On the iceberg

Although incredible progress has been made in the search for exoplanets, we're still teetering on the peak of the cloud enshrouded mountain, with little real knowledge of what lies beneath. It's instructive to take a look at where things are at today. The first image here (go on, click on it) is a plot of estimated planetary mass (typically a lower limit owing to the nature of the detection techniques used) versus the estimated distance of the stellar parent. A total of 448 planets are pulled out of the excellent exoplanet catalog. Not bad going for 15 years of effort - although it is sobering to realize that our galaxy alone almost certainly contains more than a hundred billion objects we'd term planets - and likely many more. So far the majority of our detected planetary swarm is within about 100 parsecs - about 330 light years - of the Sun. A keen eye will spot that the lowest mass planets, a few hundredths of the mass of Jupiter - the unit used on the Y-axis - also lurk preferentially in some of the closer systems. This is an artifact of the choices made in investigating systems and the nature of planet detection methods.

This 2nd plot (minus a couple of systems) reveals some more of that skewness. The low mass planets at the bottom also correlate with stars that have a slightly lower average mass than those higher up in the plot. Lower mass stars are more strongly perturbed by the gravitational tug of orbiting planets, so the telltale signs of small planets are easier to find - but they're also fainter, and so they are better targets when they're not too far away.

This lower left corner is one of the most intriguing places to go looking for planets. An astonishing 70% of all stars in our Galaxy are less than half the mass of the Sun. There are at least 300 such objects within 10 parsecs of us. They're the real dwarfs, some are a thousand times fainter than the Sun, but they can burn their nuclear fuel for a trillion years. If planets - at least the smaller rocky ones - can form efficiently around these stars, then most worlds in our galaxy, and indeed the universe as a whole, will be bathed in their reddish light. The next few years should reveal more about the closest examples, and their alien environments.