Planets abound!

Image credit: NASA.

Image credit: NASA.

It’s always fun when a government agency uses the word “bonanza” in a press release.

NASA’s Kepler space telescope spent mission spent three and a half years observing distant stars in our galaxy looking for planets crossing in front of them, until a faulty piece of equipment stopped it. It has started up again with its new K2/Second Light mission, which will do the same thing, but with brighter stars and planets with smaller orbits.

In the meantime, scientists have been working feverishly to analyze Kepler’s initial three and a half years of data. To date, somewhere around 3,600 possible planets have been identified in the data, but only a couple of hundred have been confirmed.

But today, NASA has confirmed Kepler’s discovery of 715 planets orbiting 305 stars, blowing the total number of confirmed planets from all observatories way past 1,000 to nearly 1,700. Mission scientists found a way to identify many more planets than they could by waiting around to get careful spectral observations simply by looking for stars with more than one planet. Finding one planet could be a mistake, like if there was a binary star in the background that fooled the camera, but two will only very rarely be a coincidence. And there’s a lot more coming. They found this by looking at only half the data.

If you’re technically minded, you can read the two scientific papers here and here, but here are some highlights:

106 of the new planets are “Earth-sized”, meaning that they are smaller than 1.25 times the diameter of Earth. An emerging body of research, including one of my own recent papers, suggests that it will be mostly these small planets that have visible surfaces and not very deep atmospheres, like the gas giants.

4 of the new planets are in the habitable zones of their stars, cool enough to possibly have liquid water. Since Kepler mostly looks for planets orbiting close to their stars, the others are all far too hot. We should see a lot more of these when they finish crunching the numbers.

Kepler-132 is a sci-fi writer’s dream. It’s a binary star: two stars orbiting each other at a great distance of 450 AU, and both stars have planets. (And they both orbit in the same plane where we can see them cross in front of their stars.) We can’t actually tell which planet belongs to which star, but we know they have to have one each, since two of the planets have almost identical six-day orbits. They would knock each other out of orbit if they both orbited the same star. All three planets in this system are between one and a half and two and a half times the diameter of Earth.

Kepler-296 is even more interesting. It’s another binary star, orbiting a little closer, but still far enough for each star to have its own planets. The two stars have five planets between them, ranging from Earth-sized to Neptune-sized. These are also small stars: late K-dwarfs to be precise. That means they are small, dim orange stars, and despite the close-in orbits of the planets, one of them, Kepler-296f, is in the habitable zone of its star. At 1.79 times the diameter of Earth, this is the smallest of the four new planets in the habitable zone and the most likely to be truly Earth-like.

Several stars in the sample are confirmed to have five planets, all orbiting in the same plane, like our own Solar System. Kepler-90 has at least six planets and possibly seven.

Kepler-223 has four planets in what are known as mean motion resonances. The planets interact with one another gravitationally so that the lengths of their years and whole-number ratios of one another. The lengths of the four planets orbits have a ratio of 4:6:9:12. We still don’t know much about these resonances or why they occur in some solar systems and not others, but it may have to do with how planets form and migrate over time.

About Alex R. Howe

I'm a full-time astrophysicist and a part-time science fiction writer.
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