1 November 2010

Planets Like Grains of Sand

Posted by Ryan

The other day I came across a press release announcing that nearly one in four sun-like stars could have planets as small as Earth. That’s pretty awesome! But I though it was especially interesting how they came up with this number. Current technology can’t quite see an Earth-sized planet around a sun-like star, so how do you count things that you can’t see? Well, you count everything else and then extrapolate. The analogy that they use in the press release is a great one:

“We studied planets of many masses — like counting boulders, rocks and pebbles in a canyon — and found more rocks than boulders, and more pebbles than rocks. Our ground-based technology can’t see the grains of sand, the Earth-size planets, but we can estimate their numbers,” said Andrew Howard of the University of California, Berkeley, lead author of the new study. “Earth-size planets in our galaxy are like grains of sand sprinkled on a beach — they are
everywhere.”

There are two and a half reasons that I like this analogy so much. The first reason is that the process they describe: counting large rocks to predict smaller rocks is exactly what is done when choosing a landing site on Mars! Even with HiRISE, we can’t really resolve less than a meter or so reliably, but we know enough about the size distribution of rocks that tallying up all the larger rocks visible from orbit can give a very good estimate of the smaller ones we can’t quite make out. This is a technique that has been improved over the years as new landing sites provide new “ground truth” to test the method. Matt Golombeck talked in detail about this process at the latest MSL landing site workshop, so I’ll point you to his presentation for more details.

It was only a matter of time before this sort of extrapolation was applied to planet-hunting. The problem is, it’s a lot harder to count even the big planets than it is to count the rocks in an image. You have to watch the star that they orbit for a long time so you can reliably measure the wobbles of the star caused by the planet’s motion over multiple orbits. That’s why it took years of surveying the stars to build up good enough statistics. The planet hunters involved in the survey focused on planets very close to their stars because those planets orbit in days to hours rather than years to decades. The fact that they found more small planets than large is encouraging for the search for small planets farther out from the stars in the “habitable zone”.

But you’re probably wondering what the other one and a half reasons I like the rock-counting analogy are. Well, the half reason is that it immediately reminded me of the title of a science fiction book that I’ve always wanted to read: “Stars in my Pocket Like Grains of Sand” by Samuel Delaney.

And finally, the description of earth-like planets as grains of sand on a beach called to mind this short excerpt of a poem by William Blake that I’ve always liked:

To see a World in a Grain of Sand
And a Heaven in a Wild Flower,
Hold Infinity in the palm of your hand
And Eternity in an hour.