How many planets are out there? First Results from Kepler!

“We are all captives of the pictures in our head — our belief that the world we have experienced is the world that really exists.” –Walter Lippmann

For a long time, humans have wondered about life on other worlds, and about worlds around other stars. Until the 1990s, this was mostly speculation and hope. But shortly thereafter, some clues started rolling in. In 1992, the first planet outside of our Solar System was detected, and three years later, the first planet around a solar-like star was found.

Only, something was awfully weird about this planet. You see, in our Solar System, Mercury is the closest planet to the Sun. Mercury takes 88 days to orbit the Sun, lives (on average) 58 million km from the Sun, and is very light, weighing only 5.5% as much as the Earth. And that’s what we’re used to — for an innermost planet — thanks to our Solar System.

So you can imagine the surprise when Bellerophon — or 51 Pegasi b — was announced. Instead of 88 days, it orbited its star in just 4.2 days, located a mere 7 million km from the stellar surface, and weighing in at over 150 times the mass of Earth!

When the next few extra-solar planets were discovered, they looked a lot more like this nightmarish world than like any of the ones orbiting our star. They were massive giant planets, located far closer to their stars than any of our gas giants are, and in many cases, far closer to their star than even the Earth is!

So what gives? Well, these “hot Jupiters” aren’t very common at all. The reason we found so many of them, it turns out, is that these are the easiest planets to find.

And we’ve gotten a lot better over time. As of this past Monday, the extra-solar planet catalogue had compiled 461 planets orbiting other stars!

And while most of them were still comparable to the size of Jupiter or Saturn, many of them were starting to get smaller, including a few rocky planets. (Including in the Gliese 581 system, above.)

Well, that’s all about to change. There’s the NASA Kepler mission, which is currently monitoring a small patch of the sky, looking at about 100,000 stars.

That’s all that Kepler is doing, mind you. By monitoring these same stars, accurately, from space, over a long period of time, they’re likely to discover many, many planets! How? Every so often, one of these planets will pass in front of its star, blocking just a little bit of its starlight. As this happens, periodically, we can figure out the size and distance of the planet from its star!

And, wouldn’t you know, earlier this week the Kepler team released their first results. (Steinn and Nancy have some good things to say about it, too.)

What did they find? Let’s put up the planets in our Solar System for you to look at while I spill the beans.

They have found, definitely, 306 new extra-solar planets, including 5 systems with multiple planets. Unlike previous searches, most of the planets that Kepler found are Neptune-sized (the smallest gas giant in our Solar System) or smaller!

That’s very, very promising! But they’ve left us with the ultimate teaser: they are withholding data — for a year — on 400 particularly juicy candidates!

What did they find? Are they Earth-sized planets? Are they planets in the habitable zones around their stars? Are they “just barely” detections, and they want to get some more data?

If we assume that each one is a planet, that brings us to 706 planets around 100,000 stars. Since our galaxy has around 200 billion stars, we can figure out that there ought to be — wait for it — at least 1.4 billion planets in our galaxy!

And that, mind you, is a lower limit, as this only will see planets orbiting edge-on to their stars! But it’s the biggest lower limit we’ve ever been able to set, and you’re among the first to know!

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