How Did the TRAPPIST-1 Planets Get Their Water?

In 2017, an international team of astronomers announced a momentous discovery. Based on years of observations, they found that the TRAPPIST-1 system (an M-type red dwarf located 40 light-years from Earth) contained no less than seven rocky planets! Equally exciting was the fact that three of these planets were found within the star’s Habitable Zone (HZ), and that the system itself has had 8 billion years to develop the chemistry for life.

At the same time, the fact that these planets orbit tightly around a red dwarf star has given rise to doubts that these three planets could maintain an atmosphere or liquid water for very long. According to new research by an international team of astronomers, it all comes down to the composition of the debris disk that the planets formed from and whether or not comets were around to distribute water afterward.

The team responsible for this research was led by Sebastian Marino of the Max Planck Institute for Astronomy (MPIA) and included members from the University of Cambridge, the University of Warwick, the University of Birmingham, the Harvard-Smithsonian Center for Astrophysics (CfA), and the MPIA. The study that describes their findings recently appeared in the Monthly Notices of the Royal Astronomical Society.

In terms of how the Solar System came to be, astronomers are of the general consensus that it formed over 4.6 billion years ago from a nebula of gas, dust, and volatiles (aka. the Nebular Hypothesis). This theory has it that these elements coalesced in the center first, undergoing gravitational collapse to create the Sun. Over time, the rest of the material formed a disk around the Sun that eventually accreted to form the planets.

Within the outer reaches of the Solar System, objects left over from the formation settled into a large belt containing vast amounts of iceteroids – otherwise known as The Kuiper Belt. In accordance with the Late Bombardment Theory, water was distributed to Earth and throughout the Solar System by countless comets and icy objects that were knocked out of this belt and sent hurdling inwards.

If the TRAPPIST-1 system has a Kuiper Belt of its own, then it stands to reason that a similar process was involved. In this case, gravitational perturbations would have caused objects to be kicked out of the belt that then traveled toward the seven planets to deposit water on their surfaces. Combined with the right atmospheric conditions, the three planets in the star’s HZ might have been sufficient quantities of water on their surfaces.

As Dr. Marino explained to Universe Today via email:

“The presence of a belt indicates that a system has a big reservoir of volatiles and water. This reservoir is typically located further out in the cold regions of a system, however there are diffrent processes that could bring a fraction of that water rich material near HZ planets and deliver their content. Finding a belt of comets is an indication that the reservoir existed in the first place.

Pluto and its cohorts in the icy-asteroid-rich Kuiper Belt beyond the orbit of Neptune. Credit: NASA

However, Dr. Marino also included the caveat that the absence of such a belt around stars today is not proof that a system would not have an adequate supply of water to support life. It is entirely that systems that had such a belt initially lost them after billions of years of evolution due to dynamical events. It is also possible that they could become too faint to detect since belts naturally become less massive and bright over time.

To search for a sign of an exo-Kuiper Belt around the TRAPPIST-1 system, the team relied on data collected by the Atacama Large Millimeter/submillimeter Array (ALMA). This array is renowned for its ability to detect objects that emit electromagnetic radiation between the infrared and radio wavelengths with a high degree of sensitivity.

This allows ALMA to visualize dust grains and volatile elements (like carbon monoxide) that characterize debris belts. These are generally too faint to see in visible light, but emit thermal radiation because of the heat they absorb from their respective star. Despite ALMA’s sensitivity, the team found no evidence of a exo-Kuiper Belt around TRAPPIST-1.

“Unfortunately, we did not detect this around TRAPPIST-1, but our upper limits allowed us to rule out that the system initially had a massive belt of large comets at a distance similar to the Kuiper Belt,” said Dr. Marino. “It is possible though that the system did indeed form with such a belt but it got completely disrupted by a dynamical instability in the system.”

Three of the TRAPPIST-1 planets – TRAPPIST-1e, f and g – dwell in their star’s so-called “habitable zone. CreditL NASA/JPL

They further conclude that the TRAPPIST-1 system could have been born with a planetary disk that was smaller than 40 AU in radius and had less than 20 Earth masses worth of materials. Moreover, they theorize that most of the dust grains in the disk were likely to have transported inward and used to form the seven planets that make up the planetary system.

Dr. Marino and his colleagues also used their modeling code to examine archival ALMA data on Proxima Centauri and its system of exoplanets – which include the rocky and potentially-habitable Proxima b and the newly-found super-Earth Proxima c. In 2017, ALMA data was used to confirm the existence of a cold dust and debris belt there, which was seen as a possible indication that the star had more exoplanets.

Here too, their results showed only upper limits to the gas and dust emission, which would imply that Proxima Centauri’s young disk is around one-tenth as massive as the one that formed our Solar System. As Dr. Marino explained, this study raises several questions about low-mass star systems:

“If we kept finding that this type of systems do not have massive cometary belts, it could mean that all the material used to formed these comets was used instead to form and grow planets closer in. It is very uncertain what that means for the composition of those planets since it really depends on where and how those planets formed. Just to point out, this type of belts are found around ~20% of nearby stars that are like the Sun or massive/brighter. Around low mass stars this has been much more challenging and we only know of a few belts around M stars.”

An artist’s illustration of the Proxima Centauri system. Proxima b in on the left, while Proxima C is on the right. Credit: Lorenzo Santinelli

This could be due to certain biases that make it easier to detect warmer belts around brighter stars than cold belts around M-type stars, Dr. Marino adds. It could also be the result of some intrinsic difference between the architecture of planetary systems around Sun-like stars (G-type or brighter) and those that orbit around red dwarfs.

In short, these results leave the question of how early water was transported throughout M-type star systems a mystery. At the same time, they have encouraged Dr. Marino and his colleagues to apply their techniques to younger and closer star systems in order to refine their models and increase the likelihood of detections.

These efforts will also benefit from new space-based and ground-based telescopes that will be coming online in the coming years. “Some next-generation telescopes are expected to be more sensitive and thus detect these belts if they are indeed there but not bright enough to detect them with the current telescopes,” said Dr. Marino.

As with other discoveries, these results show how exoplanet studies have made the transition from the process of discovery to the process of characterization. With improvements in instrumentation and methodology, we are beginning to see just how diverse and differentiated other types of star systems can be from our own.

Further Reading: CfA, MNRAS

2 Replies to “How Did the TRAPPIST-1 Planets Get Their Water?”

  1. Planetary system formation and especially habitable Earth analog formation is still up for grabs, as the article imply. The latest results on Earth and Moon isotope ratios agree that Earth had 40 – 60 % volatiles as Theia impacted, growing quickly from a disk pebble rain for the major part. The remaining volatiles derived from the outer system, including with the now found Theia isotopes, and was eventually slowly equilibrating in the mantle.

    That result implies Earth formation within 5 million years, and possibly within 1 million years akin to the gas giants. This just released interesting first integrative gravitic and magnetic model result agree with all of that as well as pin the observed disk magnetic field. I have high hopes (so far) that it will stand up:

    “With the aid of the “Piz Daint” supercomputer at the Swiss National Supercomputing Centre (CSCS) in Lugano, these scientists have now simulated the development of the protoplanetary disk both under the influence of gravity and in the presence of a magnetic field, thereby discovering a completely new mechanism that could explain previously unexplained observations.

    One such unexplained observation is that planets in our solar system today rotate much more slowly than the protoplanetary disk from which they must have once emerged. During the formation of planets, as well as of stars and black holes, enormous amounts of angular momentum must be lost, but how they lost this momentum has remained unclear. This so-called angular momentum problem is well-known in astrophysics. “Our new mechanism seems to be able to solve and explain this very general problem,” says Mayer.”

    “The newly developed method led to surprising results concerning the interaction between GI and the magnetic field. It was shown that the spiral arms formed by gravity in the protoplanetary disk act like a dynamo, stretching and strengthening the magnetic seed. As a result, the magnetic field grows and gains strength. At the same time, this process generates much more heat in the protoplanetary disk than previously assumed. Most surprising for the researchers, however, was the fact that the dynamo seems to have a significant influence on the motion of the matter. The dynamo pushes it vigorously both inward, to accrete on the star, and outward, away from the disk. This means that the disk is evolving much faster than previous theories had suggested.

    “The simulation shows that the energy generated by the interaction of the forming magnetic field with gravity acts outwards and drives a wind that throws matter out of the disk,” Mayer says. This would cause 90 percent of the mass to be lost in less than a million years. “If this is true, this would be a desirable prediction, because many of the protoplanetary disks studied with telescopes that are a million years old have about 90 percent less mass than predicted by the simulations of disks formation so far,” explains the astrophysicist.”

    [ ; my bold.]

    So their timescale fits and their angular momentum fits.

    But wait! There’s more! From a 2014 paper I was reminded of:

    “Now researchers at MIT, Cambridge University, and elsewhere have provided the first experimental evidence that our solar system’s protoplanetary disk was shaped by an intense magnetic field that drove a massive amount of gas into the sun within just a few million years. The same magnetic field may have propelled dust grains along collision courses, eventually smashing them together to form the initial seeds of terrestrial planets.”

    “The researchers then measured the magnetic strength of each grain, and calculated the original magnetic field in which those grains were created. Based on their calculations, the group determined that the early solar system harbored a magnetic field as strong as 5 to 54 microteslas — up to 100,000 times stronger than what exists in interstellar space today. ”

    “”Explaining the rapid timescale in which these disks evolve — in only a few million years — has always been a big mystery,” says Roger Fu, a graduate student in MIT’s Department of Earth, Atmospheric and Planetary Sciences. “It turns out that this magnetic field is strong enough to affect the motion of gas at a large scale, in a very significant way.””

    “It’s unlikely that chondrules formed via electric currents, or X-wind — flash-heating events that occur close to the sun. According to theoretical models, such events can only take place within magnetic fields stronger than 100 microteslas — far greater than what Fu and his colleagues measured.”

    “Jerome Gattacceca, research director at the European Centre for Research and Education in Environmental Sciences, says the solar system would have looked very different today if it had not been exposed to magnetic fields. “Without this kind of mechanism, all the matter in the solar system would have ended up in the sun, and we would not be here to discuss it,” says Gattacceca, who was not involved in the research. “There has to be a mechanism to prevent that. Several models exist, and this paper provides a viable mechanism, based on the existence of a significant magnetic field, to form the solar system as we know it.””

    [; my bold. ]

    The model paper show an averaged field strength within a 0.3 Gauss range in their model set [figure 6], which is a 30 micro Tesla range.

    The system formation model seems to fit very nicely together with some of the particular observations from our own system!

    1. So I forgot to put in the bold format. I also see that I misspelled “tesla” with capitals.

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