Many people quickly identify oxygen and ozone as biologically produced molecules in Earth’s atmosphere. For that reason, exoplanet researchers look for oxygen and ozone in the atmosphere of exoplanets in their search for life. Would such a discovery rank as a true signature for life?
Astrobiologists have identified many planets that they believe could conceivably be candidates to host life. However, in the search for life on such planets, the only available means will come from the spectroscopic (radiation measurements) characterization of the planets’ atmospheres. Of the many chemical species life produces, only two have the distinct spectral features and the necessary atmospheric abundance to be detectable. These two are molecular oxygen (O2), and its photochemical by-product, ozone (O3).
Until just a few years ago, astrobiologists considered the detection of molecular oxygen and ozone in an exoplanet’s atmosphere as unmistakable evidence that the exoplanet contained an abundance of life. They noted that only a long history of abundant photosynthetic life could account for a planet’s atmosphere containing detectable measures of molecular oxygen and ozone. Thanks to two papers published in the August issue of the Astrophysical Journal, the detection of molecular oxygen and/or ozone in an exoplanet’s atmosphere can no longer be considered as reliable evidence that life exists on that planet.
Possible False Positive
Several years ago, astronomers identified a way that the presence of molecular oxygen and/or ozone could be a possible false positive signature for life. A moist planet experiencing a greenhouse runaway, independent of any life, would generate enough molecular oxygen and ozone to produce a spectral signature of both.
Water vapor is a greenhouse gas. A warming event on a planet with a lot of liquid water on its surface will cause some of its liquid water to transform into water vapor. That extra water vapor in its atmosphere will trap more heat from its host star. That extra heat will evaporate more of the planet’s liquid water, which will trap even more heat from the host star, causing yet more liquid water to be evaporated. This cycle will continue until all the planet’s liquid water has been transformed into water vapor.
Once a planet accrues a lot of water vapor in its atmosphere, water vapor photolysis will occur. Radiation from the host star will break apart the water vapor, H2O, into hydrogen gas, H and H2, and oxygen gas, O and O2. The O and O2 will form molecular oxygen and ozone.
In 2011, a team of six astronomers explained that a false positive from a greenhouse runaway would be an extremely rare event.1 They showed that molecular oxygen and ozone would be abundant in a planet’s atmosphere only during the relatively short-lived runaway greenhouse event. Furthermore, runaway greenhouse events would be much rarer for planets within the liquid water habitable zone than for planets closer to their host stars than the inside edge of the liquid water habitable zone.
Probable False Positive
Now, two different teams of astronomers have shown much more likely pathways whereby an exoplanet, independent of any existing life, could possess large quantities of oxygen and ozone in its atmosphere. One team, led by Armin Kleinböhl, demonstrated that any temperate terrestrial exoplanet that has a moist atmosphere—one possessing significant quantities of water vapor and a much lower quantity of molecular nitrogen (N2) in its atmosphere than Earth’s atmosphere—will, through ongoing water vapor photolysis, possess large quantities of molecular oxygen and ozone in its atmosphere. Kleinböhl’s team showed that the reason very little abiotic molecular oxygen and ozone exist in Earth’s atmosphere is that the enormous quantity of molecular nitrogen acts as a cold trap. This feature prevents water vapor in Earth’s atmosphere from rising above the troposphere where it can be subject to photolysis.
Kleinböhl’s team showed that for temperate terrestrial exoplanets with moist atmospheres and less atmospheric molecular nitrogen than Earth, “spectral signatures of abiotic oxygen and ozone can be of comparable magnitude as in spectra of Earth seen as an exoplanet.”3 The researchers concluded that molecular oxygen and/or ozone in an exoplanet’s atmosphere cannot be used as an indicator for life on that planet.4
A second team led by Markus Scheucher first noted that compared to other known stars, the Sun has an exceptionally low level of flaring activity. Scheucher’s team also observed that the solar system is unusually well protected from the high-energy particle population of cosmic rays. Scheucher’s group then demonstrated that for typical Earth-like exoplanets, energetic particles from the host star and high-energy cosmic rays will produce a lot more nitrogen oxides (NO, NO2, NO3) and hydrogen oxides (HO, HO2) than is the case for Earth.5 Such greater quantities of nitrogen oxides and hydrogen oxides will destroy ozone in the exoplanets’ lower and upper stratospheres, but will form lots of ozone in the exoplanets’ troposphere.
Scheucher’s team concluded that unless the production rates of nitrogen and hydrogen oxides from cosmic rays and from the host star’s radiation are well known, the presence of oxygen and ozone in an exoplanet’s atmosphere cannot be considered as a signature for life on that planet.6 Since measures of these production rates lie beyond both present and foreseeable astronomical instrumentation, Scheucher’s team advises against using atmospheric oxygen and/or ozone as a biosignature.
I predict that it will take a while before the astrobiological community fully accepts that atmospheric oxygen and ozone are no longer reliable biosignatures. Nor will they agree that with present or foreseeable future astronomical instruments, scientists have no means for unambiguously detecting life on exoplanets. Thus, I will not be surprised if a paper is published or an announcement is made at a press conference claiming that life has been detected on an exoplanet. However, I hope that the research papers discussed here and scientists’ attempts to publicize reasons for skepticism about biosignatures will dampen philosophical debate on the origin of life such a paper or announcement might generate. What seems less debatable is the notion that scientific discoveries affirm the rarity, if not uniqueness, of Earth as a life-supporting planet.
Featured image: Artist’s Conception on Waterworld Planets. Image credit: NASA