In order to be considered habitable, a planet needs to have liquid water. Cells, the smallest unit of life, need water to carry out their functions. For liquid water to exist, the temperature of the planet needs to be right. But how about the size of the planet?
Without sufficient mass a planet won’t have enough gravity to hold onto its water. A new study tries to understand how size affects the ability of a planet to hold onto its water, and as a result, its habitability.
When searching for potentially habitable exoplanets, scientists are forced to take the low-hanging fruit approach. Since Earth is the only planet we know of that is capable of supporting life, this search basically comes down to looking for planets that are “Earth-like”. But what if Earth is not the meter stick for habitability that we all tend to think it is?
That was the subject of a keynote lecture that was recently made at the Goldschmidt Geochemistry Congress, which took place from Aug. 18th to 23rd, in Barcelona, Spain. Here, a team of NASA-supported researchers explained how an examination of what goes into defining habitable zones (HZs) shows that some exoplanets may have better conditions for life to thrive than Earth itself has.
The study of exoplanets has matured considerably in the last ten years. During this time, the majority of the over 4000 exoplanets that are currently known to us were discovered. It was also during this time that the process has started to shift from the process of discovery to characterization. What’s more, next-generation instruments will allow for studies that will reveal a great deal about the surfaces and atmospheres of exoplanets.
This naturally raises the question: what would a sufficiently-advanced species see if they were studying our planet? Using multi-wavelength data of Earth, a team of Caltech scientists was able to construct a map of what Earth would look like to distant alien observers. Aside from addressing the itch of curiosity, this study could also help astronomers reconstruct the surface features of “Earth-like” exoplanets in the future.
Looking to the future, NASA and other space agencies have high hopes for the field of extra-solar planet research. In the past decade, the number of known exoplanets has reached just shy of 4000, and many more are expected to be found once next-generations telescopes are put into service. And with so many exoplanets to study, research goals have slowly shifted away from the process of discovery and towards characterization.
Unfortunately, scientists are still plagued by the fact that what we consider to be a “habitable zone” is subject to a lot of assumptions. Addressing this, an international team of researchers recently published a paper in which they indicated how future exoplanet surveys could look beyond Earth-analog examples as indications of habitability and adopt a more comprehensive approach.
In recent years, the number of extra-solar planets discovered around nearby M-type (red dwarf stars) has grown considerably. In many cases, these confirmed planets have been “Earth-like“, meaning that they are terrestrial (aka. rocky) and comparable in size to Earth. These finds have been especially exciting since red dwarf stars are the most common in the Universe – accounting for 85% of stars in the Milky Way alone.
Unfortunately, numerous studies have been conducted of late that indicate that these planets may not have the necessary conditions to support life. The latest comes from Harvard University, where postdoctoral researcher Manasvi Lingam and Professor Abraham Loeb demonstrate that planets around M-type stars may not get enough radiation from their stars for photosynthesis to occur.
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.
Finding potentially habitable planets beyond our Solar System is no easy task. While the number of confirmed extra-solar planets has grown by leaps and bounds in recent decades (3791 and counting!), the vast majority have been detected using indirect methods. This means that characterizing the atmospheres and surface conditions of these planets has been a matter of estimates and educated guesses.
Similarly, scientists look for conditions that are similar to what exists here on Earth, since Earth is the only planet we know of that supports life. But as many scientists have indicated, Earth’s conditions has changed dramatically over time. And in a recent study, a pair of researchers argue that a simpler form of photosynthetic life forms may predate those that relies on chlorophyll – which could have drastic implications in the hunt for habitable exoplanets.
In of August of 2016, astronomers from the European Southern Observatory (ESO) confirmed the existence of an Earth-like planet around Proxima Centauri – the closest star to our Solar System. In addition, they confirmed that this planet (Proxima b) orbited within its star’s habitable zone. Since that time, multiple studies have been conducted to determine if Proxima b could in fact be habitable.
Unfortunately, most of this research has not been very encouraging. For instance, many studies have indicated that Proxima b’s sun experiences too much flare activity for the planet to sustain an atmosphere and liquid water on its surface. However, in a new NASA-led study, a team of scientists has investigated various climate scenarios that indicate that Proxima b could still have enough water to 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.
Since it’s discovery was announced in August of 2016, Proxima b has been an endless source of wonder and the target of many scientific studies. As the closest extra-solar planet to our Solar System – and a terrestrial planet that orbits within Proxima Centauri’s circumstellar habitable zone (aka. “Goldilocks Zone”) – scientists have naturally wondered whether or not this planet could be habitable.
Unfortunately, many of these studies have emphasized the challenges that life on Proxima b would likely face, not the least of which is harmful radiation from its star. According to a recent study, a team of astronomers used the ALMA Observatory to detect a large flare emanating from Proxima Centauri. This latest findings, more than anything, raises questions about how habitable its exoplanet could be.
For the sake of their study, the team used data obtained by the Atacama Large Millimeter/submillimeter Array (ALMA) between January 21st to April 25th, 2017. This data revealed that the star underwent a significant flaring event on March 24th, where it reached a peak that was 1000 times brighter than the star’s quiescent emission for a period of ten seconds.
Astronomers have known for a long time that when compared to stars like our Sun, M-type stars are variable and unstable. While they are the smallest, coolest, and dimmest stars in our Universe, they tend to flare up at a far greater rate. In this case, the flare detected by the team was ten times larger than our Sun’s brightest flares at similar wavelengths.
Along with a smaller preceding flare, the entire event lasted fewer than two minutes of the 10 hours that ALMA was observing the star between January and March of last year. While it was already known that Proxima Centauri, like all M-type stars, experiences regular flare activity, this one appeared to be a rare event. However, stars like Proxima Centauri are also known to experienced regular, although smaller, X-ray flares.
All of this adds up to a bad case for habitability. As MacGregor explained in a recent NRAO press statement:
“It’s likely that Proxima b was blasted by high energy radiation during this flare. Over the billions of years since Proxima b formed, flares like this one could have evaporated any atmosphere or ocean and sterilized the surface, suggesting that habitability may involve more than just being the right distance from the host star to have liquid water.”
MacGregor and her colleagues also considered the possibility that Proxima Centauri is circled by several disks of dust. This was suggested by a previous study (also based on ALMA data) that indicated that the light output of both the star and flare together pointed towards the existence of debris belts around the star. However, after examining the ALMA data as a function of observing time, they were able to eliminate this as a possibility.
As Alycia J. Weinberger, also a researcher with the Carnegie Institution for Science and a co-author on the paper, explained:
“There is now no reason to think that there is a substantial amount of dust around Proxima Cen. Nor is there any information yet that indicates the star has a rich planetary system like ours.”
To date, studies that have looked at possible conditions on Proxima b have come to different conclusions as to whether or not it could retain an atmosphere or liquid water on its surface. While some have found room for “transient habitability” or evidence of liquid water, others have expressed doubt based on the long-term effects that radiation and flares from its star would have on a tidally-locked planet.
In the future, the deployment of next-generation instruments like the James Webb Space Telescope are expected to provide more detailed information on this system. With precise measurements of this star and its planet, the question of whether or not life can (and does) exist in this system may finally be answered.
And be sure to enjoy this animation of Proxima Centauri in motion, courtesy of NRAO outreach: