By Emily Levesque, University of Washington
Astrobiology combines astronomy with other fields ranging from biology to chemistry to geology to study life in the universe. Researchers in this field examine other planets as potential biospheres that could be home to extraterrestrial organisms, and most of this potential is focused on what we call habitability. And, habitability boils down to one fundamental question: Can a planet host liquid water?
Surviving Harsh Conditions
Earth is our only frame of reference for a habitable planet, but even here we find life forms surviving and thriving in shockingly harsh conditions. We’ve discovered organisms that breathe sulfur and bacteria that can survive extreme gravity or live a half-mile deep in an Antarctic lake. Eyeless creatures live out their entire lives in pitch-dark caves, and microscopic creatures, known as tardigrades, have even survived exposures to the vacuum of space.
Yet, despite all these differences, the only universal need of all living things on Earth is liquid water. Water is, therefore, seen as a non-negotiable requirement for supporting life. So, how can we tell if a planet has liquid water?
Astronomers generally use spectroscopy to sort out the light from a star or planet, according to its wavelength, and search for the tell-tale signatures of atoms or molecules that are absorbing light. As easy as it may sound, it is still extremely challenging to find planets around other stars. Since planets don’t emit their own light, but merely block or reflect the light of their host stars, studying the chemistry of these dim and distant objects can be incredibly difficult, pushing today’s telescopes to their limits. However, we can use what we know about a planet’s orbit and host star to define something called the habitable zone.
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The Habitable Zone
The habitable zone sometimes earns the nickname the ‘Goldilocks zone’, as it represents the range of distances from a host star where a planet could host water based on its temperature. Too close to the host star and any water on the planet will sublimate into gas. Too far and it freezes into ice. Only the planets that stay at just the right distance from their host star will be able to maintain a surface temperature that allows for liquid water, marking the sweet spot where life, as we understand it, can occur.
Defining the habitability of exoplanets can be complicated. Of the more than 4,000 exoplanets that we’ve discovered, only a tiny fraction of them—less than 1%—sit in their host star’s habitable zone. But simply being in a water-friendly region doesn’t guarantee that a planet could be habitable. The planet needs to be rocky, like Earth or Mars, rather than a gas giant like Jupiter or Saturn that’s simply drifted inward from the icy outer regions of the planetary system where it likely formed. We also need to consider things like the potential atmospheric pressure and geology of the planet, and whether its environment is exposed to hostile radiation, thanks to its host star.
Potentially Habitable Exoplanet
Between 2005 and 2010, a team of astronomers discovered several exoplanets around the star Gliese 581. One of these exoplanets around the star, Gliese 581, is about 5.5-times as massive as our own Earth. It sits tantalizingly within the star’s habitable zone, and with its low mass the planet seemed, at first glance, to be potentially rocky and Earth-like. At only 20 light-years away, it was an immensely exciting candidate for the first exoplanet that might truly be habitable. Sadly, further research revealed bad news for any potential life on this planet; it had likely been bombarded with X-ray and ultraviolet light from its host star earlier in its life.
Data also suggest that the planet could be suffering from a runaway greenhouse effect, with thick clouds of greenhouse gases, like carbon dioxide and water vapor, trapping heat and driving the surface temperature past the point that would vaporize any liquid water.
Finally, the planet is close enough to its small, cold host star that it may have become what’s called tidally locked. Like the Moon around Earth, the planet’s rotation period and orbital period are the same. This means that the same face of the planet stays pointed towards its host star at all times. This would wreak havoc on the surface habitability of the exoplanet, roasting one side of it and freezing the other.
Unfortunately, a planet that, at first glance, seemed wonderfully hospitable thanks to its presence in the habitable zone, turned out instead to be a very poor bet for successfully hosting life.
How to Direct our Searches?
Today, a handful of planets are considered our best candidates for potential habitability, fulfilling all the criteria’s: Earth-sized and likely rocky, orbiting host stars that probably haven’t blasted the planets with radiation, and far enough from their hosts to avoid getting tidally locked. Still, even these data aren’t enough. Without taking a much closer look at these planets it’s hard to say what they might be like or what this implies for their surface conditions.
In conclusion one can say that although defining the habitability of exoplanets can be complicated, but we need this definition if we are to understand how to direct our searches for life on other worlds. Beyond all that, discovering a planet that can host life is quite different than identifying a planet that does host life. Even if we did find a compelling candidate planet, how could we possibly be sure that there’s life on it?
Searching for alien life might sound fanciful or unrealistic, but the field of astrobiology is actually dedicated to answering some very rigorous scientific questions. However, first, we need to understand where we should be looking!
Common Questions about Astrobiology and Habitable Zones for Extraterrestrial Life
Since planets don’t emit their own light, but merely block or reflect the light of their host stars, studying their chemistry can be incredibly difficult.
The habitable zone sometimes earns the nickname the ‘Goldilocks zone’, as it represents the range of distances from a host star where a planet could host water based on its temperature.
One of the exoplanets around the star, Gliese 581, is about 5.5-times as massive as our own Earth. It sits tantalizingly within the star’s habitable zone, and with its low mass the planet seemed, at first glance, to be potentially rocky and Earth-like.
