The search for extraterrestrial life is fairly synonymous with the search for life as we know it. We’re just not that imaginative – looking for other planets that could harbor life, we don’t know what exactly to look for, if not Earth-like conditions. Everything we know about life comes from life on Earth.
But conditions clearly favorable for life here — liquid water, surface oxygen, stratospheric ozone, possibly a magnetic field — are not necessarily prerequisites for its development elsewhere. Conversely, their presence is also no guarantee of life. So what can we look for that is an indication of life?
Skip the dwarves
Most (about seventy percent) of the stars in our galaxy are M dwarf stars, and many of them have associated planets. The search for signs of life has largely focused on these planets, especially because there are so many of them. However, the environments don’t seem to be particularly welcoming. Because M dwarf stars are faint, the hospitable zones around them are very close to the star. As a result, the planets get stuck in a gravitational lock: their orbital and rotational periods are the same. This means that (just like our moon always faces us) these planets always have the same hemisphere facing their sun.
In addition to light, this perpetual dayside is constantly bombarded with X-rays and extreme ultraviolet radiation, and the entire planet is subject to forces that would expel the atmosphere. Could life thrive, or even ever gain a foothold, in this kind of environment? “The long-term evolutionary consequences of such conditions are subjects of active debate,” writes astronomy doctoral student Paul Dalba in a recent Perspective piece. Natural Astronomy.
Rather than looking for life among the many Earth-sized planets that orbit M-dwarfs, Dalba suggests we look at the Earth-sized planets orbiting sun-like (G-type) stars. There are only about 10 percent that many of these, but he thinks they might be better bets. And instead of looking for conditions that make life possible, Dalba suggests looking for biosignatures. Specifically, atomic O+ ions at about 300 km altitude.
The atmospheres of Earth and other planets contain neutral gas molecules, as well as the ions and free electrons that form when these neutral gases absorb photons from the sun. Many of those ions accumulate in (surprise!) the ionosphere. Earth’s ionosphere – and crucially, within the solar system, only Earth’s ionosphere – has this atomic O+ ions. Like many of them. they are responsible for more than 90 percent of the ionic species above.
Oxygen and life
Like Earth, Venus and Mars are minor rock planets; they have permanent atmospheres like Earth, and their atmospheres are exposed to the same solar radiation as Earth’s. Data from the Pioneer Venus Orbiter and the Viking descent probe on Mars shows that they have very similar ionospheres – not containing much atomic O+ ions. You know what else Venus and Mars are missing? Photosynthesis.
Dalba’s thesis is that photosynthesis on a planet’s surface, which generates an excess of molecular oxygen, is the only thing that can explain this atomic O2.+ ions in a planet’s ionosphere. The mere existence of life throws a planet’s atmosphere out of chemical equilibrium. O+ would be a neat biomarker as no numerical cutoff is required – just the dominance of O+ among the ionic species in the upper atmosphere would indicate “thriving global biological activity” on the planet below.
Dalba claims that Venus and Mars act as negative controls, showing that planets like Earth but without life do not have this O+ low. Some may think that continuous volcanic activity at the surface can also generate enough oxygen, but Dalba doesn’t. Chemistry with water and UV light can also release oxygen. But the amount of water on Earth isn’t enough to account for the oxygen level required, so he thinks the presence of water on other planets wouldn’t make enough oxygen there either.
Unfortunately, we currently do not have the technology to assess the ionosphere of exoplanets for such a biosignature. Dalba concludes his piece with a plea to “the optical and radio remote sensing communities” to get started inventing the detectors that can handle this task.
Natural Astronomy2018. DOI: 10.1038/s41550-017-0375-y (About DOIs).