“Rather that simply recognizing that a planet hosts life, we may be able to say something about how the activities of its biosphere vary in space and time,” says astronomer Stephanie Olson.
When it finally launches in 2021, the JWST it will be 14 years late, orbiting the Sun 1.5 million km from Earth. As the successor to the iconic and beloved Hubble Space Telescope will have an extraordinary talent thanks to its 6.5m golden mirror and astoundingly sensitive cameras –the ability to look for signs of alien life – detecting whether atmospheres of planets orbiting nearby stars are being modified by that life.
Back on Earth, however, astronomers – including the University of Washington team and the University of California, Riverside, who propose “life-detection” observations using the telescope – are unerringly thrilled at the prospect of its launch.
Oxygen is produced by photosynthesis and is commonly thought to be a potential biosignature on other worlds, although it is also possible for oxygen to be produced from abiotic sources. Similarly, methane is produced by life and is a potential biomarker, but can also be produced by other means. Now, two recent papers discuss new ways of looking for biosignatures by studying how life can influence a planet’s atmosphere.
A paper by Stephanie Olson at the University of California, Riverside, and colleagues, discusses how seasonal changes in the atmosphere caused by life could be used as a biosignature. A second paper by astronomer Joshua Krissansen-Tottonat at the University of Washington, along with Olson and David Catling, and looked at potential biosignatures produced by atmospheric gases that can only co-exist in the presence of life.
Krissansen-Totton and his team have looked into whether the telescope could detect signs of what they call “biosignatures” in the atmospheres of planets that are orbiting a nearby star. Seasonal changes and the presence of disequilibrium gases in such a planet’s atmosphere could indicate the existence of life there. Biosignatures that vary in time and atmospheric gases that shouldn’t exist without life to replenish them could be two possible ways to detect life on exoplanets.
“We could do these life-detection observations in the next few years,” says Krissansen-Totton. The basis for this search may lie in JWST’s cameras being so sensitive to light that it could pick up so-called “atmospheric chemical disequilibrium”.
“Rather that simply recognizing that a planet hosts life, we may be able to say something about how the activities of its biosphere vary in space and time,” says Olson.
Earth’s atmosphere would change dramatically if all life was suddenly removed –an idea with a long heritage, promoted by celebrated scientists James Lovelock,who coined the gaia theory of Earth’s biosphere, and Carl Sagan. The thinking is that if all life on Earth disappeared tomorrow, the many gases which make up our atmosphere would undergo natural chemical reactions, and the atmosphere would slowly revert to a different chemical mixture.
Both Krissansen-Totton and Olson examined the seasonal variations in carbon dioxide on Earth, a signal that could be detectable on other planets assuming that life elsewhere is also carbon-based. Carbon dioxide is an important atmospheric component on habitable worlds due to the role it plays in climate regulation via weathering.
They found that the seasonal carbon dioxide (CO2) signal would be dominated by land-based ecosystems, which are in direct contact with the atmosphere, indicating that CO2variability might not be detectable on ocean worlds. This is seen on Earth, where the ocean-dominated Southern Hemisphere has a weaker CO2variability signal than the Northern Hemisphere. Carbon dioxide seasonality would be difficult to detect on other planets, but it is a powerful indicator of the presence of life since it is unlikely to occur on planets with an ocean unless life is present.
Because of this, searching for signs of oxygen (or its chemical cousin ozone) has long been thought to be a good way of finding life. But this does rest on the assumption that extraterrestrial life runs by the same biological rules as our own.
They also looked at the scenario of an exoplanet that is an analog of the early-Earth, where life existed but where there was still very little oxygen in the atmosphere. Weak oxygen signals are difficult to detect, but a varying ozone signature (ozone is a molecule built from three oxygen atoms) might be more visible in the spectrum of an exoplanet. Such a signal is more likely to be detected for a planet with less oxygen than the present day Earth because ozone can create a stronger signal than oxygen.
“Seasonality would be difficult to detect for a planet resembling the present-day Earth, at least in the case of oxygen,” explains Olson. “The reason is that baseline levels of oxygen are really high today, and so small seasonal fluctuations are very challenging to measure at our planets surface, and would be even more so on a distant planet.”
Ozone is a proxy for oxygen in the search for the existence of oxygen in the atmospheres of potentially habitable planets. (Lynette Cook).
Krissansen-Totton, Olson and Catling also simulated early-Earth atmospheres, but this time looking for signatures of disequilibrium, meaning the presence of gases that would not ordinarily exist in an atmosphere without some active process, such as life, producing them. Earth has a large atmospheric disequilibrium today, but they calculated that a disequilibrium has existed since life formed on Earth and that the evolution of disequilibrium follows the rise in biogenic atmospheric oxygen.
In the Archean eon (4 to 2.5 billion years ago), a disequilibrium existed via the coexistence of carbon dioxide, nitrogen, methane, and liquid water, which ordinarily would react to create ammonium and bicarbonate, quickly removing the methane from the atmosphere without the presence of life to replenish it. Carbon dioxide and methane should be detectable in exoplanet spectra by JWST, particularly on planets orbiting red dwarfs. If these are detected, but no carbon monoxide is found, it could be a strong biosignature. This is because many of the non-biological scenarios that replenish methane would also be expected to produce carbon monoxide (CO), and because surface life consumes CO.
“This is a very easy metabolism to do; if there’s CO and water around, then microbes can make a living by combining these species to make CO2and molecular hydrogen (H2),” says Krissansen-Totton.
The largest source of disequilibrium in the Proterozoic eon (2.5 to 0.54 billion years ago) was the coexistence of nitrogen, water and oxygen. Both oxygen and nitrogen are produced by life, and without life to replenish the oxygen, it would be converted to nitric acid in the ocean.
Recognizing signs of life that use different metabolic pathways might also be possible if the atmospheric gases are in an unusual disequilibrium, but it would be challenging to detect.
“Detecting microbes that oxidize iron in the ocean might be challenging since this particular metabolism does not generate any gaseous waste products,” says Krissansen-Totton. “Among the possible metabolisms that do produce waste gases are some promising possibilities. For example, laughing gas (N2O) is a biogenic gas that we would not expect to see in equilibrium in the atmospheres of lifeless planets. Similarly, various sulfur metabolisms might be detectable since they modify the abundances of organic molecules in a planet’s atmosphere to be out of equilibrium.”
Finding early-Earth analogues with signs of seasonality or disequilibrium might indicate that life is not only present, but has evolved in a similar manner to life on our own planet.
So, what might the JWST’s first target for alien life be?
Mr Krissansen-Totton simulated the data that would be obtained if JWST were to look at planets orbiting a small Jupiter-sized star called TRAPPIST-1, about 39.6 light-years away from our Sun. This star caused a sensation in 2017 when it was discovered to host seven Earth-sized planets, several of which could possess liquid water, and hence might be a good bet for hosting life.
This artist’s impression shows an imagined view from the surface one of the three planets orbiting an ultracool dwarf star just 40 light-years from Earth that were discovered using the TRAPPIST telescope at ESO’s La Silla Observatory. These worlds have sizes and temperatures similar to those of Venus and Earth and are the best targets found so far for the search for life outside the Solar System. They are the first planets ever discovered around such a tiny and dim star. In this view one of the inner planets is seen in transit across the disc of its tiny and dim parent star.
The Washington researcher predicts that James Webb could measure the amounts of methane and carbon dioxide in the atmosphere of the fourth planet, TRAPPIST-1e, from the dips in light at wavelengths affected by these gases.
It would be a measurement of an unimaginably tiny signal, but Cornell University astronomer Prof Jonathan Lunine, who was not involved in this study, is excited by the prediction, saying “they make the case that this can really be done with JWST”.
Once the measurement is made, though, Krissansen-Totton observed that, “you can then ask the question: do we know of any non-biological processes” that could produce that effect?”
If the atmosphere of TRAPPIST-1e (at top of page) was found to be out of sync, researchers would then need to rule out any non-biological effects, such as volcanic activity before declaring the existence of extraterrestrial life. “That kind of confirmation is going to require multiple observations, to really make a totally solid case,” says Krissansen-Totton.
“But if we detect something that we don’t have an alternative explanation for, I think that would be an incredibly exciting discovery.”
But in the interim, the JWST’s golden mirror remains securely locked in a lab in California, and astronomers must continue to wait for these possibilities to be explored.
JWST will be joining a host of new facilities that will subject planets around other stars to some serious scrutiny over the next few decades.
“I think that we’re in a remarkable time for understanding our Universe and exploring the cosmos, and James Webb is going to take the next step in that,” says Lunine. “It is going to be truly worth it.”
NASA Image top of page: Trappist 1f
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