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Part of Kepler's CCD array  (Source: NASA)
Kepler spacecraft will detect small planets close to the size of earth.

Humanity has wondered about the heavens above since before recorded history. Recently, the discovery of hundred of planets in other star systems has sparked extraordinary interest in determining the odds of extraterrestrial life.

The Kepler mission will seek to explain one part of the puzzle by observing the brightness of over 100,000 stars over the next forty-two months. In doing so, it will be able to track  earth-sized planets, generating future targets of interest for more advanced future space observatories like the Terrestrial Planet Finder and the Laser Interferometer Space Antenna.

Nearly all of the extrasolar planets detected thus far are giant planets the size of Jupiter or larger. Kepler will look for planets 30 to 600 times less massive, closer to the size of Earth and more likely to support life.

All planets in stable orbits transit across their star during their own unique annual cycle. This causes a dip in the star's apparent magnitude for an observer in the same plane. By timing these transits, the orbit and length of year can be calculated. The orbit of a planet can be used to determine if it lies within the "zone of life", where it is close enough to the sun to support liquid water, yet far enough that potential life is not destroyed by it.

"Kepler's mission is to determine whether Earth-size planets in the habitable zone of other stars are frequent or rare; whether life in our Milky Way galaxy is likely to be frequent or rare", said William Borucki, NASA's Principal Investigator on the Kepler Mission.

While Kepler will only focus on a small area of the sky, its results will be enough to enable accurate estimates of the number of earth-sized planets in our galaxy.

Kepler will use an array of 42 CCD (charge-coupled device) cameras, each measuring 50x25 mm. With a resolution of 1024x2200 each, Kepler has a total resolution of approximately 95 megapixels.
CCD cameras are used in most digital cameras and optical scanners. They are also used in astronomy and in night-vision devices due to their sensitivity to the ultraviolet and infrared ranges of light.

Mission operations will be conducted by NASA's Ames Research Center in Moffett Field, California, and are included as part of the $600 million total mission cost. Ames will contact the Kepler spacecraft twice a week using the X-band for command updates as well as system status updates. Scientific data is only downloaded once a month using the Ka-band, at a data rate of up to 4.33 Mb/s. To conserve bandwidth, Kepler will conduct partial analysis on board and only transmit data of interest to researchers.

The Kepler spacecraft will be launched at 2250 Eastern Standard Time from Cape Canaveral Air Force Station in Florida. It will use the Delta II multi-stage rocket, which has flown 140 missions while achieving a success rate of almost 99 percent.
Instead of a typical earth orbit, it will launch Kepler into an earth trailing orbit in order to block light from the sun and the moon. This orbit also avoids gravitational perturbations inherent in an Earth orbit, thus allowing for additional platform stability.

The Kepler Mission is named for Johannes Kepler, best known for his Laws of Planetary Motion.

Updated 3/8/2009

The Kepler spacecraft was launched successfully aboard a Delta II rocket in the D2925-10L launch configuration from pad 17B at 22:49:57 EST on Friday March 6th. The three-stage launch vehicle had nine additional solid rocket boosters, six for the first stage and three for the second stage. The third stage boosted  the Kepler payload to its heliocentric orbit trailing Earth. Two months of testing and systems verification will occur for the next two months before Kepler begins its inspiring mission.


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Starlight dims
By sonoran on 3/6/2009 2:46:28 PM , Rating: 2
Ok, so they can catch it when the light emitted by a star dims. How do they know we're not just looking at something akin to sunspots? Don't they dim the sun's light emissions?

I can see this finding many stars which "probably" have planets. But it would seem much longer observation, to see if the dimming repeats at regular intervals (as the planet repeatedly passes between us and the star),would be required to confirm the findings?

RE: Starlight dims
By General Disturbance on 3/6/2009 3:21:24 PM , Rating: 2
Yes you're absolutely right about sunspots dimming the light, and they will more than likely create a very large number of false detections.
However, more detailed analysis can be used to filter out the real planetary detections from the false ones, such as the depth, length, periodicity and overall photoemtric profile of the dimming. Sunspots have charasteric features for all those, which have been studied extensively and are relatively well known.
So yah it does present a problem, but there are indeed insightful ways to dig the needles out of the haystack. Repeated observation as you suggest, for example.

RE: Starlight dims
By Goty on 3/8/2009 5:21:08 PM , Rating: 2
Sunspots are actually an indicator of increased solar activity, so sunspots will not decrease the flux of radiation detected by Kepler, but rather increase it.

RE: Starlight dims
By General Disturbance on 3/8/2009 8:16:32 PM , Rating: 2
Sorry, that's wrong, but for the right reasons perhaps. Sunspots are an indicator of increased solar activity in that there are more sunspots, QED.
But sunspots themselves are actually cooler than the surrounding material, and hence they appear darker because luminosity generally goes as temperature to the fourth power.
So, sunspots do indeed cause a decrease in the total luminosity of the star. But this brings up a good point in regards to differentiating sunspots from planetary transits: because sunspots are cool, you will see a significant colour (temperature) change in the star when a sunspot first appears, while for a planetary transit the colour (temperature) of the star will remain the same because the transit blocks all wavelengths light equally - a transit does not change the temperature of the star. This is the primary method for distinguishing sunspot activity from a transit.

RE: Starlight dims
By Goty on 3/9/2009 12:23:38 AM , Rating: 2
While I congratulate your rudimentary application of the Stefan-Boltzmann law, you need to realize that the implications you drew only apply to the region of the sunspot itself and that the total luminosity of the star will increase with the number of sunspots.

Sunspots are regions of intense magnetic activity produced by the motion of the ions in the star's interior, motion which is created by convection currents which, in turn, are driven by the fusion of hydrogen into helium (in our sun's case) in the core of the star. Increased activity in the core of the star drives the convection currents faster, creating a larger magnetic field which, in some places, will actually inhibit the movement of warm material from the inner layers of the star to the surface (creating the cool region we identify as a sunspot). This increase in convection leads to an increase in the luminosity everywhere else.

So, while yes, the luminosity of the area occupied by the sunspot will decrease, the overall luminosity of the star will increase. There is much empirical evidence for this, including the correlation between the Maunder Minimum and the Little Ice Age.

RE: Starlight dims
By General Disturbance on 3/9/2009 1:29:04 AM , Rating: 2
Those are long period (multi year) processes. Planetary transits last on the order of hours. So, your objection doesn't really apply here.

If a sunspot develops while looking at the star, or, if a sunspot rotates around the limb of the star and into view, this will cause a relatively abrupt (order of hours) change in the brightness and colour of the star - the brightness will decrease, and the colour (temperature) will cool. This may look similar to a transit upon first look because transits also last around that long. However the depth of the photometric minimum will generally be much larger than that for a transit, and the colour will also change (become cooler) which it doesn't for a transit. Applied astrophysics...

Since you don't seem to appreciate it:
F = sigma*Teff^4. A small drop in Teff (effective temperature for you non-astrophysics majors), causes a significant drop in total flux (F), due to the fourth power dependence.

You are generally correct about sunspot cycles (multi-year processes keep in mind) affecting luminosity, but those effects don't really apply here because they're looking for order-hour changes in brightness.

And even for stars that are undergoing significant sunspot activity, the light curves for those stars are still relatively smooth and well behaved. And so a transit will still stick out like a little blip, and will not be colour-dependent. You could easily detect a transit around a short period Cepheid, for example (short period ~3d for Cepheids).
Certainly, stars of very poorly behaved photometric profiles will simply need to be filtered. They will look for the easy ones first.

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