets more strongly than any other planet in the system.

Tides from the other planets in a planetary system are rarely seen as a factor, say the scientists, but heating due to tidal deformation is definitely in play here. From the paper:

Such tides are typically negligible because the mass of the central tide raising body is usually far greater than other bodies in the system, and also because the distances between these bodies are vast and the strength of tidal forces decreases with the distance between them cubed. The seven planet extrasolar system TRAPPIST-1…is the first system to be discovered where this is not the case. The separation distance at conjunction is small enough that tides raised by neighbouring planets can become significant, and heating must occur as a result.

Similarly, TRAPPIST-1’s two inner planets come close enough to raise powerful tides on each other, possibly sustaining volcanic activity on worlds that would be too hot on the day side to support life. An atmosphere maintained by volcanic eruptions could move heat to the night side, assuming tidal lock.

Image: An artist’s concept for a view of the TRAPPIST-1 system from near TRAPPIST-1f. The system is located in the constellation Aquarius and is just under 40 light-years away from Earth. Credit: NASA/JPL-Caltech.

The Trouble with TRAPPIST-1e

We’ve also recently looked at Lisa Kaltenegger’s work on the effect of intense radiation on M-dwarf planets (see M-Dwarfs: Weighing UV Radiation and Habitability). Kaltenegger (Cornell University/Carl Sagan Institute) has been investigating possible ways for life to survive the intense flares and ultraviolet radiation that pummel such worlds. Various mechanisms suggest themselves, enough to keep open the possibility that planets like these could sustain life.

What Federico Fraschetti (Harvard Smithsonian Center for Astrophysics) and colleagues have been studying is the ability of a star so much cooler and less massive than the Sun to emit such quantities of radiation. The scientists have simulated the path of high-energy protons through the magnetic field of the star, finding that the first of the three TRAPPIST-1 planets thought to be in the habitable zone (TRAPPIST-1e) is receiving up to 1 million times more flux than Earth.

We’re fortunate, of course, in being protected by our planet’s magnetic field from our star’s energetic proton bath, but Fraschetti’s calculations show that to have the same effect at TRAPPIST-1e, the planet’s magnetic field would need to be hundreds of times more powerful than Earth’s. The conclusion is based on the star’s most likely field alignment, which brings its energetic protons directly to the surface of TRAPPIST-1e, where damaging biological effects could occur. But much depends upon how the star’s magnetic field is angled away from its axis of rotation, making this a key datapoint for future investigations. From the paper:

Based on the scaling relation between far-UV emission and energetic protons for solar flares by Youngblood et al. (2017), we estimate that the innermost putative habitable planet, TRAPPIST-1e, is bombarded by a proton flux up to 6 orders of magnitude larger than experienced by the present-day Earth. Such a bombardment of planets in this study is found to result largely from the misalignment of the B-field/rotation axis assumed for the star-proxy. Since the exact magnetic morphology and alignment of the magnetic field is currently unknown for TRAPPIST-1, and for M dwarfs in general, our results indicate that determination of these quantities for exoplanet hosts would be of considerable value for understanding their radiation environments.

TRAPPIST-1e, then, may need some of Lisa Kaltenegger’s proposed solutions to the radiation flux problem if it is to be considered habitable. Lithophilic life, or perhaps life beneath an ocean, is one solution among those that Kaltenegger has proposed, and of course there is the possibility of tidal lock, which could keep the ‘dark’ side of the planet free of the flux. Habitability, as we continue to learn, is by no means an easy call, no matter where a planet is located within or without the putative habitable zone of its host.

The papers are Fraschetti et al., “Stellar Energetic Particles in the Magnetically Turbulent Habitable Zones of TRAPPIST-1-like Planetary Systems,” Astrophysical Journal Vol. 874, No. 1 (18 March 2019) (abstract / preprint); and Hay & Matsuyama, “Tides Between the TRAPPIST-1 Planets,” Astrophysical Journal Vol. 875, No. 1 (9 April 2019) (abstract / preprint).

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{ 35 comments… add one }
  • Thomas R Mazanec April 18, 2019, 13:53

    no matter where a star is located within or without the putative habitable zone of its host.

    You mean where the planet is located?

    • Paul Gilster April 18, 2019, 15:32

      Tom, thanks for catching my typo. Just fixed it in the text.

      • ljk April 22, 2019, 9:17

        But does it have habitable life? From The Black Hole, 1979. :^)

  • Gary Wilson April 18, 2019, 16:54

    Are we even sure that Trappist-1e has a magnetic field, before talking about its morphology and alignment?

    • Harold Shaw April 19, 2019, 13:16

      The comments concerning magnetic field morphology and alignment are directed at the star.

      • Gary Wilson April 22, 2019, 18:42

        Oh I see Harold. Thank you. I should have been able to pick that up.

  • Gary Wilson April 18, 2019, 16:56

    I should probably have also asked are we sure every rocky planet has an iron core? Do we have an exact enough density for 1e to speculate that it therefore must have a magnetic field? Even if it does have an iron core could it possibly have cooled to a solid, not a rotating liquid?

    • Nicky April 18, 2019, 22:48

      Fe, Mg, and Si have condensation temperatures very close to each other, meaning if rocky planets contain silicates they likely contain iron as well. A pure silicate planet with no iron content is rather unlikely from our understanding of planet formation.

      • Gary Wilson April 19, 2019, 14:32

        Thank you Nicky. That explains a great deal. But if the hypothetical iron core had cooled and solidified there would be no significant magnetic field would there? I’m just thinking of the example of Mars. I realize Mars is much smaller and it’s core cooling processes may have occurred much more rapidly but even so the question remains I think..

        • Nicky April 20, 2019, 1:55

          We still need a complete picture of core dynamo, especially why Venus lacks a magnetic field. From planetary dynamic perspective, magnetism is likely a natural outcome of terrestrial planet core cooling instead of anthropic selection, but we don’t know the frequency.

          • Gary Wilson April 22, 2019, 18:48

            If Venus, which is roughly the same size as Earth has no significant magnetic field it might point to a much wider diversity of cores than we suspect for rocky planets. Is it just a case of varying rates of core cooling? I have always wondered how long our molten core will remain molten. I know there are radioactive heat producing species in our Earth’s core. Are they significantly contributing to the length of time the core will remain molten? Also, how long will the core continue to rotate and how variable are molten core rotation rates going to turn out to be? Perhaps Venus had a very slowly rotating core to begin with? Some very interesting questions to address.

    • Bruce D. Mayfield April 19, 2019, 10:45

      Iron is the most common metallic element in space, and it is what causes core-collapse supernova to blow. Without Fe could there even be such a thing as a rocky planet? Quite likely not I would think.

  • Michael April 18, 2019, 17:41

    It’s nice to see the red dwarf depicted in its natural colours, not red but an orange hue.

  • Nicky April 18, 2019, 22:43

    We should treat these numbers with extra caution, as the authors themselves said it. They are likely the *overestimated upper* limits of proton flux. The proton events and CMEs of active stars are probably not as devastating as we once thought.

    Proton events are accelerated in two ways through flares and CMEs, which the later are often closely associated with strong flaring on our Sun. Flares have been observed on other stars, but proton events and CMEs have not been confidently identified other than these of the Sun. The proton flux given in Fraschetti et al 2019 is not based on numerical simulation but empirical extrapolation derived from our solar activity.
    However, more and more studies have shown that for the highly active stars such extrapolation is no longer valid.

    An early attempt of solar-CMEs extrapolation in Drake et al 2013 found that for the active stars (young or low-mass stars), the extrapolated required kinetic energy for CME is a staggering fraction of 10% of the stellar bolometric luminosity, which is apparently unphysical. They thus concluded that CME mass versus flare energy must flatten off or drop significantly after reaching a certain activity level.
    The same conclusion was reached 8 years earlier in 2005 by Wood et al. Although individual CME is undetectable, the collective effect can be measured as total mass-loss rate which is a blend of stellar wind and CMEs. The measured mass-loss rates do increase with higher activity level but eventually reach a threshold where they drop significantly.
    Recent numerical simulation by Alvarado-Gómez et al 2018 shows that a strong enough stellar magnetic field (~>75 G) can almost fully suppress CMEs, explaining why stellar mass-loss rate decreases after reaching a certain activity level.

    Numerical simulation presented by Fraschetti et al 2019 demonstrates that almost all proton events accelerated by flaring are trapped and collapse back within a short distance from Trappist-1 star, and only the ones accelerated by CMEs are able to impact the planets. Yet Trappist-1 has measured magnetic field reaching 600 G, thus it is not known how much protons would actually the star to affect the planets and most of them probably fail.
    Fraschetti et al 2019 discussed CMEs confinements and proton trapping in section 7.2, and they concluded that the calculations are highly uncertain:
    “However, recent findings that CMEs can be strongly suppressed by
    strong stellar magnetic fields (Drake et al. 2016; Alvarado-Gómez et al.
    2018) point to a consequent large uncertainty in our understanding of the
    EP fluxes that exoplanets around active stars sustain.”

    All these studies are interesting to look at!

  • Nicky April 19, 2019, 1:17

    For Trappist-1 planets, space weather and stellar radiation are probably not the main obstacles to habitability. Because all the habitable-zone planets seem to have relatively low density (Grimm et al 2018), the planets likely contain > 5 wt% water, and this amount of water is equivalent to an ocean with depth of several hundred km (Dorn et al 2018). It also means that the atmosphere and water stripped by stellar wind and radiation are insignificant compared to the planets’ total volatile inventories (Bourrier et al 2017). Being in the habitable-zone, lack of carbonate-silicate cycle on these oceanplanets would result a globally frozen surface. A thick surface ice-shell is excessively safe enough to shield the bottom ocean life from stellar and cosmic radiation. However, it is still debatable if abiogenesis can happen at the bottom of a thick ocean like that where the nutrient flux is strongly limited. Regardless, this kind of life is undetectable without in situ flyby or even submarines, so I don’t put much hope into Trappist-1 for detecting alien life.

    • Michael C. Fidler April 19, 2019, 9:26

      Nicky, wonder if you could give the info to find these papers on CME’s? The big question is what would be the effect of tidal locking and exposure to high temperature on the sun facing ocean worlds. The possibilities of crustal tides and volcanic mountain building could make areas of exposed land on even deep ocean worlds. What of ALMA, have they done any imaging for dust belts?

      • Nicky April 20, 2019, 1:27

        Hi Fidler, I’m glad that you’re interested.

        The CMEs papers are listed here:
        Jeremy J. Drake et al 2013 ApJ 764 170
        Wood B. E., Müller H.-R., Zank G. P., Linsky J. L. and Redfield S. 2005a ApJL 628 L143
        Odert P., Leitzinger M., Hanslmeier A. and Lammer H. 2017 MNRAS 472 876
        Alvarado-Gómez, J. D., Drake, J. J., Cohen, O., Moschou, S. P., & Garraffo, C. 2018, ApJ, 862, 93

        I don’t believe the day-side hemisphere has any negative consequence for facing the sun permanently, because ice albedo is high enough to keep the ocean surface frozen.

        The mean elevation difference between continents and ocean basins on Earth is 4-5 km. Compared to the ocean depth on these water-rich planets, the exo-mountains or islands need to be 100 times thicker to emerge above sea-level, because we are talking depth of several hundred km. This is obviously unphysical even considering the strong tidal heat flux. Young Earth was also heated by intense tides and radioactivity yet the emergence of large landmasses did not happen until 3 billion years ago even under very little presence of water with mass fraction at least 50-100 times smaller than Trappist-1 planets. Unless the new TTV measurements obtained by Spitzer Space Telescope in March-April 2019 will refine the planets to higher density or rockier composition, water-rich structure remains the best interpretation of the current TTV data.

        ALMA’s sensitivity is not high enough to detect dust belts around Trappist-1.

        • Michael C. Fidler April 20, 2019, 10:02

          Thanks Nicky, One issue I’ve been wondering about is just how much water could be stored in the mantle of these planets. We only have our earth to see what could be, but these planets formed under completely different conditions from our solar system. Modeling the internal characteristics of such world is still in its infancy as is understanding the chemical, mineral and geology of them.

          https://www.quantamagazine.org/the-hunt-for-earths-deep-hidden-oceans-20180711/

          https://www.astrobio.net/news-exclusive/scientists-detect-evidence-oceans-worth-water-earths-mantle/

          • Nicky April 21, 2019, 0:50

            Yes, H2O is structurally bonded into mantle silicate minerals, composing of mainly magnesium and silicon. Earth’s mantle H2O storage capacity is estimated to be around 10 oceans worth of water, less than 0.3 wt% of Earth mass.
            Trappist-1 planets are most likely made of the same elements, because many stellar abundance measurements have shown that iron, magnesium and silicon are the most abundant refractory elements in making up planets. Therefore, we can safely argue that these planets cannot store more than 0.5 wt% of water in terms of mass fraction, but here we are talking about planets with water mass fractions ≥ 5%, so they must have some significant volatile layer.

            Two scenarios that would avoid using massive ocean layer to explain the density data are that the planets are either iron-poor or core-less. Both scenarios are rare but considering the vast diversity of planet structures they are still possible.

            • Michael C. Fidler April 22, 2019, 3:03

              So could the UV flares reflection be used to probe the atmosphere and oceans on these planets?

              • Nicky April 22, 2019, 23:57

                Currently, direct detection of rocky planet atmosphere is beyond our capability. Well, there is other way of doing it with less direct method.

                Water loss on Earth today is mainly driven by Jeans’ escape, which a small fraction of hydrogens attain enough kinetic energy to escape Earth. The evaporation of atmosphere forms an extended hydrogen exosphere. When a water- or hydrogen-rich planet is highly irradiated, hydrodynamic escape (runaway greenhouse) creates a far-extended exosphere that is way easier to be detected in Lyman-alpha line (the spectral line of hydrogen) than the compact lower atmospheres at longer wavelengths. Two well studied samples are GJ 3470 b and GJ 436 b (Bourrier et al., 2018; Ehrenreich et al., 2015).
                For Trappist-1, HST observations already rule out hydrogen-dominated atmospheres on planet b, c, d, e, f and g (de Wit et al., 2016; de Wit et al., 2018; Wakeford et al., 2018). The search for absorption signatures in Lyman-alpha line during the transits of planet b and c with HST has yielded *tentative evidence* of extended hydrogen exospheres (Bourrier et al., 2017; Bourrier et al., 2017). More observation is needed to confirm this detection. If the discovery is proven, it not only means that planet b and c still hold a large amount of water as their radius-mass implies, outer habitable-zone planets e, f and g most certainly contain even more water.

                The tentative detection of hydrogen exosphere is presented in two papers:
                Section 3.3 in V. Bourrier et al 2017 AJ 154 121
                Section 5 in Bourrier V., Ehrenreich D., Wheatley P. J. et al 2017 A&A 599 L3

                • Michael C. Fidler April 23, 2019, 9:35

                  But what I’m pointing at is the large UV flares lighting up these planets and their ozone layer. The intense flares around Proxima Centauri would be close enough and bright enough. If c or any other planets exist they should show as separate reflections with c being over 1 arcsecond away and 12 minutes later then the original flare.

                  • Nicky April 23, 2019, 17:08

                    I believe you’re talking about detection of light echoes. A very preliminary feasibility study (Sparks et al., 2018) has shown that this concept can be used to characterize temperate planets around flaring M-dwarfs. Right now it is very new and has not been actually tested. It still lacks detailed calculation and feasibility study regarding of telescope sizes, integration times, S/N, specific target stars, etc. At this point it might seem to be a viable method, but we don’t know specifically how it would be achieved with HST.
                    I would say transmission spectrum remains the best way to characterize exoplanet atmospheres.

                    • Michael C. Fidler April 24, 2019, 4:06

                      Thank you so much Nicky,, that is exactly what I’m talking about! The only other paper that one of the commenters brought up is the “Stellar Echo Imaging of Exoplanets” 2016 NASA NIAC Phase I Final Report.
                      Which is a much more complicated long range project. I found the paper that you mentioned and it is very interesting.

                      Direct detection and characterization of M-dwarf planets using light echoes.

                      William B. Sparks, Richard L. White, Roxana E. Lupu, Holland C. Ford
                      (Submitted on 3 Jan 2018)
                      Exoplanets orbiting M dwarf stars are a prime target in the search for life in the Universe. M dwarf stars are active, with powerful flares that could adversely impact prospects for life, though there are counter-arguments. Here, we turn flaring to advantage and describe ways in which it can be used to enhance the detectability of planets, in the absence of transits or a coronagraph, significantly expanding the accessible discovery and characterization space. Flares produce brief bursts of intense luminosity, after which the star dims. Due to the light travel time between the star and planet, the planet receives the high intensity pulse, which it re-emits through scattering (a light echo) or intrinsic emission when the star is much fainter, thereby increasing the planet’s detectability. The planet’s light echo emission can potentially be discriminated from that of the host star by means of a time delay, Doppler shift, spatial shift, and polarization, each of which can improve the contrast of the planet to the star. Scattered light can reveal the albedo spectrum of the planet to within a size scale factor, and is likely to be polarized. Intrinsic emission mechanisms include fluorescent pumping of multiple molecular hydrogen and neutral oxygen lines by intense LyAlpha and LyBeta flare emission, recombination radiation of ionized and photodissociated species, and atmospheric processes such as terrestrial upper atmosphere airglow and near infrared hydroxyl emission. We discuss the feasibility of detecting light echoes and find that under favorable circumstances, echo detection is possible.
                      https://arxiv.org/abs/1801.01144

                      But the new Proxima c is much further away, about 1.15 arcsecond from Proxima Centauri and the flare takes longer to reach it so it should be much easier to detect. Of course this is if it actual exist, but is there any instruments on the ground that will work???

                  • Nicky April 25, 2019, 14:44

                    Current instruments would not be able to detect planet c around Proxima even it turns out to be a Jupiter-size planet, see Table 2 in “Stellar Echo Imaging of Exoplanets 2016 NASA NIAC Phase I Final Report”

                    • Michael C. Fidler April 25, 2019, 22:40

                      I believe we are talking about two different techniques:
                      “Table 2 provides a few representative calculations. The time required to detect an echo is minimized by
                      enhancing the coronagraph contrast, maximizing the size of the telescope collection area, and maximizing
                      the available spectral bandwidth, provided that the fluctuations occur across the entire bandwidth (which
                      depends on the phenomenology—a rapidly fluctuating narrow bandwidth can be more valuable than a more
                      stable wide-band measurement). As indicated by these calculations, the stellar echo detection technique is
                      directly viable for detecting exoplanets that are too close to their host star to be resolved by direct imaging
                      techniques. For wider separations or smaller planet-star contrast ratios, a coronagraph becomes necessary.
                      However, it is not necessary to have a state-of-the-art 10 9 blocking power coronagraph to produce useful
                      echo data that cannot be obtained through other means”

                      DIRECT DETECTION AND CHARACTERIZATION OF M-DWARF PLANETS USING
                      LIGHT ECHOES
                      “M dwarf stars are active, with powerful flares that could adversely impact prospects for life, though there
                      are counter-arguments. Here, we turn flaring to advantage and describe ways in which it can be used to enhance the detectability of planets, in the absence of transits or a coronagraph, significantly expanding the accessible discovery and characterization space. Flares produce brief bursts of intense luminosity, after which the star dims. Due to the light travel time between the star and planet, the planet receives the high intensity pulse, which it re-emits through scattering (a light echo) or intrinsic emission when the star is much fainter, thereby increasing the planet’s
                      detectability. The planet’s light echo emission can potentially be discriminated from that of the
                      host star by means of a time delay, Doppler shift, spatial shift, and polarization, each of which can
                      improve the contrast of the planet to the star. Scattered light can reveal the albedo spectrum of the planet to within a size scale factor, and is likely to be polarized. Intrinsic emission mechanisms
                      include fluorescent pumping of multiple molecular hydrogen and neutral oxygen lines by intense Lyα and Lyβ flare emission, recombination radiation of ionized and photodissociated species, and atmospheric processes such as terrestrial upper atmosphere airglow and near infrared hydroxyl
                      emission.”

                      If Hubble can do it it can be done from the ground and one arcsecond is easily resolved with my 16″ telescope, so all this argument is about is having a relatively small telescope at high enough altitude to for UV observation. This does depend on the inclination of exoplanet to our line of sight from Proxima Centauri and the phase the planet is in when the flare takes place. BUT the most important and why a small scope will work is that it can monitor Proxima Centauri continuously in the UV and record it in video with a cheap astro camera.

                    • Michael C. Fidler April 26, 2019, 10:25

                      Meredith MacGregor

                      Space Telescope Live.
                      And so it begins! On 11 nights in the next 2 weeks, we will be doing a multi-wavelength monitoring campaign of Proxima Cen using @NASAHubble @NASASwift @almaobs @NASA_TESS @chandraxray @EvryscopeCTIO @LCOAstro to study flaring activity!!!

                      From Twitter.

                    • Nicky April 26, 2019, 12:45

                      Angle of separation is not a problem, but the sensitivity of current telescopes do not reach the required star-planet contrast ratio of 10^-7

        • Michael April 22, 2019, 13:54

          @Nicky

          ‘I don’t believe the day-side hemisphere has any negative consequence for facing the sun permanently, because ice albedo is high enough to keep the ocean surface frozen.’

          Ice is surprisingly absorbing of infrared radiation.

          • Nicky April 22, 2019, 22:31

            Ice is less reflective when the spectrum is redder, but it doesn’t mean that these planets can deglaciate themselves, especially planet f and g. Planet e might be able to keep liquid ocean surface right blew the substellar point, but it is basically still a snowball.

  • Andrew Palfreyman April 19, 2019, 2:38

    The magnetic characteristics of Proxima are yet one more interesting facet for Starshot-type probes to investigate. This is a good example of the only way to get the data is to actually go there. It’s motivating.

  • wdk April 19, 2019, 20:10

    A grim picture of charged particle bombardment. Wonder how it compares with Jovian satellites? The authors also note an assumption
    of stellar magnetic field perpendicular to planetary orbital plane.
    Could be the case, but I wonder if there is much observational data to support this for the M dwarfs in general or this particular case? After all,
    until the 1990s, our assumptions for potential nearby planetary systems
    were arbitrary orientations, rotational data for the stars being what they may…

  • Paul451 April 23, 2019, 6:35

    “there is the possibility of tidal lock, which could keep the ‘dark’ side of the planet free of the [proton] flux.”

    Not necessarily. Because of the interaction between flares/plasma and the magnetic field, solar wind rarely flows in a nice straight line. Instead, it tends to approach isotropic, if not uniformly so.

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