Galaxy filled with at least 100 billion planets

Friday, 4 January 2013 - 12:12pm IST | Agency: ANI
When you look up at the night sky you’ll see stars for sure, but you are also seeing billions and billions of planets at least, researchers say.

When you look up at the night sky you’ll see stars for sure, but you are also seeing billions and billions of planets at least, researchers say.

The finding by astronomers at the California Institute of Technology (Caltech) that provides yet more evidence that planetary systems are the cosmic norm.

The team made their 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,” John Johnson, co-author of the study, said.

“That’s mind-boggling,” he said.

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, 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, Jonathan Swift, lead author of the paper, 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 -- and because the planets represent the vast majority of planets that are thought to populate the galaxy, the team says, 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,” he said.

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, says Swift, a more accurate estimate that includes data from other analyzes 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.

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 says. Instead, the planets must have started their orbits farther away from the star before moving inward over time and settling into their current configuration.

The study will be published in the Astrophysical Journal.

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