Using NASA's orbiting Kepler observatory, astronomers have found a complete solar system of six planets orbiting a sun-like star... and it's really weird: five of the six planets huddle closer to their star than Mercury does to the Sun!

None of them is what I would call precisely earth-like -- they're all more massive and much hotter than Earth -- but their properties are intriguing, and promise that more wonderful discoveries from Kepler are coming. The star, called Kepler-11 for convenience, is very sun-like, with a mass, size, age, and temperature very close to that of our own Sun. Located 2000 light years away, it's way too faint to be seen without a good telescope, though. The planets, called Kepler-11 b, c, d, e, f, and g, were detected using the transit method: their orbits are edge-on as seen from Earth, so when they pass in front of (transit) their star they block a bit of its light. That dip in brightness is what Kepler detects. Over time, multiple dips are seen and those are used to determine the periods of the planets. The amount of light blocked by the planet gives you its size; the bigger the planet the more light it blocks. So the transit method tells you how many planets there are, how long it takes them to orbit, and their size.

But in the case of the minisystem Kepler-11, the planets are huddled together near their star like a close-knit family. And like any such family, the kids poke and prod each other: the planets are so tightly packed near the star that they all interact gravitationally! Each one pulls and tugs on the others, subtly changing their orbits, which in turn affects the timing of the transits. Applying a complex mathematical model to the timings allowed the astronomers to actually calculate the masses of the planets, something usually not possible in systems with fewer planets or more widely spaced orbits. What they found was that the five inner planets have periods from 10 to 46 days (Mercury, by comparison, takes 88 days to orbit the Sun), meaning they lie from 13.6 million to 37.4 million km (8.4 million to 23 million miles) from the star. The planet sizes range from twice the Earth's size to 4.5 times, and have 4 to 14 times the Earth's mass. Once you have the size and mass, you can calculate the density, and that's where things get interesting. The density of an object is a strong clue to its composition; low density objects may be mostly gas (meaning a thick atmosphere) whereas high densities imply a rocky and metallic object like the Earth. These planets are all over the place, from fairly dense to fairly puffy:

That graph shows the size of the planet (vertical axis) versus its mass (horizontal axis). Earth and Venus, being small and dense, are at the lower left. Neptune and Uranus, which are more larger and puffier, are to the upper right. The ellipses represent the planets of Kepler-11; uncertainties in the sizes and masses mean the real values could be anywhere in those ellipses (for example, Kepler-11 c has the least well-known mass, so its ellipse is quite long horizontally). The lines represent where you'd see planets of certain compositions, so a planet anywhere on that bottom solid line could be Earthlike in its makeup -- though I'll point out that doesn't mean it is, just that it could be. All of the newly found planets are more massive than the Earth, but Kepler-11 b has the highest density, putting it very near the Earth's density, about 60% of ours, in fact. This means that it most likely does not have a super-thick atmosphere like a gas giant, but may be rocky with lots of water.

Again, none of these planets is earth-like in the sense of really being like our home planet; they are far too hot and massive for that. But the important thing to note here is that this system is, apparently, stable over long periods of time. The star is billions of years old, and assuming the planets formed at the same time as the star (which is kinda how it has to go) then the system must be stable. That doesn't mean it hasn't changed since it formed, however. Most likely the planets formed farther out and migrated inwards over time, as detailed physical models have shown can happen. Most likely the inner planets were also more massive when they formed, and lost a lot of their atmospheres as they got closer to their star. I'll admit, I was pretty skeptical when I first heard of this system. With planets packed that closely together they can interact pretty strongly, and I'd think that would be unstable over billions of years. At some point one of the planets would get a bit too close to another and get chucked out of the system or dropped into the star, making a hash of the other orbits at the same time. However, after reading the paper, the detection of these planets looks really solid, so clearly nature is telling us we have to be careful -- once again -- with our assumptions and prejudices. But this news is very cool. Kepler will probably find quite a few systems similar to this one, and this new technique of using the slight changes in the orbital periods to get the planets' masses is really amazing and useful. I'll note that finding a solar system like ours, with well-spaced-out planets, will take years to confirm since the planets take much longer to orbit their star; we see fewer transits from them. But this shows that Kepler can easily see planets not too much different than Earth when they transit a star very much like our Sun, and that stokes my hope that soon, maybe very soon, we'll detect a truly earth-like world orbiting another star.

Image credits: transit art: ESO/L. Calçada; graphs: Nature magazine. Related posts: - Gallery of exoplanets: real pictures of alien worlds - Big news: first solid exoplanet found! - Exoplanet found from another galaxy! - Kepler works!