"I usually try not to call things 'Rosetta stones,' but this is as close to a Rosetta stone
as anything I've seen," says John Johnson, assistant professor of
planetary astronomy at Caltech. "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. There's at least 100 billion planets in the galaxy—just
our galaxy," says Johnson, coauthor of the study, which was recently
accepted for publication in the Astrophysical Journal. "That's mind-boggling."
The conclusion of a new study by astronomers at the California Institute of Technology 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.
"It's a staggering number, if you think about it," adds Jonathan
Swift, a postdoc at Caltech and lead author of the paper. "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 have
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, Swift says. 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.
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 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 says. 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 say.
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 further 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 says 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 proto-planetary 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 proto-planetary 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, when 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 says.
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 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. "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
explains.
The implications of a galaxy chock full of planets are far-reaching,
the researchers say. "It's really fundamental from an origins
standpoint," says 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"
The image at the top of the page shows the Kepler Mission Star Field by
Carter Roberts of the Eastbay Astronomical Society in Oakland, CA,
showing the Milky Way region of the sky where the Kepler
spacecraft/photometer will be pointing. Each rectangle indicates the
specific region of the sky covered by each CCD element of the Kepler
photometer. There are a total of 42 CCD elements in pairs, each pair
comprising a square.
Journal reference: Astrophysical Journal
Source: The Daily Galaxy via California Institute of Technology
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