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TRAPPIST-1: Seven Worlds of Similar Compostion

Image: Artist’s depiction of the TRAPPIST-1 star and its seven worlds. Credit: NASA/JPL-Caltech/R. Hurt (IPAC).

As if we needed another indication that the TRAPPIST-1 system is utterly different from our own, consider new work led by Eric Agol (University of Washington), which examines this seven-planet system in terms of the planets’ density. The planets here are all similar in size to Earth. Compare that with the huge range in planetary size we see in our own system. The new work tightens orbital dynamics and calculates their densities, showing they are all about 8 percent less dense than the rocky planets around Sol.

In the paper, the TRAPPIST-1 densities are calculated through analysis of abundant data (over 1,000 hours of observation by Spitzer alone, along with significant contributions from Kepler and ground-based telescopes like TRAPPIST and SPECULOOS). They are refined through computer simulations on planetary orbits, showing a system where planetary composition begs for an explanation.

We’re dealing with a system under intense scrutiny, one about which we have ever-tightening parameters on planetary diameter, mass and density, so we have much to work with. The work pauses briefly to consider a possible eighth planet:

Our mass precisions are predicated on a complete model of the dynamics of the system. We ignore tides and general relativity, which are too small in amplitude to affect our results at the current survey duration and timing precision (Bolmont et al. 2020). Should an eighth planet be lurking at longer orbital periods, which has yet to reveal itself via significant TTVs or transits, this may modify our timing solution and shift the masses slightly. In our timing search for an additional planet, however, we found that such a planet might only cause shifts at the ≈1σ level.

Agol worked with colleagues Zachary Langford and Victoria Meadows at the University of Washington, and an international team including scientists based in Switzerland, France, the UK and Morocco. Says Agol:

“This is one of the most precise characterizations of a set of rocky exoplanets, which gave us high-confidence measurements of their diameters, densities and masses. This is the information we needed to make hypotheses about their composition and understand how these planets differ from the rocky planets in our solar system.”

Image: Shown here are three possible interiors of the TRAPPIST-1 exoplanets. The more precisely scientists know the density of a planet, the more they can narrow down the range of possible interiors for that planet. All seven planets have very similar densities, so they likely have a similar compositions. Credit: NASA/JPL-Caltech.

The researchers examine the TRAPPIST-1 planets in terms of the proportion of iron, oxygen, magnesium and silicon thought to make up rocky worlds in any system. The 8 percent difference in density is explicable through several competing hypotheses.

A lower percentage of iron — 21 percent as compared to Earth’s 32 percent — would do the trick, possibly indicating a core with a lower relative mass (most of Earth’s iron is found in its core). Alternatively, planets enriched with oxygen compared to the Earth could produce iron oxide — rust — in abundance, possibly extending to the core (Earth, Mars, Mercury and Venus have cores of unoxidized iron). A combination of both these scenarios is possible, according to Agol, with the TRAPPIST-1 planets having less iron than our system’s rocky worlds but with a larger amount of it being oxidized.

A third possibility is that these planets are enriched with water compared to the Earth. Surfaces covered with water would change a planet’s overall density. This would demand amounts of water totalling 5 percent of the total mass of the four outer planets (by comparison, water makes up less than 0.1 percent of the Earth’s total mass). This would also imply hot, dense atmospheres for the three inner worlds at TRAPPIST-1.

Each hypothesis has consequences in terms of planet formation. Water worlds imply origins beyond the snow line, an environment rich in ice, with the planets subsequently migrating inward toward the host star. Long-term planetary configurations can form from migration, and the stability of TRAPPIST-1’s system has been examined in earlier work. Tightening up the constraints on planetary mass and orbital eccentricity allows the authors to again consider the question. They find that the idea of migration also fits the hypothesis of a fully oxidized planetary core. From the paper:

…the lower measured bulk densities of the TRAPPIST-1 planets relative to Earth-like composition might be explained by core-free interiors (Elkins-Tanton & Seager 2008) in which the oxygen content is high enough such that all iron is oxidized. If the refractory elements (Mg, Fe, Si) follow solar abundances, a fully oxidized interior would contain about 38.2 wt% of oxygen, which lies between the value for Earth (29.7 wt%) and CI chondrites (45.9 wt%). Such an interior scenario can easily describe the observed bulk densities…, and this may bolster the long-range migration scenario in which the planets formed in a highly oxidizing environment that enabled the iron to remain in the mantle even after migration.

One thing that would drive this work further is more information about the composition of the M8 red dwarf that hosts the planetary system. The authors point out that measurements of the Mg/Fe and Fe/Si ratios would affect the interpretation of their results on planetary cores and mantle composition.

And as we see so often in such work, a functioning James Webb Space Telescope is pointed to as a possibility for producing even more precise constraints on system dynamics and planetary bulk densities, which would aid in choosing among the competing hypotheses for composition. For all of this, the TRAPPIST-1 system is an ideal testing ground, as co-author Caroline Dorn (University of Zurich) notes:

“The night sky is full of planets, and it is only within the last 30 years that we have been able to begin to unravel their mysteries. The TRAPPIST-1 system is fascinating because around this unique star we can learn about the diversity of rocky planets within a single system. And we can also learn more about a planet by studying its neighbours, so this system is perfect for that.”

The paper is Agol et al., “Refining the Transit-timing and Photometric Analysis of TRAPPIST-1: Masses, Radii, Densities, Dynamics, and Ephemerides,” Planetary Science Journal Vol. 2, No. 1 (22 January 2021). Full text.

{ 24 comments… add one }
  • Brett January 26, 2021, 11:36

    I’m inclined towards the “smaller iron core” argument. The “heavy water fraction” argument would require the innermost planets to hold on to all that water in spite of intense flaring and light from the parent star – I doubt they’d be so similar to the outer planets in volatile fraction.

    • Edwin January 26, 2021, 16:39

      Yes I’m with the smaller core argument too

    • Ashley Baldwin January 26, 2021, 17:26

      ALL the planets are close in to what is still a very active star ( the reason the authors give for the difficulties characterising its make up- – ‘know the star to know the planet’) despite being eight billion plus years old . That’s a lot of volatile stripping stellar wind , EUV/X-ray flares – and charged particles. Even allowing for delayed inward planetary migration. All combined with the extended pre main sequence luminosity and activity theorised for late M dwarfs championed by Rory Barnes et al.

      Could the similar densities and sizes of the planets be because they are all desiccated and atmosphere free ‘evaporated cores’ ? A depressing thought and a question I’ve asked Michael Gillon , one of the authors. This scenario is not discuss in the text of the article – which was optimistic – and hopefully means it either wasn’t felt reasonable to simulate. Or was simulated without showing a high sigma probability.

      Fingers crossed.

      • Andrei January 27, 2021, 11:23

        The first one uniform shown is indeed a low sigma variety, and only possible if the planets have formed very recently and not differentiated yet. The second least likely was already pointed out by Brett and I only concur, the innermost worlds are unlikely to be water worlds. These planets pass each others so close that they would be visible from the surface. The orbits are stable right now, but if they were the slightest off at the time of formation, which is more likely than not. There might have been significant tidal flexing that also could have driven off volatiles. Taken together this mean that the small iron core is very much a frontrunner.

  • Alex Tolley January 26, 2021, 13:48

    So we have a system with:

    1. An old M-dwarf that has a long lifetime for maintaining life
    2. Fully seven worlds, all rocky, of similar size
    3. 3 are in the HZ (possibly as many as 6) for terrestrial type life.

    Why not speculate that this may be an artificial system constructed to support various advanced species? Are we like the proverbial ants unaware that there are buildings and a highway near the nest? The 7 worlds are all suitable for human-scale ETIs, with 3 planets possibly having open to the sky biospheres, and 4 with enclosed spaces programmed for supporting tailored biospheres or living spaces for biological or machine ETIs. Technology could protect the open sky worlds from any remaining intense solar flares.

    There is lots of living room, easy transport, and communication between the worlds (certainly better than a globular cluster, even if space is far more restricted). Lots of opportunities for cultural diversity. Any rogue ETI culture could be contained by the remaining 6 worlds, maintaining stable, peaceful, political conditions.

    Have these worlds been constructed from other planets now dismantled? Should we look for techno-signatures there? Trappist-1 is only 40 ly away, so a probe at 10% c could be sending back flyby data well within the next millennium.

    • Paul Gilster January 26, 2021, 14:42

      Some SF writer needs to tackle this! Ingenious…

      • Wojciech J February 14, 2021, 19:30

        Already done in the SF game Terra Genesis I believe ;)

    • Michael January 26, 2021, 15:20

      Don’t forget the gravilens only been around 55 AU’s away.

  • Michelangelo Pantaleoni January 26, 2021, 14:00

    The European Southern Observatory with ESA’s Cheops mission has just discovered a 6 planet system similar yo Trappist-1, were 5 of them are in a chain of orbital resonances. It is quite a paradoxical system; on one hand the resonance is extremely delicate and thus this system might have formes very differently than ours (with less chaotic dynamics). On the other hand the densities of the planets make no sense and seem to have formed on very different scenarios.


    • Paul Gilster January 26, 2021, 14:38

      Yes, and this one is in queue as the next post. Putting the finishing touches on it now.

  • Robert Flores January 26, 2021, 14:26

    Out of many studies posted in CDreams, on planet formation and composition, few attack the Stellar Nursery Conditions. Sure, the usual words about collapsing nebulas created from the remains massive progenitor stars are out there.
    But few postings about the effects of the following:
    1) The position of protoplanetary disk within a stellar nursery
    2) Nearby interaction with sibling protoplanetary disk
    3) powerful F or A class stellar neighbors influencing stars such as Trappist

    It can’t believe, that the 3 above had NO influence in the final architecture and composition of the Trappist system.
    This system seems to be an outlier, this seems to imply an unusual
    formation history.

    • Robert Flores January 28, 2021, 0:07

      On second thought, maybe stellar nurseries are poorly
      understood. Maybe there are no good models on what happens
      in them in detail enough to give significant insights into the protostars interactions in them.

  • Michael January 26, 2021, 15:19

    These are Hades world’s, tortured by tidal flexing.

  • Edwin January 26, 2021, 16:02

    Hi Paul

    I was looking forward to your post on this one, its one interesting system and something I have read a lot about.

    I read this from Planet S last night

    I’ll look forward to reading the comments and your post

  • ljk January 26, 2021, 16:43
  • Dave Moore January 26, 2021, 16:49

    It’s interesting that they all have a common density. In our system, we have Mercury with a density of 5.4 and Mars with a density of 4. If the system had this sort of variation, it would presumably be enough to show up in the measurements.

    Looking at the interiors of these planets, if we assume a 5% water, then the base of a steam atm. on these planets would be at 17.5 kilobar, putting it over a molten rock surface. The Oxygen left over from Hydrogen escape would then oxidize the mantle, so there would be no O2 build up as with Venus. At 375 deg C, the superfluid steam changes phase into water, which could still allow mixing between the atm. and mantle. Once however the bottom of the ocean approaches freezing point, a high pressure ice layer would form cutting the atmosphere and mantle off from each other. This could allow a build up of the Oxygen in the atm. from the disassociation of water and Hydrogen escape, so atmospheric spectrography might be use to tell the internal state of the planets.

    Conversely, a lack of Oxygen in outer Trappist planets could be used as way to constrain their water inventory. (This assumes there is no photosynthetic life on the planets to mess us the signal.)

  • Michael Fidler January 26, 2021, 23:15

    This system is not like our solar system but very similar to Jupiter’s. The planets b and c are active volcanic worlds with very similar activity to Io. These two large dense planets are nearest to the M dwarf and may act as Io’s plasma torus does at Jupiter and create trillions of kilowatts of power that are cause flaring and CME’s near the M dwarfs poles. This would keep the hard radiation away from the planet’s orbital plane and may also cause plasma and charge particles to be contained in the large flux tubes that form between the volcanic planets and the M dwarf. Large scale magnetic reconnection may take place when the two volcanically active planets flux tubes pass each other. There may be a way to prove this, with the flaring patterns that may relate to the motions of planet b and c and possibly d and the magnetic reconnections taking place with the high energy flux tubes from the planets to the M dwarf.

    • Michael Fidler January 28, 2021, 9:22

      Well another brilliant idea, and all of you are going to wish you had thought of it! The planets b, c and d are giant Flux Capacitor’s that control the space-time continuum around Trappist 1 and that’s how they transport themselves to our solar system. They jump to Io’s giant Flux Capacitor to reach Jupiter and then Earth! Now how do I get the patent on this??? Maybe we should be keeping an eye on Io……. ;-}

  • Harold Shaw January 27, 2021, 15:49

    Red dwarf systems are long shots for life but this opens an intriguing possibility. Consider how our Moon shapes our path into space. In such a compact system, a space faring capable people could emerge and have nearly hospitable planets as close as our Moon. They could potentially always know there is life on those worlds. Seasonal changes would be visible to the naked eye.

  • torque_xtr January 28, 2021, 6:13

    The luminosity of central star was about 30-100 times higher than current at the time of planetary system formation, 1-10 MYr. If the orbits have not changed, it means the snow line in the disk was several times farther than the outermost currently seen planet. No single drop of water for all seven, in the abscence of material transfer from outer system! And, given the uniform composition and undisrupted orbital state, this is very likely. The inner disk was completely dried up at the stage of planetesimal assembly already if this is the case.

    We’re probably looking at seven Ios, parched to the point of being bone-dry. It also tells that UCD dwarf systems are not exactly scaled-up Galileans. There should be big gaps between inner dry worlds and outer water worlds, which form beyond the initial snow-line. If we find an eighth planet at Trappist-1, it’s mass and orbital separation will tell much about the system’s formation!

    • Michael January 28, 2021, 13:46

      Some of them may be Aridian worlds, bone dry and with huge atmospheres of liquid CO2 !

  • Geoffrey Hillend January 29, 2021, 17:11

    I would rule out the no core idea for rocky worlds because of differentiation; the heavy elements always sink to the core. I would like to know how the iron stays uniformly mixed and does not sink into the core. Earth has a large iron core as theorized by the giant impact hypothesis. Theia’s core went into the Earth.

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