Questions 31-40 are based on the following passage.
It took humans thousands of years to explore our
NATURAL SCIENCE: This passage is adapted from the article “Worlds Apart Seeking New Earths" by Timothy Ferris (©2009 by National Geographic Society).
own planet and centuries to comprehend our neighbor
ing planets, but nowadays new worlds are being discov
ered every week. To date,astronomets have identified
5 more than 370 “exoplanets,” worlds orbiting stars other
than the sun. Many are strange. There's an Icarus - like
"hot Saturn" 260 light-years from Earth, whirling
around its parent star so rapidly that a year there lasts
less than three days. Circling another star 150 light-
10 years out is a scorched “hot Jupiter,” whose upper
atmosphere is being blasted off to form a gigantic
comet- like tail. Three benighted planets have been
found orbiting a pulsar-the remains of a once mighty
star snrunk into a spinning atomic nucleus the size of a
15 city - while untold numbers of worlds have evidently
fallen into their suns or been flung out of their systems
to become ‘floaters” that wander in eternal darkness.
Amid such exotica, scientists are eager for a hint
of the famibar: planets, resembling Earth, orbiting their
20 stars at just the right distance - neither too hot nor too
cold-to support life as we know it. No planets quit
like our own have yet been found, presumably beause
they’re inconspicuou. To see a planet as small and dim
as ours amid the glare of its star is like trying to see a
25 firely in a fireworks display; to detect its gravitational
influence on the star is like listening for a cricket in a
tornado. Yet by pushing technology to the limits,
astronomers are rapidly approaching the day when they
can find another Earth and interrogate it for signs of
30 life.
Only 11 exoplanets, all of them big and bright and
conveniently far away from their stars, have as yet had
their pictures taken. Most of the others have been
detected by using the specttoscopic Doppler techniqu,
35 in which starlight is analyzed for evidence that the star
is being tugged ever so slightly back and forth by the
gravitational pull of its planets. In recent years
astronomers have refined the Doppler technique so
exquisitely that they can now tell when a star is pulled
40 from its appointed rounds by only one meter a
second - about human walking speed. That s sufficient
to detect a giant planet in a big orbit, or a small one if
it’s very close to its star, but not an Earth at anything
like our Earth's 93-million-mile distance from its star.
45 The Earth tugs the sun around at only one-tenth walk
ing speed, or about the rate that an infant can crawl;
astronomers cannot yet prize out so tiny a signal from
the light of a distant star.
Another approach is to watch a star for the slight
50 periodic dip in its brightness that will occur should an
orbiting planet circle in front of it and block a fraction
of its light. At most a tenth of all planetary systems are
likely to be oriented so that these mini-eclipses, called
transits, are visible from Earth, which means that
55 astronomers may have to monitor many stars patiently
to capture just a few transits. The French COROT satel
lite, now in the third and final year of its prime mission,
has discovered seven transiting explanets, one of
which is only 70 percent larger than Earth.
60 The United States’ Kepler satellite is COROT’S
more ambitious successor. Launched from Cape
Canaveral in March 2008, Kepler is essentially just a
big digital camera with a 95 meter aperture and a
95 - megapixel detector. It makes wide-field pictures
65 every 30 minutes, capturing the light of more than
100,000 stars in a single patch of sky between the
bright stars Deneb and Vega. Computers on Earth moni-
tor the brightness of all those stars over time, alerting
humans when they detect the slight dimming that could
70 signal the transit of a planet.
Because that dimming can be mimicked by other
phenoillena, such as the pulsations of a variable star or
a large sunspot moving across a star's surface, the
Kepler scientists won’ t announce the presence of a
75 planet until they have seen it transit at least three
times - a wait that may be only a few days or weeks for
a planet rapidly circling close to its star but years for a
terrestrial twin. By combining Kepler results with .
Doppler observations, astronomers expect to determine
80 the diameters and masses of transiting planets. If they
manage to discover a rocky planet roughly the size of
Earth orbiting in the habitable zone - not so close to the
star that the planet's water has been baked away, nor so
far out that it has frozen into ice-they will have found
85 what biologists believe could be a promising aboae for
life.