rbital periods. This orientation therefore provides an opportunity to study the system in great detail. Because the planets represent the vast majority of planets that are thought to populate the galaxy, the team said, the system also can help astronomers better understand planet formation in general.
“I usually try not to call things ‘Rosetta stones,’ but this is as close to a Rosetta stone as anything I’ve seen,” Johnson said. “It’s like unlocking a language that we’re trying to understand — the language of planet formation.”
One of the fundamental questions regarding the origin of planets is how many of them there are. Like the Caltech group, other teams of astronomers have estimated that there is roughly one planet per star, but this is the first time researchers have made such an estimate by studying M-dwarf systems, the most numerous population of planets known.
To do that calculation, the Caltech team determined the probability that an M-dwarf system would provide Kepler-32’s edge-on orientation. Combining that probability with the number of planetary systems Kepler is able to detect, the astronomers calculated that there is, on average, one planet for every one of the approximately 100 billion stars in the galaxy. But their analysis only considers planets that are in close orbits around M dwarfs — not the outer planets of an M-dwarf system or those orbiting other kinds of stars. As a result, they say, their estimate is conservative. In fact, said Swift, a more accurate estimate that includes data from other analyses could lead to an average of two planets per star.
M-dwarf systems like Kepler-32’s are quite different from our own solar system. For one, M dwarfs are cooler and much smaller than the Sun. Kepler-32, for example, has half the mass of the Sun and half its radius. The radii of its five planets range from 0.8 to 2.7 times that of Earth, and those planets orbit extremely close to their star. The whole system fits within just over a tenth of an astronomical unit (the average distance between Earth and the Sun) — a distance that is about a third of the radius of Mercury’s orbit around the Sun. The fact that M-dwarf systems vastly outnumber other kinds of systems carries a profound implication, according to Johnson, which is that our solar system is extremely rare. “It’s just a weirdo,” he said.
The fact that the planets in M-dwarf systems are so close to their stars doesn’t necessarily mean that they’re fiery, hellish worlds unsuitable for life, the astronomers said. Indeed, because M dwarfs are small and cool, their temperate zone — also known as the “habitable zone,” the region where liquid water might exist — is also farther inward. Even though only the outermost of Kepler-32’s five planets lies in its temperate zone, many other M-dwarf systems have more planets that sit right in their temperate zones.
As for how the Kepler-32 system formed, no one knows yet. But the team said its analysis places constraints on possible mechanisms. For example, the results suggest that the planets all formed farther away from the star than they are now and migrated inward over time.
Like all planets, the ones around Kepler-32 formed from a protoplanetary disk — a disk of dust and gas that clumped up into planets around the star. The astronomers estimated that the mass of the disk within the region of the five planets was about as much as that of three Jupiters. But other studies of protoplanetary disks have shown that three Jupiter masses can’t be squeezed into such a tiny area so close to a star, suggesting to the Caltech team that the planets around Kepler-32 initially formed farther out.
Another line of evidence relates to the fact that M dwarfs shine brighter and hotter when they are young and planets would be forming. Kepler-32 would have been too hot for dust — a key planet-building ingredient — to even exist in such close proximity to the star. Previously, other astronomers had determined that the third and fourth planets from the star are not very dense, meaning that they are likely made of volatile compounds su