NASA's Kepler telescope has discovered many strange, new worlds. None are stranger than the planetary systems that are commonly seen orbiting very close to their host star. These planets are typically Earth to "super-Earth"-size.

Some of these planets are almost 100 times closer to their star than the Earth is to the Sun, and many of these orbits are much smaller than those of Mercury. The relative inclination between the orbits in these systems is even lower than our solar system making these systems very flat. Understanding the origin of these close-in super-Earths is a major challenge for astronomers.

In a recent paper by Sourav Chatterjee and Jonathan C. Tan, appearing in the Astrophysical Journal Letters, a new model of planet formation is tested against the properties of the innermost of these planets, nicknamed "Vulcan planets" by the authors.

Vulcan was the name given to the once-sought innermost planet of our solar system that was thought might exist inside Mercury's orbit, but was never found. The new study finds extrasolar Vulcan planets have a property that the larger their distance from their parent star, the larger their mass.

This property is a prediction of a theory of "inside-out planet formation" proposed last year by Chatterjee and Tan in the Astrophysical Journal, which involves planets forming at their current locations from a ring of pebbles and small rocks delivered there after spiraling in from an extended gas and dust disk.

Such "in situ" formation of planets at very close distances to the star using material that may come from large regions of the planet-forming disk is a radical departure from most previous theories of the formation of these planets, which involved formation in the outer regions of disks followed by planetary migration to the current orbits.

Proving the existence of different modes of planet formation would be a key breakthrough for understanding the diversity of worlds that are now being discovered by planet hunters.