One of the biggest questions in astronomy today is, are there any other Earths out there, and if so, how many? We know that there are over 100 billion stars in the galaxy, and that a lot of them have planets. But we're still groping round trying to nail down the frequency of planets, and how many are rocky bodies like Earth (as opposed to gas giants like Saturn and Jupiter).
A new study using the Spitzer Space Telescope has revealed that planets like Earth may be common! The procedure was actually rather simple. They looked at over 300 sun-like stars, and grouped them by ages (very young, young, middle aged, etc). They then used Spitzer to look for the presence of dust around the stars; dust glows in the infrared when warm, and the temperature (and thus the distance of the dust from the star) can be found. What they found was striking: young stars had lots of dust that was at about the same distance from the star as the Earth is from the Sun, but as stars get older, the amount of dust drops. The timescale for the dust to disappear -- a few hundred million years -- is roughly the same timescale it takes planets to form. The obvious conclusion is that as time goes on, planets are forming around those stars, and they hoover up the dust. This process would make rocky planets, much like the Earth! In other words, this study did not directly detect planets, but it found that planet-making material disappears with age. That's incredibly provocative. They found that 10-20% of young stars had these disks of dusty debris around them. As it happens, about 10% of the stars in the Milky Way can be categorized as sun-like, which is about 10 billion stars. If 10% of them have rocky planets, as this study indicates, then there may be a billion Earths orbiting stars in our galaxy alone! And that's only for stars like the Sun; lower mass stars also can form planetary systems, and there are far more of them then stars like the Sun. It is entirely possible that there are many billions of terrestrial planets in the galaxy... and there are hundreds of billions of galaxies in the Universe. The Universe may be buzzing with life. We still don't know, but this is another big step forward.