 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 such as carbon dioxide, methane, or other ices and gases, the Caltech team said. However, those volatile compounds could not have existed in the hotter zones close to the star.
Finally, the Caltech astronomers discovered that three of the planets have orbits that are related to one another in a very specific way. One planet’s orbital period lasts twice as long as another’s, and the third planet’s lasts three times as long as the latter’s. Planets don’t fall into this kind of arrangement immediately upon forming, Johnson said. Instead, the planets must have started their orbits farther away from the star before moving inward over time and settling into their current configuration.
“You look in detail at the architecture of this very special planetary system, and you’re forced into saying these planets formed farther out and moved in,” Johnson said.
The implications of a galaxy chock full of planets are far-reaching. “It’s really fundamental from an origins standpoint,” said Swift, who notes that because M dwarfs shine mainly in infrared light, the stars are invisible to the naked eye. “Kepler has enabled us to look up at the sky and know that there are more planets out there than stars we can see.”