How did the TRAPPIST-1 planets get their water

(ORDO NEWS) — In 2017, an international team of astronomers announced an important discovery. Based on years of observations, they found that the TRAPPIST-1 system (an M-type red dwarf located 40 light-years from Earth) contains at least seven rocky planets.

Equally exciting was the fact that three of these planets were found in the star’s habitable zone (HZ), and the system itself had 8 billion years to give rise to the chemical elements for life.

At the same time, the fact that the planets orbit the red dwarf star raises doubts that these three planets can maintain an atmosphere or liquid water for a long time.

According to a new study by an international team of astronomers, it all comes down to the composition of the debris disk from which the planets formed and whether there were comets around to distribute the water.

The team responsible for this study was led by Sebastian Marino of the Institute of Astronomy. Max Planck (MPIA).

The group also included members of the University of Cambridge, the University of Warwick, the University of Birmingham, the Harvard-Smithsonian Center for Astrophysics (CfA) and MPIA. A study that describes their findings recently appeared in the monthly notices of the Royal Astronomical Society.

In terms of how the solar system came into being, astronomers are generally of the view that it formed over 4.6 billion years ago from a nebula composed of gas, dust, and volatiles (the Nebular Hypothesis).

This theory says that these elements first combine at the center, undergoing gravitational collapse to create the Sun. Over time, the rest of the material formed a disk around the Sun, which eventually grew to form the planets.

Within the outer reaches of the solar system, the objects left over from the formation settled into a large belt containing a huge number of objects – otherwise known as the Kuiper Belt.

In accordance with the late bombardment theory, water was distributed across the Earth and throughout the solar system by countless comets and icy objects that were knocked out of this belt and thrown into the system.

If the TRAPPIST-1 system has its own Kuiper belt, then it is quite understandable that a similar process was involved there too.

In this case, gravitational perturbations would cause objects to be knocked out of the belt, which would then be directed towards the seven planets to fall on their surfaces. Combined with the right atmospheric conditions, three planets in the star’s habitable zone could have ample water on their surfaces.

As Dr. Marino explained, “The presence of the belt indicates that there is a large reservoir of volatiles and water in the system.

This reservoir is typically located farther in the cold regions of the system, but there are other processes that could bring some of this water-rich material to habitable zone planets and deliver its contents. The belt of comets is a sign that water existed.”

However, Dr. Marino also added that the absence of such a belt around the stars today is not proof that the system will not have enough water to support life.

It is quite possible that systems with such a belt lost it after billions of years of evolution due to dynamical events. It is also possible that it may have become too faint to be detected, as the belts naturally become less massive and brighter over time.

To find a sign of the exo-Kuiper belt around the TRAPPIST-1 system, the team used data collected with the Atacama Large Millimeter/Submillimeter Array (ALMA). It is known for its ability to detect objects that emit electromagnetic radiation between infrared and radio waves with a high degree of sensitivity.

This allows ALMA to visualize dust particles and volatile elements (such as carbon monoxide) that characterize debris lanes. They are generally too faint to be seen in visible light, but emit thermal radiation that they absorb from the star. Despite ALMA’s sensitivity, the team found no evidence for an exo-Kuiper belt around TRAPPIST-1.

“Unfortunately, we did not detect it in TRAPPIST-1, but our upper bounds allowed us to rule out that the system originally had a massive belt of large comets at a distance similar to the Kuiper belt,” Dr. Marino said. “It is possible, however, that the system did form with such a belt, but it was completely destroyed by dynamic instability in the system.”

They also concluded that the TRAPPIST-1 system could have been born with a planetary disk less than 40 AU in radius and material with a mass less than 20 Earth masses. Moreover, they suggest that most of the dust particles in the disk were probably transported into the system and used to form the seven planets that make up the planetary system.

Dr. Marino and his colleagues also used their simulation code to study archival ALMA data on Proxima Centauri and its exoplanet system, which includes the rocky and potentially habitable Proxima b and the newly discovered superterrestrial Proxima c. In 2017, ALMA data was used to confirm the existence of a cold belt of dust and debris there, which was taken as a possible sign that the star had more exoplanets.

Their results also showed only upper limits for gas and dust emissions, which would mean that the young disk of Proxima Centauri is about one-tenth smaller than the one that formed our solar system. As Dr. Marino explained, this study raises several questions about low-mass star systems:

“If we continued to find that this type of system does not have massive comet belts, it could mean that all the material that would have been used to form these comets was used to form and grow planets instead.

It is not clear what this means for these planets, as it all depends on where and how these planets formed. This type of belt is found in about ~20% of nearby stars that are similar to or brighter than our sun. Around low-mass stars, it was much more difficult, and we only know of a few belts around M-type stars.”

This may be due to certain distortions that make it easier to detect warmer belts around bright stars than cold belts around M-type stars, adds Dr. Marino. It may also be the result of some intrinsic difference between the architecture of planetary systems around stars of type G or brighter and those orbiting red dwarfs.

In short, how early water was transported through M-type star systems remains a mystery. At the same time, the results encouraged Dr. Marino and his colleagues to apply their methods to younger and closer star systems in order to improve their models and increase the likelihood of detection.

These efforts will also benefit from new space and ground-based telescopes coming up in the coming years. “It is expected that some next-generation telescopes will be more sensitive, and thus detect these belts if they do exist, but are not bright enough to be detected with current telescopes,” Dr. Marino said.

As with other discoveries, these results show how exoplanet research has moved from a process of discovery to a process of characterization. As tools and methodology improve, we are beginning to see how diverse and differentiated other types of star systems can be.


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