Monday, March 5, 2012

How We Know: GJ 1214 b, a Planet 40 Lightyears from Earth, Has Water

In December 2009, astronomers noticed a star they had been observing, known as GJ 1214 (there are billions of stars in our galaxy; they can't all have imaginative names) gave off a little less light about once every 38 hours.  This was a telltale sign that a planet was orbiting, and it was exactly what these astronomers were looking for.

Artist's impression of GJ 1214 b passing its star, by ESO/L. Cal├žada.

These astronomers were working on the MEarth Project, looking at thousands of red dwarf stars, named for their color and size. (Yes, astronomers call a star many times larger than Jupiter a "dwarf."  You should see what they call "giants.")  GJ 1214 is one of these red dwarfs, and its only known planet earned the name GJ 1214 b (the star itself would be considered GJ 1214 a, if you were wondering).

By 2009, planet-hunting had become a science (ha ha).  Get a powerful telescope (the more, the merrier), aim at a patch of sky, feed the data into a computer program, and wait.  The computer will scan the images, looking for changes in brightness in all the stars in view.  When a planet gets in between its star and us, it blocks some of the light, and the stars brightness dips.  Since a planet has a regular orbit, it will cause these dips on a regular schedule.  Anyone out in space looking at our star from the right angle will see it dip a bit once every year as we pass by.  They'd see a much bigger dip every 12 years as Jupiter passes by, since it blocks much more light.

But even though we're getting the process down, the number of planets we know of is still only about 760, much fewer than the 160 billion we estimate are out there orbiting other stars, not to mention the trillions we think are free-floating throughout the galaxy.

Cartoonish view of planets in the galaxy, by ESO/M. Kornmesser.

With only 760 confirmations (so far), we can take a closer look at each, and really try to find out as much as possible about them.

But what can we learn from a dip of starlight?  You might be surprised.