This artist’s illustration represents the variety of planets being detected by NASA’s Kepler spacecraft. A new analysis has determined the frequencies of planets of all sizes, from Earths up to gas giants. Key findings include the fact that one in six stars hosts an Earth-sized planet in an orbit of 85 days or less, and that almost all sun-like stars have a planetary system of some sort. (Hat tip to Robert Hurt for inspiring this illustration.) Courtesy C. Pulliam & D. Aguilar (CfA)
Every January, I journey out to the American Astronomical Society for its annual Winter Meeting. And, every time, I’m amazed at new bit of information about the universe. Today’s revelation (and it’s only Day 1 of the meeting), is that the Milky Way Galaxy is populated with many planets — in fact, one team of scientists estimates that at least one out of every six stars in the galaxy has an Earth-sized planet.
That, my friends, is pretty profound.
If you postulate that the Milky Way has about a hundred billion stars, that means there are at least 17 BILLION Earth-sized planets in our galaxy. Again, that’s pretty profound. Now, the next question everybody will ask is, “How many of those are capable of supporting life?” And to answer that question requires a lot more observation. First, to support life, those planets need to be orbiting close enough to their stars that liquid water will be available to sustain life on them. Then, scientists need to look at the other conditions on the planets, and look for “bio signatures” in the planets’ atmospheres that indicate life could be there. So, even though there could be the potential for 17 billion Earth-sized worlds out there, that doesn’t say they are Earth-like… or that they have life. But, there are 17 BILLION Earth-sized worlds out there. Up until the last decade of the 20th century, we didn’t know of any.
Thanks to the Kepler Mission, which has been cranking out planetary candidate discoveries for some time now, the hunt for planets is now an understood and successful ongoing project.
Want to read more details about how the scientists came up with their numbers? Check it out here. And, stay tuned for more AAS news!
This artist's conception illustrates Kepler-22b, a planet known to comfortably circle in the habitable zone of a sun-like star. Image credit: NASA/Ames/JPL-Caltech
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.
