If Friday's
launch goes according to plan and successfully lobs NASA's new Kepler space
telescope into orbit, the mission stands to potentially change the way we look
at the universe.
Kepler is
designed to turn its eye on thousands of stars in our own Milky Way galaxy and
look for signs
of Earth-sized planets orbiting in a region conducive to supporting life.
"Kepler
will push back the boundaries of the unknown in our patch of the Milky Way
galaxy, and its discoveries may fundamentally alter humanity's view of itself,"
said Jon Morse, director of NASA's Astrophysics Division at the agency's headquarters
in Washington, D.C.
The probe
is slated
to launch on Friday from Cape Canaveral Air Force Station in Florida aboard a Delta 2 booster. NASA delayed the mission by one day for extra rocket
checks after the loss of another probe just after its launch last week.
The $600 million
Kepler mission is named for Johannes Kepler, the 17th century German scientist
who pioneered the fields of optics and planetary motion.
"Now,
400 years later, we're using his discoveries in order to answer a profound and
fundamental question about our place in the universe: Are there other
Earth-like planets out there?" Morse said.
Kepler will
use an unprecedented combination of light detectors (adding up to about 95
million pixels) to capture the subtle shifts in light that characterize an extrasolar
planet orbiting its star. For comparison, a high-end digital camera on
Earth might have 10 megapixels, but Kepler's detectors add up to 95-megapixel
array, mission researchers said.
Looking
for Goldilocks
Over the
last two decades, scientists have spotted more than 300 extrasolar planets
circling other stars in our Milky Way galaxy. Most of these planets have been
about the size of Jupiter or larger, making it unlikely they would harbor life.
But those aren't what Kepler is aiming to find.
"We're
interested in planets like
that of Earth, rocky planets in an orbit where life might be possible,"
said principal investigator for Kepler science William Borucki of NASA's Ames Research Center in Moffett Field, Calif.
Kepler will
be pointing its 0.95-meter diameter telescope and array of 42 charge-coupled
devices (light-sensitive microchips also found in standard digital cameras) at
a pre-selected target group of 100,000 stars.
Kepler will
be looking for fluctuations in the light of each star that result from a planet
transiting – or moving in front of – its parent star as seen from the
perspective of Earth. These fluctuations can tell scientists how big the planet
is, as well as how far away its orbit is from its parent star.
"When
a planet goes across a star, it blocks some light," Borucki explained.
"The bigger the planet, the more light it blocks, so we get the size of
the planet from the size of the dimming."
By looking
for multiple transits, the Kepler team can determine the planet's orbital
period, or how long it takes the planet to circle its star. Each planet will
need to be observed through at least three transits, Borucki said, to determine
the period and to make sure the dimming isn't due to some other astronomical
phenomenon, such as a spot on the star.
"We
don't want to have false discoveries; we want to be very, very sure that when
we say it's an Earth, it really is an Earth," Borucki said.
If the
potential planet has a short orbital period (a few days or weeks), that means
it orbits very closely to its sun. A long period (of several years) would mean
it sits closer to the edge of the star's gravitational grip. In general, such
extreme orbits would make the planet too hot or too cold, respectively, for
life to arise there.
Borucki and
the rest of the Kepler team are interested in finding a planet with a more
middle-of-the-road orbit, one that is "not too hot, not too cold, but just
right," he said. Such orbits, which fall in what is called the sun's
"habitable
zone," would mean that the temperature of the planet was mild enough
for liquid water to exist on the surface.
Stellar
continuum
Of course,
this Goldilocks orbit won't be in the same place for every star, which can have
large variations in the amount of light they put out.
Kepler will
be looking at three main types of stars: A-type stars, G-type stars (the group
our sun belongs to), and M-dwarfs.
A-type
stars are the hottest of the bunch, which shifts their habitable zone further
away from the star.
They "are
very luminous, they put out a lot of energy, and their habitable zone is
actually much further away from the star than for our Sun," said
astronomer Debra Fischer of the San Francisco State University, who is not
directly involved with the Kepler mission.
The
habitable zone for G-type stars would lie in the range of Earth's orbit, so any
likely exoplanet candidates
would have an orbital period of about one year.
M-dwarfs,
meanwhile, lie on the other extreme.
"These
very low luminosity stars are not giving out as much energy," Fischer
explained. "To be in the habitable zone around one of these low-mass
stars, you have to move in a little bit so you can warm up, basked in the
starlight."
So planets
in the habitable zone of an M-dwarf will have much faster orbital periods than
those in the habitable zones of G-type or A-type stars.
"And
so what Kepler will be doing is watching as the planet goes across, blinks the
starlight out, and with the M-dwarfs, the frequency of the blinks of planets in
the habitable zone is going to be much higher," Fischer explained.
Hot
Jupiters and twin Earths
Kepler
won't be sending back news of a twin Earth right away though. Once it is up in
orbit and properly calibrated, it will likely churn out detections of bigger
planets first.
The first
planets to "roll out the Kepler assembly line" will likely be the
so-called 'hot
Jupiters,' Fischer said. The planets are intriguing to astronomers because
they are the size of Jupiter but sit at the orbit of Mercury.
"The
shear number of these objects that Kepler will find is going to help us learn a
lot about these systems," Fischer said.
Next to
roll out will likely be the similarly-named 'hot Neptunes," and then
finally, "the hardest detection, and by far the most exciting, is going to
be the detection of bonafide Earths," Fischer said.
While
Kepler won't be able to tell astronomers what the planets look like, Fischer
thinks there will be great variety in the other
potential Earths out there, for example, 'water worlds' covered entirely by
oceans.
"I
think that the science fiction writers are going to be challenged to imagine
the diversity that we could expect to find even in these types of planets,"
she said.
Just how
many other Earth's Kepler will find is anybody's guess though. "because we
don't really know what's out there," Borucki said.
"Kepler's
designed to find hundreds of Earth-size planets if such planets are common
around stars; dozens of these planets if they're in the habitable zone,"
Borucki said. "If we find that many, it certainly will mean that life may
well be common throughout our galaxy, because it's an opportunity for life to
have a place to evolve."
"If on
the other hand, we don't find any, that will be another profound discovery. It
will mean that Earths must very rare, we may be the only extant life in our
universe," Borucki added. "It'll mean no 'Star Trek.'"
Borucki
emphasized that no matter what Kepler finds, it won't find little green men.
"Although
Kepler will not find E.T., it's helping to find E.T.'s home," he said.
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