detect planets that briefly cross, or transit, their stars’ faces.
NASA launched the telescope in
March 2009 into a “trailing” orbit. Sitting slightly farther from the sun than
Earth does, Kepler makes one trip
around the sun every 372 days, gradually falling farther behind Earth. In its
solitude, the spacecraft keeps its eye on
hordes of stars, checking for planets.
It is finding them, too, in scads. There
are more than 1,200 entries on Kepler’s
list of candidate planets (SN: 2/26/11,
p. 18) betrayed by small, repetitive and
distinctively shaped dips in the brightness of their parent stars — the shadow of
a planet crossing a star’s face as seen by
the craft. While further analysis surely
will reveal some as false alarms, chances
are that 90 percent are real. That will
triple the number of known exoplanets.
With time, scientists hope to confirm a
few with Earthlike size, orbit and other
conditions suitable for life as we know
it to arise.
But the majority of stars don’t have
planets lined up to block light headed
toward Kepler. These stars too are worthy of study. Still, who is based at Ames,
hopes outside astronomers will take a
close look at Kepler’s new data on stars
without signs of planets, plus look at
those stars with other scientific instruments while the craft is still operational.
Eye for the weird
“There are so many stars that show
bizarre, utterly unexplainable brightness variations that I don’t know where
to begin,” says Geoff Marcy of the University of California, Berkeley. Marcy gained
fame in the mid-1990s when he helped
pioneer, with ground-based instruments,
discovery of extrasolar planets; he joined
the Kepler team to help expand the
planet-finding toolbox. After the detection of far more than planets began
overwhelming the Kepler program,
Marcy hired an undergraduate statistics
major to scan the plots of varying brightness for tens of thousands of stars. He
trained her to spot what Marcy calls the
WTF objects, which might politely be
rendered “What The Flip is that?”
Astronomers have already
spotted stars with remarkable pulsation modes, double
stars orbiting so closely that
streams of white-hot plasma
flow between them, immense
star spots whose movements
hint at unlikely rotations,
collapsed white dwarf stars
in eclipsing orbits around
large and seemingly
younger stars, and more.
“These phenomena have
never been seen before,
or never with such clarity.
This is a gold mine,” Marcy says.
Ironically, the bounty comes from a
telescope that Borucki has often called,
tongue in cheek, the most boring space
mission in history. All Kepler does is
look, occasionally pausing to realign
its solar panels, at one starry patch of
sky about as big as a hand held at arm’s
length. The target is roughly bet ween the
Northern Hemisphere constellations
Cygnus and Lyra.
About 4 million stars in Kepler’s view
are bright enough to study closely. Unlike
most telescopes, its camera gathers no
spectra that directly reveal chemical
composition. The main objective is to
measure star brightness, an astronomical
procedure called photometry. Stellar
photometry has never been done before
on this scale or with this accuracy. With
42 CCD photo chips, or about 95 mega-pixels in all, Kepler’s camera — the largest
ever put in orbit — can detect changes in
brightness to well under 0.01 percent,
good enough for Earth-sized planets
orbiting sun-sized stars. There is capacity
on the radio link to Earth for about 6 percent of the data avalanche. “We throw
away the rest,” Still says. Thus choosing
which stars’ data to hold onto is essential.
Most of Kepler’s attention is on
156,000 stars selected because they
appeared from pre-Kepler data to be
more or less sunlike, fairly stable in their
output. Even among these stars, most
light curves are not checked by human
eye; computer programs recognize
and flag those with the regular, slight
dips suggestive of planets. Researchers
b
n
Since launch in March 2009,
Kepler has identified more
than 1,200 exoplanet candi-
dates; 15 are confirmed.
can request up to 512 extra
objects for detailed study:
stars already excluded from
planet study, perhaps, but
interesting for other reasons,
or background galaxies.
Already, general astro-
physics papers (mean-
ing papers about stars)
published from the mission
outnumber those dealing with planets.
Some papers are on single stars that
turned out to be double or triple systems,
or stars orbited by — and sometimes consuming — whirling disks of gas, plasma
and dust. Such things were already
known to exist, but to have so many in
such detail is new. Anybody can go to
the Kepler website, pick at random one
of the 156,000-plus or so stars on the target list, and command the Kepler server
to plot and display the star’s light curve.
An amateur helps himself
Even amateurs are welcome to try. One
such amateur is Kevin Apps, an engineer
in Surrey, England. He works days managing natural gas pipeline flows but has
published several astrophysics papers
with professionals. Shortly after Kepler
went into orbit, Apps looked in the telescope’s data file for a red dwarf about
120 light-years away that he knew about
from star catalogs. To his surprise it had
not been included in the list of 156,000
for Kepler’s close attention.
From his home computer Apps found
that he could retrieve the light curve
using data gathered during the telescope’s initial commissioning phase. The
light curve had four dips spaced 12.71
days apart, suggestive of planet-sized
transits. He contacted John A. Johnson,
an assistant professor at Caltech whom
Apps had worked with before.
Intrigued, Johnson recruited eight
other professional astronomers. They
obtained spectra with telescopes in California and Hawaii. Analysis revealed
