Look up at the night sky, and you’ll see stars. But you’re also seeing planets — billions and billions of them.
That’s the conclusion of a new study by astronomers at the California Institute of Technology that provides yet more evidence that planetary systems are the cosmic norm. The team made its estimate while analyzing planets orbiting a star called Kepler-32 — planets that are representative, they say, of the vast majority in the galaxy and thus serve as a perfect case study for understanding how most planets form.
“There’s at least 100 billion planets in the galaxy — just our galaxy,” said John Johnson from Caltech. “That’s mind-boggling.”
“It’s a staggering number, if you think about it,” said Jonathan Swift from Caltech. “Basically, there’s one of these planets per star.”
The planetary system in question, which was detected by the Kepler space telescope, contains five planets. The existence of two of those planets has already been confirmed by other astronomers. The Caltech team confirmed the remaining three and then analyzed the five-planet system and compared it to other systems found by the Kepler mission.
The planets orbit a star that is an M dwarf — a type that accounts for about three-quarters of all stars in the Milky Way. The five planets, which are similar in size to Earth and orbit close to their star, are also typical of the class of planets that the telescope has discovered orbiting other M dwarfs, Swift said. Therefore, the majority of planets in the galaxy probably have characteristics comparable to those of the five planets.
While this particular system may not be unique, what does set it apart is its coincidental orientation: The orbits of the planets lie in a plane that’s positioned such that Kepler views the system edge-on. Due to this rare orientation, each planet blocks Kepler-32’s starlight as it passes between the star and the Kepler telescope.
By analyzing changes in the star’s brightness, the astronomers were able to determine the planets’ characteristics, such as their sizes and orbital 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