Exoplanet Kepler 62f would need an atmosphere rich in carbon dioxide for water to be in liquid form. Artist's Illustration: NASA Ames/JPL-Caltech/T. Pyle
Upcoming telescopes will give us more power to search for biosignatures on all the exoplanets we’ve found. Much of the biosignature conversation is centred on biogenic chemistry, such as atmospheric gases produced by simple, single-celled creatures. But what if we want to search for technological civilizations that might be out there? Could we find them by searching for their air pollution?
If a distant civilization was giving our planet a cursory glance in its own survey of alien worlds and technosignatures, they couldn’t help but notice our air pollution.
This artist’s impression shows the planet K2-18b, it’s host star and an accompanying planet in this system. K2-18b is now the only super-Earth exoplanet known to host both water and temperatures that could support life. UCL researchers used archive data from 2016 and 2017 captured by the NASA/ESA Hubble Space Telescope and developed open-source algorithms to analyse the starlight filtered through K2-18b’s atmosphere. The results revealed the molecular signature of water vapour, also indicating the presence of hydrogen and helium in the planet’s atmosphere.
The ultra-powerful James Webb Space Telescope will launch soon. Once it’s deployed, and in position at the Earth-Sun Lagrange Point 2, it’ll begin work. One of its jobs is to examine the atmospheres of exoplanets and look for biosignatures. It should be simple, right? Just scan the atmosphere until you find oxygen, then close your laptop and head to the pub: Fanfare, confetti, Nobel prize.
Of course, Universe Today readers know it’s more complicated than that. Much more complicated.
In fact, the presence of oxygen is not necessarily reliable. It’s methane that can send a stronger signal indicating the presence of life.
Artist's concept of Earth-like exoplanets, which (according to new research) need to strike the careful balance between water and landmass. Credit: NASA
Welcome back to our Fermi Paradox series, where we take a look at possible resolutions to Enrico Fermi’s famous question, “Where Is Everybody?” Today, we examine the possibility that the reason for the Great Silence is that many planets out there are just too watery!
In 1950, Italian-American physicist Enrico Fermi sat down to lunch with some of his colleagues at the Los Alamos National Laboratory, where he had worked five years prior as part of the Manhattan Project. According to various accounts, the conversation turned to aliens and the recent spate of UFOs. Into this, Fermi issued a statement that would go down in the annals of history: “Where is everybody?“
This became the basis of the Fermi Paradox, which refers to the disparity between high probability estimates for the existence of extraterrestrial intelligence (ETI) and the apparent lack of evidence. Since Fermi’s time, there have been several proposed resolutions to his question, which includes the possibility thatmany exoplanets are Waterworlds, where water is so plentiful that life will be less likely to emerge and thrive.
In September, an international team announced that based on data obtained by the Atacama Millimeter-submillimeter Array (ALMA) in Chile and the James Clerk Maxwell Telescope (JCMT) in Hawaii, they had discovered phosphine gas (PH3) in the atmosphere of Venus. The news was met with its fair share of skepticism and controversy since phosphine is considered a possible indication of life (aka. a biosignature).
Shortly thereafter, a series of papers were published that questioned the observations and conclusions, with one team going as far as to say there was “no phosphine” in Venus’ atmosphere at all. Luckily, after re-analyzing the ALMA data, the team responsible for the original discovery concluded that there is indeed phosphine in the cloud tops of Venus – just not as much as they initially thought.
Artist's impression of the exoplanet GJ 1132 b, which orbits the red dwarf star GJ 1132. Astronomers have managed to detect the atmosphere of this Earth-like planet. Credit: MPIA
We’re waiting patiently for telescopes like the James Webb Space Telescope to see first light, and one of the reasons is its ability to study the atmospheres of exoplanets. The idea is to look for biosignatures: things like oxygen and methane. But a new study says that exoplanets with hydrogen in their atmospheres are a good place to seek out alien life.
Artist's rendition of a white dwarf from the surface of an orbiting exoplanet. Astronomers have found two giant planet candidates orbiting two white dwarfs. More proof that giant planets can surve their stars' red giant phases. Image Credit: Madden/Cornell University
Some very powerful telescopes will see first light in the near future. One of them is the long-awaited James Webb Space Telescope (JWST.) One of JWST’s roles—and the role of the other upcoming ‘scopes as well—is to look for biosignatures in the atmospheres of exoplanets. Now a new study is showing that finding those biosignatures on exoplanets that orbit white dwarf stars might give us our best chance to find them.
Artist’s impression of a sunset seen from the surface of an Earth-like exoplanet. Credit: ESO/L. Calçada
In their efforts to find evidence of life beyond our Solar System, scientists are forced to take what is known as the “low-hanging fruit” approach. Basically, this comes down to determining if planets could be “potentially habitable” based on whether or not they would be warm enough to have liquid water on their surfaces and dense atmospheres with enough oxygen.
This is a consequence of the fact that existing methods for examining distant planets are largely indirect and that Earth is only one planet we know of that is capable of supporting life. But what if planets that have plenty of oxygen are not guaranteed to produce life? According to a new study by a team from Johns Hopkins University, this may very well be the case.
Artist impression of the early Earth. Credit: NASA Goddard Space Flight Center Conceptual Image Lab
The question how life began on Earth has always been a matter of profound interest to scientists. But just as important as how life emerged is the question of when it emerged. In addition to discerning how non-living elements came together to form the first living organisms (a process known as abiogenesis), scientists have also sought to determine when the first living organisms appeared on Earth.
According to a new study, evidence of life (aka. biosignatures) could be found by examining ejecta from extra-solar planets caused by asteroid impacts. Image: NASA/JPL-Caltech/Univ. of Arizona
In recent years, the number of confirmed extra-solar planets has risen exponentially. As of the penning of the article, a total of 3,777 exoplanets have been confirmed in 2,817 star systems, with an additional 2,737 candidates awaiting confirmation. What’s more, the number of terrestrial (i.e. rocky) planets has increased steadily, increasing the likelihood that astronomers will find evidence of life beyond our Solar System.
Unfortunately, the technology does not yet exist to explore these planets directly. As a result, scientists are forced to look for what are known as “biosignatures”, a chemical or element that is associated with the existence of past or present life. According to a new study by an international team of researchers, one way to look for these signatures would be to examine material ejected from the surface of exoplanets during an impact event.
Artist’s impression of what an asteroid hitting the Earth might look like. Credit: NASA/Don Davis.
As they indicate in their study, most efforts to characterize exoplanet biospheres have focused on the planets’ atmospheres. This consists of looking for evidence of gases that are associated with life here on Earth – e.g. carbon dioxide, nitrogen, etc. – as well as water. As Cataldi told Universe Today via email:
“We know from Earth that life can have a strong impact on the composition of the atmosphere. For example, all the oxygen in our atmosphere is of biological origin. Also, oxygen and methane are strongly out of chemical equilibrium because of the presence of life. Currently, it is not yet possible to study the atmospheric composition of Earth-like exoplanets, however, such a measurement is expected to become possible in the foreseeable future. Thus, atmospheric biosignatures are the most promising way to search for extraterrestrial life.”
However, Cataldi and his colleagues considered the possibility of characterizing a planet’s habitability by looking for signs of impacts and examining the ejecta. One of the benefits of this approach is that ejecta escapes lower gravity bodies, such as rocky planets and moons, with the greatest ease. The atmospheres of these types of bodies are also very difficult to characterize, so this method would allow for characterizations that would not otherwise be possible.
And as Cataldi indicated, it would also be complimentary to the atmospheric approach in a number of ways:
“First, the smaller the exoplanet, the more difficult it is to study its atmosphere. On the contrary, smaller exoplanets produce larger amounts of escaping ejecta because their surface gravity is lower, making ejecta from smaller exoplanet easier to detect. Second, when thinking about biosignatures in impact ejecta, we think primarily of certain minerals. This is because life can influence the mineralogy of a planet either indirectly (e.g. by changing the composition of the atmosphere and thus allowing new minerals to form) or directly (by producing minerals, e.g. skeletons). Impact ejecta would thus allow us to study a different sort of biosignature, complementary to atmospheric signatures.”
Another benefit to this method is the fact that it takes advantage of existing studies that have examined the impacts of collisions between astronomical objects. For instance, multiple studies have been conducted that have attempted to place constraints on the giant impact that is believed to have formed the Earth-Moon system 4.5 billion years ago (aka. the Giant Impact Hypothesis).
While such giant collisions are thought to have been common during the final stage of terrestrial planet formation (lasting for approximately 100 million years), the team focused on impacts of asteroidal or cometary bodies, which are believed to occur over the entire lifetime of an exoplanetary system. Relying on these studies, Cataldi and his colleagues were able to create models for exoplanet ejecta.
As Cataldi explained, they used the results from the impact cratering literature to estimate the amount of ejecta created. To estimate the signal strength of circumstellar dust disks created by the ejecta, they used the results from debris disk (i.e. extrasolar analogues of the Solar System’s Main Asteroid Belt) literature. In the end, the results proved rather interesting:
“We found that an impact of a 20 km diameter body produces enough dust to be detectable with current telescopes (for comparison, the size of the impactor that killed the dinosaurs 65 million years ago is though to be around 10 km). However, studying the composition of the ejected dust (e.g. search for biosignatures) is not in the reach of current telescopes. In other words, with current telescopes, we could confirm the presence of ejected dust, but not study its composition.”
Perspective view looking from an unnamed crater (bottom right) towards the Worcester Crater. The region sits at the mouth of Kasei Valles, where fierce floodwaters emptied into Chryse Planitia. Credit: ESA/DLR/FU Berlin
In short, studying material ejected from exoplanets is within our reach and the ability to study its composition someday will allow astronomers to be able to characterize the geology of an exoplanet – and thus place more accurate constraints on its potential habitability. At present, astronomers are forced to make educated guesses about a planet’s composition based on its apparent size and mass.
Unfortunately, a more detailed study that could determine the presence of biosignatures in ejecta is not currently possible, and will be very difficult for even next-generation telescopes like the James Webb Space Telescope (JWSB) or Darwin. In the meantime, the study of ejecta from exoplanets presents some very interesting possibilities when it comes to exoplanet studies and characterization. As Cataldi indicated:
“By studying the ejecta from an impact event, we could learn something about the geology and habitability of the exoplanet and potentially detect a biosphere. The method is the only way I know to access the subsurface of an exoplanet. In this sense, the impact can be seen as a drilling experiment provided by nature. Our study shows that dust produced in an impact event is in principle detectable, and future telescopes might be able to constrain the composition of the dust, and therefore the composition of the planet.”
In the coming decades, astronomers will be studying extra-solar planets with instruments of increasing sensitivity and power in the hopes of finding indications of life. Given time, searching for biosignatures in the debris around exoplanets created by asteroid impacts could be done in tandem with searchers for atmospheric biosignatures.
With these two methods combined, scientists will be able to say with greater certainty that distant planets are not only capable of supporting life, but are actively doing so!
Artist's concept of Kepler-69c, a super-Earth-size planet in the habitable zone of a star like our sun, located about 2,700 light-years from Earth in the constellation Cygnus. Credit: NASA
When looking for potentially-habitable extra-solar planets, scientists are somewhat restricted by the fact that we know of only one planet where life exists (i.e. Earth). For this reason, scientists look for planets that are terrestrial (i.e. rocky), orbit within their star’s habitable zones, and show signs of biosignatures such as atmospheric carbon dioxide – which is essential to life as we know it.
This gas, which is the largely result of volcanic activity here on Earth, increases surface heat through the greenhouse effect and cycles between the subsurface and the atmosphere through natural processes. For this reason, scientists have long believed that plate tectonics are essential to habitability. However, according to a new study by a team from Pennsylvania State University, this may not be the case.
The study, titled “Carbon Cycling and Habitability of Earth-Sized Stagnant Lid Planets“, was recently published in the scientific journal Astrobiology. The study was conducted by Bradford J. Foley and Andrew J. Smye, two assistant professors from the department of geosciences at Pennsylvania State University.
The Earth’s Tectonic Plates. Credit: msnucleus.org
On Earth, volcanism is the result of plate tectonics and occurs where two plates collide. This causes subduction, where one plate is pushed beneath the other and deeper into the subsurface. This subduction changes the dense mantle into buoyant magma, which rises through the crust to the Earth’s surface and creates volcanoes. This process can also aid in carbon cycling by pushing carbon into the mantle.
Plate tectonics and volcanism are believe to have been central to the emergence of life here on Earth, as it ensured that our planet had sufficient heat to maintain liquid water on its surface. To test this theory, Professors Foley and Smye created models to determine how habitable an Earth-like planet would be without the presence of plate tectonics.
These models took into account the thermal evolution, crustal production and CO2 cycling to constrain the habitability of rocky, Earth-sized stagnant lid planets. These are planets where the crust consists of a single, giant spherical plate floating on mantle, rather than in separate pieces. Such planets are thought to be far more common than planets that experience plate tectonics, as no planets beyond Earth have been confirmed to have tectonic plates yet. As Prof. Foley explained in a Penn State News press release:
“Volcanism releases gases into the atmosphere, and then through weathering, carbon dioxide is pulled from the atmosphere and sequestered into surface rocks and sediment. Balancing those two processes keeps carbon dioxide at a certain level in the atmosphere, which is really important for whether the climate stays temperate and suitable for life.”
Map of the Earth showing fault lines (blue) and zones of volcanic activity (red). Credit: zmescience.com
Essentially, their models took into account how much heat a stagnant lid planet’s climate could retain based on the amount of heat and heat-producing elements present when the planet formed (aka. its initial heat budget). On Earth, these elements include uranium which produces thorium and heat when it decays, which then decays to produce potassium and heat.
After running hundreds of simulations, which varied the planet’s size and chemical composition, they found that stagnant lid planets would be able to maintain warm enough temperatures that liquid water could exist on their surfaces for billions of years. In extreme cases, they could sustain life-supporting temperatures for up to 4 billion years, which is almost the age of the Earth.
As Smye indicated, this is due in part to the fact that plate tectonics are not always necessary for volcanic activity:
“You still have volcanism on stagnant lid planets, but it’s much shorter lived than on planets with plate tectonics because there isn’t as much cycling. Volcanoes result in a succession of lava flows, which are buried like layers of a cake over time. Rocks and sediment heat up more the deeper they are buried.”
Image of the Sarychev volcano (in Russia’s Kuril Islands) caught during an early stage of eruption on June 12, 2009. Taken by astronauts aboard the International Space Station. Credit: NASA
The researchers also found that without plate tectonics, stagnant lid planets could still have enough heat and pressure to experience degassing, where carbon dioxide gas can escape from rocks and make its way to the surface. On Earth, Smye said, the same process occurs with water in subduction fault zones. This process increases based on the quantity of heat-producing elements present in the planet. As Foley explained:
“There’s a sweet spot range where a planet is releasing enough carbon dioxide to keep the planet from freezing over, but not so much that the weathering can’t pull carbon dioxide out of the atmosphere and keep the climate temperate.”
According to the researchers’ model, the presence and amount of heat-producing elements were far better indicators for a planet’s potential to sustain life. Based on their simulations, they found that the initial composition or size of a planet is very important for determining whether or not it will become habitable. Or as they put it, the potential habitability of a planet is determined at birth.
By demonstrating that stagnant lid planets could still support life, this study has the potential for greatly extending the range of what scientists consider to be potentially-habitable. When the James Webb Space Telescope (JWST) is deployed in 2021, examining the atmospheres of stagnant lid planets to determine the presence of biosignatures (like CO2) will be a major scientific objective.
Knowing that more of these worlds could sustain life is certainly good news for those who are hoping that we find evidence of extra-terrestrial life in our lifetimes.
