We look up at the stars and wonder if we humans are alone in this vast universe. It’s not a question we can answer yet (unless you’re a true believer in UFOs), but there sure are a lot of people working to estimate how much galactic real estate might support life.
Our Milky Way galaxy alone is thought to include 200 billion stars—maybe many more than that, and at least tens of billions of those are yellow-orange stars similar to our sun. But until the last 20 years or so, we really had no way to know if many of those stars had planets orbiting them. If we’re hoping to find life, the first step is to not only find planets, but planets in the habitable zone of their stars: the so-called Goldilocks zone: not too hot, not too cold. There are other considerations, like gravity and atmosphere, but even just finding rocky planets the right distance from their sun is a good start.
Since 2009 the Kepler spacecraft has been monitoring the brightness of more than 145,000 stars. Why? Because if a planet crosses in front of the star (between it and Earth) the light will dim just a tiny bit. Kepler can measure that. Another astonishing instrument called High Accuracy Radial velocity Planet Searcher (HARPS) in an observatory in Chile uses a high-precision spectrograph to also measure light, but in such a way as to detect motion. A planet orbiting a star will cause a tiny wobble in the star that HARPS can see. The Keck Observatory in Hawaii has been using a similar method to great effect for a couple of decades now. There are other efforts underway in the UK, Spain and elsewhere.
So with all this searching going on, how many planets have scientists really discovered around other stars? The number is approaching 700, and might even be higher than that by the time you read this. More than 1200 other possible candidates have been identified and are just waiting to be confirmed. Admittedly many of those are probably gas giants like Jupiter, but many are also smaller than Neptune and might be much closer to Earth in size and composition. But it’s the ones that appear to be in the habitable zone of their star that grab headlines. That number is still small, but growing. And even if only a small percentage of sun-like stars have a habitable planet, in a place as big as the Milky Way that still means potentially tens of millions of planets where life similar to Earth’s could exist.
It’s still not proof, but it sure does improve the odds. So if you’re getting tired of the Caribbean or Europe, have we got a long distance vacation destination for you!