A Near-Earth-sized Planet in the Goldilocks Zone

The search for extra-solar planets that the Kepler mission is doing just keeps cranking out discoveries. The latest one is special: the first planet that is close to the size of Earth that orbits its star in the so-called “Goldilocks zone”. Essentially, that’s the region around almost any star where liquid water could exist on the surface of a planet that happens to be orbiting ‘in the zone’.

The planet is called Kepler-22b, and it is about 600 light-years away from us. While the planet is larger than Earth, its orbit of 290 days around a Sun-like star resembles that of our world. The planet’s host star belongs to the same class as our Sun—a G-type star which is actually slightly smaller and cooler.

Now, this discovery of a planet in the right place around its star is interesting because—as we all know—water is one of the three requirements necessary for life to exist: water, warmth, and organic material (food). So, finding a planet in the sweet spot is a big first step in locating life on other worlds.

It does NOT mean that Kepler has found life. It just means it has found a planet in the right place to support conditions that might allow life. That sounds hand-wavy, but this is the way discoveries work. You have to figure out if the environment is right for life, and then go about trying to understand that environment. Now, we have to study the planet further to see if water exists there.  It could be done by watching as the planet orbits between us and the star, and studying the star’s light as it passes through the planet’s atmosphere.  That is a technique called spectroscopy, and it means that astronomers detect the light, let it pass through a “super-prism” that breaks up the light into its component wavelengths, and then study the data to see if it indicates that water is present.  It would most likely be the presence of water vapor.  The amount you find, along with some other characteristics, tell you about the amount of water in the system.  So, Kepler’s discovery is  a big first step.

Kepler is an interesting observatory. It doesn’t take pictures.  It’s mainly interested in something called “light variation”.  That is, it discovers planets and planet candidates by measuring dips in the brightness of more than 150,000 stars.  If a it sees a periodic dip in the light intensity coming from a star, then there’s a very good possibility that a planet is crossing in front of the star (from our point of view), or “transiting” it.  Kepler requires at least three transits to verify a signal as a planet. (And, by signal, we mean “a dip in the light intensity”.)

Once these candidate planets are announced, then a series of ground-based telescopes and the infrared-sensitive Spitzer Space Telescope look at them and provide data that helps astronomers verify that these things are planets. It’s a long-term task and one that’s keeping astronomers busy. Kepler finds many candidate planets, and each one needs to be meticulously checked out.  Kepler has found 2,326 planet candidates. Of these, 207 are approximately Earth-size, 680 are super Earth-size, 1,181 are Neptune-size, 203 are Jupiter-size and 55 are larger than Jupiter. Today’s announced discovery is one of only 48 stars (of the many thousands  that Kepler has studied) that have planets in their habitable zones. It is the first planet in a habitable zone that is near-Earth-sized, and that’s exciting. I hope it also means that there are many more of them out there, just waiting to be detected!  Stay tuned.