The search for extraterrestrial life is fairly synonymous with the search for life as we know it. We're just not that imaginative—when looking for other planets that could host life, we don’t know what to look for, exactly, if not Earth-like conditions. Everything we know about life comes from life on Earth.
But conditions that clearly favor life here—liquid water, surface oxygen, ozone in the stratosphere, possibly a magnetic field—may not necessarily be prerequisites for its development elsewhere. Conversely, their presence does not guarantee life, either. So what can we look for that's an indication of life?
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, primarily because there are so many of them. However, the environments do not seem to be especially welcoming. Because M dwarf stars are dim, the hospitable zones around them are very close to the star. As a result, the planets get stuck in a gravitational lock: their orbital period and their rotational period are the same. This means that (just like our moon) these planets always have the same hemisphere facing their sun.
In addition to light, this perpetual-day side is constantly barraged with X-rays and extreme ultraviolet radiation, and the whole planet is subject to forces that would drive off its atmosphere. Could life thrive, or even ever get a toehold, in this type of environment? “The long-term evolutionary consequences of such conditions are topics of active debate,” writes astronomy PhD candidate Paul Dalba in a recent Perspective piece in Nature Astronomy.
Instead of looking for life among the many Earth-size planets orbiting M dwarfs, Dalba suggests we look at the Earth-size planets orbiting Sun-like (G-type) stars. There are only about 10 percent as many of these, but he thinks they might be better bets. And instead of looking for conditions that might support life, Dalba suggests looking for biosignatures. Specifically, atomic O+ ions at about 300km up.
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