Mercury, the closest planet to our Sun, is often thought of as being super-close to the Sun and super hot. But really, it’s 40% of Earth’s distance from the Sun, and it’s only 430 degrees Celsius (800 Fahrenheit) on the day side. We already knew about hot jupiters, of course. These giants can be as close as 5% of Earth’s distance from the Sun and as hot as 1000 degrees Celsius (1800 Fahrenheit).
But the thing with hot jupiters is that they have to migrate to get where they are, and that migration would throw off the gravitational balance of the whole system and scatter the other planets into space. (Well, that’s what we think, anyway.) So we wouldn’t expect to see five planets all orbiting a star closer than Mercury, would we? Better check that picture again.
Kepler-11 is a yellowish star not too different from our Sun, but it does, indeed, have five planets orbiting closer than Mercury, and one more a little outside them. And these aren’t small planets, either. They’re not Jupiter-sized, but they’re all super-Earths or bigger. Some of them look like water worlds, but some of them are less dense than water, meaning that they have thick hydrogen atmospheres, like Neptune. That means they probably migrated from farther out in the solar system.
How did so many planets wind up in such a small space without getting scattered around by each other’s gravity? This is one of the many questions about planet formation that we’re still trying to understand. What we do know is that these planets are about as close together as they can be without being gravitationally scattered (this is called being dynamically packed). We also know that this situation is fairly common, and Kepler-11 is only the most vivid example.
We also know of solar systems that are more spread out, like ours, but they are harder to find with current technology. It will be interesting to see what gets turned up by the next generation of planet hunters.