When it comes to the search for extra-terrestrial life, scientists have a tendency to be a bit geocentric – i.e. they look for planets that resemble our own. This is understandable, seeing as how Earth is the only planet that we know of that supports life. As result, those searching for extra-terrestrial life have been looking for planets that are terrestrial (rocky) in nature, orbit within their stars habitable zones, and have enough water on their surfaces.
In the course of discovering several thousand exoplanets, scientists have found that many may in fact be “water worlds” (planets where up to 50% of their mass is water). This naturally raises some questions, like how much water is too much, and could too much land be a problem as well? To address these, a pair of researchers from the Harvard Smithsonian Center for Astrophysics (CfA) conducted a study to determine how the ratio between water and land masses can contribute to life.
We’re not learning that the vast majority of potentially habitable worlds out there are actually icy moons like Europa and Enceladus. Good news, there are hundreds, if not thousands of times more of them than worlds like Earth. Bad news, they’re locked in ice. What have we learned about water worlds and their potential for habitability?
Ever since the first exoplanet was confirmed in 1992, astronomers have found thousands of worlds beyond our Solar System. With more and more discoveries happening all the time, the focus of exoplanet research has begun to slowly shift from exoplanet discovery to exoplanet characterization. Essentially, scientists are now looking to determine the composition of exoplanets to determine whether or not they could support life.
In February of 2017, the world was astounded to learn that astronomers – using data from the TRAPPIST telescope in Chile and the Spitzer Space Telescope – had identified a system of seven rocky exoplanets in the TRAPPIST-1 system. As if this wasn’t encouraging enough for exoplanet-enthusiasts, it was also indicated that three of the seven planets orbited within the stars’ circumstellar habitable zone (aka. “Goldilocks Zone”).
Since that time, this system has been the focus of considerable research and follow-up surveys to determine whether or not any of its planets could be habitable. Intrinsic to these studies has been the question whether or not the planets have liquid water on their surfaces. But according to a new study by a team of American astronomers, the TRAPPIST planets may actually have too much water to support life.
For the sake of their study, the team used data from prior surveys that attempted to place constraints on the mass and diameter of the TRAPPIST-1 planets in order to calculate their densities. Much of this came from a dataset called the Hypatia Catalog (developed by contributing author Hinkel), which merges data from over 150 literary sources to determine the stellar abundances of stars near to our Sun.
Using this data, the team constructed mass-radius-composition models to determine the volatile contents of each of the TRAPPIST-1 planets. What they noticed is that the TRAPPIST planets are traditionally light for rocky bodies, indicating a high content of volatile elements (such as water). On similarly low-density worlds, the volatile component is usually thought to take the form of atmospheric gases.
But as Unterborn explained in a recent SESE news article, the TRAPPIST-1 planets are a different matter:
“[T]he TRAPPIST-1 planets are too small in mass to hold onto enough gas to make up the density deficit. Even if they were able to hold onto the gas, the amount needed to make up the density deficit would make the planet much puffier than we see.”
Because of this, Unterborn and his colleagues determined that the low-density component in this planetary system had to be water. To determine just how much water was there, the team used a unique software package developed known as ExoPlex. This software uses state-of-the-art mineral physics calculators that allowed the team to combine all of the available information about the TRAPPIST-1 system – not just the mass and radius of individual planets.
What they found was that the inner planets (b and c) were “drier” – having less than 15% water by mass – while the outer planets (f and g) had more than 50% water by mass. By comparison, Earth has only 0.02% water by mass, which means that these worlds have the equivalent of hundreds of Earth-sized oceans in their volume. Basically, this means that the TRAPPIST-1 planets may have too much water to support life. As Hinkel explained:
“We typically think having liquid water on a planet as a way to start life, since life, as we know it on Earth, is composed mostly of water and requires it to live. However, a planet that is a water world, or one that doesn’t have any surface above the water, does not have the important geochemical or elemental cycles that are absolutely necessary for life.”
These findings do not bode well for those who believe that M-type stars are the most likely place to have habitable planets in our galaxy. Not only are red dwarfs the most common type of star in the Universe, accounting for 75% of stars in the Milky Way Galaxy alone, several that are relatively close to our Solar System have been found to have one or more rocky planets orbiting them.
Unfortunately, these latest findings indicate that the planets of the TRAPPIST-1 system are not favorable for life. What’s more, there would probably not be enough life on them to produce biosignatures that would be observable in their atmospheres. In addition, the team also concluded that the TRAPPIST-1 planets must have formed father away from their star and migrated inward over time.
This was based on the fact that the ice-rich TRAPPIST-1 planets were far closer to their star’s respective “ice line” than the drier ones. In any solar system, planets that lie within this line will be rockier since their water will vaporize, or condense to form oceans on their surfaces (if a sufficient atmosphere is present). Beyond this line, water will take the form of ice and can be accreted to form planets.
From their analyses, the team determined that the TRAPPIST-1 planets must have formed beyond the ice line and migrated towards their host star to assume their current orbits. However, since M-type (red dwarf) stars are known to be brightest after the first form and dim over time, the ice line would have also moved inward. As co-author Steven Desch explained, how far the planets migrated would therefore depend on when they had formed.
“The earlier the planets formed, the farther away from the star they needed to have formed to have so much ice,” he said. Based on how long it takes for rocky planets to form, the team estimated that the planets must have originally been twice as far from their star as they are now. While there are other indications that the planets in this system migrated over time, this study is the first to quantify the migration and use composition data to show it.
This study is not the first to indicate that planets orbiting red dwarf stars may in fact be “water worlds“, which would mean that rocky planets with continents on their surfaces are a relatively rare thing. At the same time, other studies have been conducted that indicate that such planets are likely to have a hard time holding onto their atmospheres, indicating that they would not remain water worlds for very long.
However, until we can get a better look at these planets – which will be possible with the deployment of next-generation instruments (like the James Webb Space Telescope) – we will be forced to theorize about what we don’t know based what we do. By slowly learning more about these and other exoplanets, our ability to determine where we should be looking for life beyond our Solar System will be refined.
When hunting for potentially habitable exoplanets, one of the most important things astronomers look for is whether or not exoplanet candidates orbit within their star’s habitable zone. This is necessary for liquid water to exist on a planet’s surface, which in turn is a prerequisite for life as we know it. However, in the course of discovering new exoplanets, scientists have become aware of an extreme case known as “water worlds“.
Water worlds are essentially planets that are up to 50% water in mass, resulting in surface oceans that could be hundreds of kilometers deep. According to a new study by a team of astrophysicists from Princeton, the University of Michigan and Harvard, water worlds may not be able to hang on to their water for very long. These findings could be of immense significance when it comes to the hunt for habitable planets in our neck of the cosmos.
This most recent study, titled “The Dehydration of Water Worlds via Atmospheric Losses“, recently appeared in The Astrophysical Journal Letters. Led by Chuanfei Dong from the Department of Astrophysical Sciences at Princeton University, the team conducted computer simulations that took into account what kind of conditions water worlds would be subject to.
This study was motivated largely by the number of exoplanet discoveries have been made around low-mass, M-type (red dwarf) star systems in recent years. These planets have been found to be comparable in size to Earth – which indicated that they were likely terrestrial (i.e. rocky). In addition, many of these planets – such as Proxima b and three planets within the TRAPPIST-1 system – were found to be orbiting within the stars habitable zones.
However, subsequent studies indicated that Proxima b and other rocky planets orbiting red dwarf stars could in fact be water worlds. This was based on mass estimates obtained by astronomical surveys, and the built-in assumptions that such planets were rocky in nature and did not have massive atmospheres. At the same time, numerous studies have been produced that have cast doubt on whether or not these planets would be able to hold onto their water.
Basically, it all comes down to the type of star and the orbital parameters of the planets. While long-lived, red dwarf stars are known for being variable and unstable compared to our Sun, which results in periodic flares up that would strip a planet’s atmosphere over time. On top of that, planets orbiting within a red dwarf’s habitable zone would likely be tidally-locked, meaning one side of the planet would be constantly exposed to the star’s radiation.
Because of this, scientists are focused on determining just how well exoplanets in different types of star systems could hold onto their atmospheres. As Dr. Dong told Universe Today via email:
“It is fair to say that the presence of an atmosphere is perceived as one of the requirements for the habitability of a planet. Having said that, the concept of habitability is a complex one with myriad factors involved. Thus, an atmosphere by itself will not suffice to guarantee habitability, but it can be regarded as an important ingredient for a planet to be habitable.”
To test whether or not a water world would be able to hold onto its atmosphere, the team conducted computer simulations that took into account a variety of possible scenarios. These included the effects of stellar magnetic fields, coronal mass ejections, and atmospheric ionization and ejection for various types of stars – including G-type stars (like our Sun) and M-type stars (like Proxima Centauri and TRAPPIST-1).
With these effects accounted for, Dr. Dong and his colleagues derived a comprehensive model that simulated how long exoplanet atmospheres would last. As he explained it:
“We developed a new multi-fluid magnetohydrodynamic model. The model simulated both the ionosphere and magnetosphere as a whole. Due to the existence of the dipole magnetic field, the stellar wind cannot sweep away the atmosphere directly (like Mars due to the absence of a global dipole magnetic field), instead, the atmospheric ion loss was caused by the polar wind.
“The electrons are less massive than their parent ions, and as a result, are more easily accelerated up to and beyond the escape velocity of the planet. This charge separation between the escaping, low-mass electrons and significantly heavier, positively-charged ions sets up a polarization electric field. That electric field, in turn, acts to pull the positively charged ions along behind the escaping electrons, out of the atmosphere in the polar caps.”
What they found was that their computer simulations were consistent with the current Earth-Sun system. However, in some extreme possibilities – such as exoplanets around M-type stars – the situation is very different and the escape rates could be one thousand times greater or more. The result means that even a water world, if it orbits an red dwarf star, could lose its atmosphere after about a gigayear (Gyr), one billion years.
Considering that life as we know it took around 4.5 billion years to evolve, one billion years is a relatively brief window. In fact, as Dr. Dong explained, these results indicate that planets that orbit M-type stars would be hard pressed to develop life:
“Our results indicate that the ocean planets (orbiting a Sun-like star) will retain their atmospheres much longer than the Gyr timescale as the ion escape rates are far too low, therefore, it allows a longer duration for life to originate on these planets and evolve in terms of complexity. In contrast, for exoplanets orbiting M-dwarfs, they could have their oceans depleted over the Gyr timescale due to the more intense particle and radiation environments that exoplanets experience in close-in habitable zones. If the atmosphere were to be depleted over the timescale less than Gyr, this could prove to be problematic for the origin of life (abiogenesis) on the planet.”
Once again, these results cast doubt on the potential habitability of red dwarf star systems. In the past, researchers have indicated that the longevity of red dwarf stars, which can remain in their main sequence for up to 10 trillion years or longer, make them the best candidate for finding habitable exoplanets. However, the stability of these stars and the way in which they are likely to strip planets of their atmospheres seems to indicate otherwise.
Studies such as this one are therefore highly significant in that they help to address just how long a potentially habitable planet around a red dwarf star could remain potentially habitable. As Dr. Dong indicated:
“Given the importance of atmospheric loss on planetary habitability, there has been a great deal of interest in using telescopes such as the upcoming James Webb Space Telescope (JWST) to determine whether these planets have atmospheres and, if so, what their composition are like. It is expected that the JWST should be capable of characterizing these atmospheres (if present), but quantifying the escape rates accurately requires a much higher degree of precision and may not be feasible in the near-future.”
The study is also significant as far as our understanding of the Solar System and its evolution is concerned. At one time, scientists have ventured that both Earth and Venus may have been water worlds. How they made the transition from being very watery to what they are today – in the case of Venus, dry and hellish; and in the case of Earth, having multiple continents – is an all-important question.
In the future, more detailed surveys are anticipated that could help shed light on these competing theories. When the James Webb Space Telescope (JWST) is deployed in Spring of 2018, it will use its powerful infrared capabilities to study planets around nearby red dwarfs, Proxima b being one of them. What we learn about this and other distant exoplanets will go a long way towards informing our understanding of how our own Solar System evolved as well.