But the dominant methods for studying exoplanet atmospheres are not intended for objects as distant, dim and complex as planets trillions of miles from Earth, Burrows said. They were instead designed to study much closer or brighter objects, such as planets in Earth’s solar system and stars.
Nonetheless, scientific reports and the popular media brim with excited depictions of Earth-like planets ripe for hosting life and other conclusions that are based on vague and incomplete data, Burrows wrote in the first in a planned series of essays that examine the current and future study of exoplanets. Despite many trumpeted results, few “hard facts” about exoplanet atmospheres have been collected since the first planet was detected in 1992, and most of these data are of “marginal utility.”
The good news is that the past 20 years of study have brought a new generation of exoplanet researchers to the fore that is establishing new techniques, technologies and theories. As with any relatively new field of study, fully understanding exoplanets will require a lot of time, resources and patience, Burrows said.
“Exoplanet research is in a period of productive fermentation that implies we’re doing something new that will indeed mature,” Burrows said. “Our observations just aren’t yet of a quality that is good enough to draw the conclusions we want to draw.
“There’s a lot of hype in this subject, a lot of irrational exuberance. Popular media have characterized our understanding as better than it actually is,” he said. “They’ve been able to generate excitement that creates a positive connection between the astrophysics community and the public at large, but it’s important not to hype conclusions too much at this point.”
The majority of data on exoplanet atmospheres come from low-resolution photometry, which captures the variation in light and radiation an object emits, Burrows reported. That information is used to determine a planet’s orbit and radius, but its clouds, surface, and rotation, among other factors, can easily skew the results. Even newer techniques such as capturing planetary transits — which is when a planet passes in front of its star, and was lauded by Burrows as an unforeseen “game changer” when it comes to discovering new planets — can be thrown off by a thick atmosphere and rocky planet core.
All this means that reliable information about a planet can be scarce, so scientists attempt to wring ambitious details out of a few data points. “We have a few hard-won numbers and not the hundreds of numbers that we need,” Burrows said. “We have in our minds that exoplanets are very complex because this is what we know about the planets in our solar system, but the data are not enough to constrain even a fraction of these conceptions.”
Burrows emphasizes that astronomers need to acknowledge that they will never achieve a comprehensive understanding of exoplanets through the direct-observation, stationary methods inherited from the exploration of Earth’s neighbors. He suggests that exoplanet researchers should acknowledge photometric interpretations as inherently flawed and ambiguous. Instead, the future of exoplanet study should focus on the more difficult but comprehensive method of spectrometry, wherein the physical properties of objects are gauged by the interaction of its surface and elemental features with light wavelengths, or spectra. Spectrometry has been used to determine the age and expansion of the universe.
Existing telescopes and satellites are likewise vestiges of pre-exoplanet observation. Burrows calls for a mix of small, medium and large initiatives that will allow the time and flexibility scientists need to develop tools to detect and analyze exoplanet spectra. He sees this as a challenge in a research environment that often puts quick-payback results over deliberate research and observation. Once scientists obtain high-quality spectral data, however, Burrows predicted, “Many conclusions reached recently about exoplanet atmospheres will be overturned.”
A longtime exoplanet researcher, Burrows predicted the existence of “hot-Jupiter” planets — gas planets similar to Jupiter but orbiting very close to the parent star — in a paper in the journal Nature months before the first such planet, 51 Pegasi b, was discovered in 1995.
Since the launching of the Kepler Mission, astronomers have discovered hundreds of distant alien planets in the past two decades. Future missions could detect potential signs of life called biosignatures on those worlds, such as oxygen or methane in their atmospheres.
In 2012, astrobiologist Jack O'Malley-James at the University of St. Andrews in Fife, Scotland and his colleagues noted that biosignatures of life on Earth have not remained the same over time, but have altered considerably over its history. This led the researchers to speculate about how Earth and other planets might look in the future.
"Astrobiology as a field seems to put a lot more focus on the origins of life and how to find life beyond Earth, but less emphasis is put on the end of life, which is what got me interested in finding out more about how biospheres on other planets might meet their ends, and by extension, how long we could expect to detect life on a habitable planet over the course of its habitable lifetime," said O'Malley-James, the lead author of the study.
The scientists were testing a computer model of the climates and biospheres — the overall life — of possible exoplanets.
"That was when the idea came about to run this model forward in time to see when all water and all life would disappear from the planet," O'Malley-James said.
The Sun is a middle-aged star, currently about 4.6 billion years old. In the later stages of its evolution, about 2 billion to 3 billion years from now, the Sun will grow much hotter, leading to much higher surface temperatures on the future Earth and thus far harsher environments for any last life to grow and survive on the planet.
The research team modeled the biosignature gases Earth's biosphere would generate up to 2.8 billion years from the present.
"The most exciting thing about these results is that they suggest that we could potentially detect the presence of life on a planet even at the very end of its habitable lifetime, when the diversity of life and population sizes are considerably reduced compared to what we see on Earth today," O'Malley-James told Astrobiology Magazine.
The death of Earth's biosphere as it exists today would start with plants dying off. Rising temperatures cause silicon-loaded rocks known as silicates to wear away, increasing their absorption of carbon dioxide. The resulting drop in atmospheric carbon dioxide, which plants need in order to generate energy from sunlight, would eventually bring an end to the age of plants.
The extinction of plants would both curtail atmospheric oxygen levels and remove the primary source of food from most ecosystems, leading to the simultaneous extinction of animals, from large vertebrates to smaller ones, with invertebrates having the longest stay of execution. All in all, the researchers calculated Earth's surface would become largely uninhabitable between 1.2 billion and 1.85 billion years from the present.
Still, life is hardy, so microbes could last for much longer than more complex organisms on a dying Earth. After the extinction of plants and animals, the scientists reasoned the planet's future biosphere will be much like its early biosphere in consisting mainly of single-celled microbes.
Without plants to help generate oxygen, atmospheric oxygen would eventually reach negligible levels, triggering a relatively quick shift — within a few million years — toward microbes that can survive without oxygen. The final survivors of Earth could persist either in caves, deep underground, or in relatively cool refuges at high altitudes until roughly 2.8 billion years from now, when the Sun will probably make the planet too hot for astronomers to detect any life from a distance.
The scientists calculated the extinction of higher plants would lower atmospheric oxygen and ozone levels to concentrations undetectable by astronomers by about 1.11 billion years from now. Still, this drop in oxygen could mean levels of the volatile compound isoprene could build up in the air, potentially serving as a biosignature until plants go extinct. Isoprene is a biological substance that normally has a very short lifetime in the atmosphere, since it quickly reacts with oxygen.
The death of plants and animals would also generate large amounts of decaying matter that would release compounds such as methanethiol into the atmosphere. This gas is only known to come from biological sources — although sunlight rapidly breaks this gas down, the resulting gas, ethane, could serve as a potential biosignature until all plants and animals go extinct.
Methane could also be a biomarker when all other biomarker gases become undetectable in a dying planet's atmosphere. In fact, far-future levels of methane in Earth's atmosphere could be 10 times higher than the present — methane-producing bacteria get more of the carbon dioxide they need as fuel because plants are no longer there to remove the carbon dioxide. Still, the researchers caution life is not the only source of methane — volcanoes and chemical reactions involving volcanic rocks can generate the gas as well.
The scientists also conjecture that clouds might serve as homes to potential biosignatures on a dying planet. Once the planet's surface becomes too hot to live, microbes could find refuge in the clouds — microorganisms are known to exist in Earth's atmosphere today, although it remains uncertain whether they are just passing through before falling back down or whether they actively live in the sky. Airborne microorganisms could help generate unexpectedly large cloud droplets in the atmospheres of arid planets, the researchers say. In addition, vegetation could serve as a detectable biosignature until higher plants go extinct — leaves cause a red edge to appear in the spectrum of light reflected off Earth.
One major confounding factor into how a dying alien planet might look could be the influence of extraterrestrial intelligence.
"Intelligent life is difficult to factor in when making these kind of predictions," O'Malley-James said. "It's certainly possible that intelligent life could play a role in mitigating these changes to the far-future environment, perhaps by some form of geoengineering [artificial changes to the land, sea or air], or even moving the planet out to orbit in a cooler position. Predicting what that would do to a planet's biosignatures would be quite a challenge, but it may simply make the planet's biosphere appear younger than we would expect given the age of the planet."
All in all, when astronomers start finding habitable-zone planets circling older stars, "it will be useful to know if we could expect to see any signs of life and, if we can, what signatures that life might leave for us to detect, because the biosphere on a dying planet would be very different to the life we are familiar with on Earth today," O'Malley-James said.
The next step with this avenue of research is to start applying it to real examples astronomers have discovered of older, habitable-zone planets around Sun-like stars, O'Malley-James said. "There are not very many of these yet, so this may involve some modeling of theoretical planets around chosen nearby examples of older stars," he noted. "It's likely that these worlds would not be nice exact copies of Earth, so this may impact the timeline of events that lead up to the end of life on that particular planet."
O'Malley-James is also investigating whether Mars could serve as a template for an alien planet that has reached the end of its habitable lifetime — "in this case, by becoming cold and dry," he said. The researchers would adapt their existing computer model "to simulate Mars and populate all the potentially habitable regions on the planet with microbes that could live there, with the aim of adding to the suite of possible biosignatures for dying biospheres."
O'Malley-James and his colleagues detailed their findings in the International Journal of Astrobiology.
The Daily Galaxy via PNAS and Astrobio.net
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