August 14, 2008

The Summer Triangle: Target of Kepler Mission

evening, I go outside and look overhead at the Summer Triangle: three bright
stars high above my home in California. Then I dream about what will happen
when we find another Earth, a habitable planet around a distant sun. In less
than a year, the first actual observations to make this dream come true will be
underway with NASA's Kepler mission.

Kepler is
NASA's first mission capable of finding Earth-size and smaller planets in the
habitable zone of distant stars. The Kepler mission team expects to discover
hundreds of planets
other stars. Some of those will be in the habitable zone of their
parent stars where water is liquid on the surface of the planet(s). To find the
small, Earth-size planets, the mission will continuously monitor 100,000 stars
for periodic decreases in stellar brightness caused by planets transiting their
stars. Transits happen when a planet crosses its star as viewed from our solar
system. The transits last for a few hours to about a day.

precise photometry (brightness measurement), the Kepler spacecraft will
detect the small decreases in light as a planet transits its star. Three transits
with a consistent period, brightness change, and duration provide a rigorous
method for detection of extrasolar planets. Planets the size of Earth and even
smaller can be found this way, but this requires that we go to space where the
high-precision (1 part in 20,000) measurements can be made. On the ground, such
precision is denied by atmospheric blurring.

So, where
will Kepler look and how long will it stare to find Earth-size planets?

The science
team chose a
large field of stars in the Summer Triangle
, an asterism composed of Deneb,
Vega and Altair. These three first magnitude stars are easily seen in northern
skies from spring until fall. The field is near, but not in the Milky Way where
the star density is higher and could cause confusion. It's above the plane of
our solar system, away from the co-planar asteroids and dust. It's easily
visible to northern hemisphere observatories that can make follow-up
spectroscopic observations of stars that may have planets. Finally, the field
is easy for everyone to find as it's centered between Deneb and Vega. Download a planisphere
(rotating star map)
marked with the Kepler field of view (FOV) and stars
known to have host planets.

The FOV was
selected to sample a
portion of the Milky Way galaxy
that is populated with stars similar to our
Sun in size and age. The FOV encompasses millions of stars and distant
galaxies, and from those, about 100,000 stars will be selected for observation.
Statistically, a small percentage of the 100,000 star systems will be lined up
so that we can discover their planets in transit. Kepler has to stare at
a large number of stars to find those with transiting planets.

with other telescopes, Kepler has a very large FOV. The heart of the Kepler telescope
detector system is an array of 21 CCD pairs. Each CCD
pair covers 5 square degrees on the sky; the array collects starlight from over
100 square degrees of sky. (The pairs are spaced so that light from the
brightest stars will fall between the detectors because the bright stars would
overload the system.) Overall, the CCD array spans more than half the way
between Deneb and Vega. At arm's length, your two hands (side by side, with
your index fingers touching) span the FOV. In contrast, the FOV of most deep-space
telescopes is akin to looking through a tiny straw — like a coffee stirrer. Kepler
will search a big piece of the sky and sample a large population of stars to
find habitable worlds.

When you
look at the Summer
some evening soon, think about what we might discover with the Kepler
mission: another place like Earth. This space mission should be able to answer
a fundamental question: Are planets like Earth rare or common?

Tell us
what you think about finding other worlds.

The Kepler mission
invites you to participate in our "Names
in Space
" project where you can submit your opinion (up to 500 words)
plus your name, city, state and country. Your information will be recorded on a
DVD, attached to the Kepler spacecraft, and launched. A copy of the DVD will be
provided to the Smithsonian Air and Space Museum as an historic document of
public opinion about the Kepler mission's search for other Earths. (The project
will remain open until November 1, 2008.)