Kepler-11 and its six known planets represent one of the most extraordinary planetary systems we have discovered. A new analysis challenges the theory these planets were strange hybrids between rocky objects like the Earth and gas giants, instead, putting them firmly in the Earth-like class, but also suggests surprisingly closely bunched planets can orbit stars similar to the Sun.
Even among the host of planets discovered by the Kepler Space Telescope, the Kepler-11 system stands out. All the planets we have identified around this star are far too hot to support life, but the presence of six planets orbiting so close to their star challenges our ideas about planetary formation. Astronomers continue to debate whether so many planets can survive in the long term so close together.
A new analysis concludes Kepler-11 is almost identical to what the Sun was like a billion years ago, implying that unusual systems are not confined to freakish stars. Instead, stars just like the Sun can host planetary systems that look nothing at all like our own.
As with all Kepler's discoveries, the planets of Kepler-11 were detected through dips in the light coming from the parental star, thanks to the good fortune that their orbits cause them to pass in front of it, as seen from Earth. Five of these planets orbit closer to their star than Mercury does to the Sun, and the last one is not much further out.
If Kepler-11 was a faint red dwarf, worlds at this distance might support water, or even life. Instead, it is fractionally brighter than the Sun, baking any objects at such close distances. The interest lies in what the existence of so many planets, packed so tightly together, can tell us about the way planets form, and what other systems might exist elsewhere.
University of Chicago graduate student Megan Bedell has investigated Kepler-11. Her paper rejecting the previous categorization as an old, low-density star is available to view on arXiv.org. Bedell and her co-authors note that different methods for measuring Kepler-11's size contradict each other. Although they cannot explain this discrepancy, they argue the smaller estimates are more credible, suggesting it is a star very like the Sun in size, mass, and composition, but somewhat younger.
Besides revealing that so many planets can exist close to Sun-like stars, Bedell's work, if confirmed, would resolve a puzzling feature of Kepler-11's planets. The sizes of planets Kepler finds are estimated based on the portion of the star they block out.
Original estimates suggested Kepler-11's planets had some of the lowest densities of planets too small to be gas giants. Bedell's estimates of Kepler-11's size suggest its planets are 8 percent heavier, but with 5 percent smaller radii, than originally thought, bringing their density much closer to those of familiar rocky planets.
If Bedell is right, the galaxy might not have inexplicable “puffy” planets, but could contain plenty more examples of Sun-like stars accompanied by planets closer to each other than we previously thought possible.