New data from the European Space Agency shows that there may be tens of billions of rocky "super-Earths" orbiting in the habitable zones of red dwarf stars in our galaxy, and 100 of them are probably with just 30 light years of us. But how many of these planets are really "Earth-like?" Probably not nearly that many.
The ESO's HARPS planet-finding spectrograph spent the last six years surveying a sample of about 100 red dwarf stars, which are smaller and cooler than our sun but incredibly common, making up about 80% of all stars in our galaxy. The astronomers discovered that a whopping 41% of the stars that they looked at had super-Earths orbiting in the stars' habitable zones. Using this sample as a proxy for the entire red dwarf population, that works out to anywhere from tens of billions to about a hundred billion stars playing host to habitable zone super-Earths. Wow.
But how excited should we really be? Does this mean that there are tens of billions of worlds out there ripe for colonization or bursting with aliens? Maybe, but probably not.
In a staggeringly lucky coincidence, humans managed to evolve on a planet that's perfect for us. The temperature is just right, there's water all over the place, and the gravity is exactly 1g. What are the odds! We've been pampered, though, and we'd likely have a tough time adapting to other worlds, especially if those worlds are bigger and warmer as opposed to smaller and cooler (like, say, Mars).
When we're talking about exoplanets, the distinction between "Earth-like" and "habitable" is an important one. From what we can tell, astronomers use the term "habitable" to describe a planet that could potentially be inhabited by something, the criteria being a planet with a rocky surface that's the right distance from its parent star such that liquid water could be present. Into this category fall "super-Earths," which are rocky planets in habitable zones that are anywhere between one and ten times the mass of Earth.
And this is where things start to take an unfortunate turn for us humans, since we don't deal very well with increased gravity. If you found yourself on a super-Earth exoplanet with twice the mass of Earth, you'd weigh twice as much on its surface. And it's not just like putting on your own body mass in fat all at once: everything weighs double, from your blood to your eyelids to your hair, while your muscles haven't gotten any bigger. Yeah, it would suck.
And that's just 2g: these super-Earths are still called super-Earths all the way up to 10g. This isn't to say that 10g isn't technically survivable: untrained humans can deal with between 4g and 6g before blacking out, and trained fighter pilots wearing specialized g suits can remain conscious at over 9g. But again, there's a huge difference between "technically survivable for extremely short periods of time" and "habitable by humans," and while it's fantastic to learn that there are so many potentially habitable (by something) planets out there, we've still got some work to do finding our species a nice little vacation planet that we'd all enjoy visiting.