Chart comparing the smallest known exoplanets with Mars and Earth. via
Sara Seager, a researcher from MIT, is calling for scientists to keep an open mind when it comes picking exoplanets to target in the search for alien life. Astronomers tend to focus on Earth-like exoplanets, but life may exist on exoplanets and even exomoons that aren’t Earth-like at all. It may sound like a bold suggestion, but it’s not so far fetched an idea. We might see a shift in the way we look for alien like.
As far as we know, Earth is the only planet with life. Our planet is teeming with animal, plant, and especially microbial life. And we owe that plethora of life to the Earth’s orbit. We’re in the Solar System’s sweet spot where water exists in a liquid form on the surface and the Sun’s light is able to promote and sustain life. It’s a zone astronomers have come to call the “Goldilocks zone.” The Earth is also the right size for life as we know it. Unlike Mars, our planet has managed to hold on to its life-sustaining atmosphere and we have a wonderful protective magnetic field thanks to our dense iron core.
Since we’re all we know about life in the Universe, it’s natural that we extend the case of Earth outwards and apply it to other bodies. But so far we haven’t found a twin Earth. The closest possible Earth-like exoplanets are nearly twice as big as our world and don’t, as far as we know, have the same composition. Most of the exoplanets we know about are more similar to Jupiter, which we know can’t support Earth life. And there are some other really strange bodies out there. Take hot Jupiters, for example, Jupiter-sized planets that orbit their star closer than Mercury orbits the Sun, and rogue planets that drift through space without a parent star.
1,235 exoplanet candidates and their host stars, all discovered by the Kepler telescope. via
It’s the amazing diversity of exoplanets that has led Seager to suggest we stop focussing on Earth-like planets in the search for life. That we’re finding so many different kinds and sizes of planets is teaching astronomers a lot about how planets form and migrate within their own systems. So why should we apply rules from our system to theirs?
Seager’s paper argues that looking for life on Earth-like planets alone is too narrow a view. By looking exclusively for Earth-like planets, we might be excluding or ignoring some really interesting potentially life-supporting exoplanets. The search for life is one where it doesn’t necessarily pay to be picky; astronomers can only hope to get in depth looks at a handful of exoplanets in the foreseeable future. It’s probably a good idea to explore a variety of planets instead of one type.
We’d do well, says Seager, to extend the parameters of the habitable zone. The conventional definition assumes a planet in this zone will have an Earth-like atmosphere dominated by nitrogen, carbon dioxide, and water vapor. But what about huge, cold planets that could hold on to gaseous molecular hydrogen, a powerful greenhouse gas that could make a planet warm enough for liquid water to exist on its surface well outside the star’s traditionally defined habitable zone. There could also be dry, rocky planets whose atmospheres have much less water vapor than Earth's, but enough to sustain life.
There’s also the argument that life on exoplanets or exomoons won’t be the kind of life that we have on Earth, that is carbon-based life that needs liquid water to survive. That scientists have found life around the hydrothermal vents deep in the ocean far from the Sun’s reach suggests life could thrive off different ingredients than we have on Earth. There could be lifeforms that use liquid methane like we use water, a gas that stays in a liquid form at much colder temperatures than water. Looking for different building blocks for life is another way to extend and expand the habitable zone.
A false colour view of the liquid ethane or methane lakes on Titan's surface. via
So while an extended habitable zone and broader scope for “life” might seem like a far cry in an already impossible endeavour of finding aliens on exoplanets, it’s not really that far fetched. It’s worth remembering that astronomers only found the first exoplanet in 1995. Since then the number of confirmed exoplanets has risen to around 800 with thousands more candidates. And only a handful of those are Earth-like. Astronomers have started measuring the atmospheres of some of these alien worlds with the Hubble Space Telescope, a task the James Webb Space Telescope will pick up when it launches in 2018 (or so). Shouldn’t we pick a variety of planets for these telescopes’ observations and not just one type?
Seager’s proposal of expanding the habitable zone also has some interesting implications for our own Solar System. If planets or moons in faraway orbits could host the ingredients for life, or if life can use different building blocks, there are some good candidates for alien life in our own cosmic backyard. If I’ve said it once I’ve said it a hundred times: it’s time we go fishing on Europa.