This page is moving to a new address.
It will appear in 3 seconds.
Please update your bookmarks or favorites accordingly.
Updated 3/19/02
Frequently Asked Questions
Why can't Earth-size planetary transits be observed from the
ground?
There
are two major reasons why these observations can't be done from
the ground:
- The motions in the atmosphere are constantly
bending the rays of light from each star into different directions.
This is why stars appear to twinkle. If you can see the change
with your eye, you already know that the apparent brightness
is changing by more than 50% (one stellar magnitude). With a
lot of effort and for a very small region of the sky, astronomers
have been able to measure changes as small as one part in 1,000
by comparing each star in a group to the whole group. This precision
is still not good enough to find Earth-size planets, but should
still be okay for detection of giant planets from the ground.
- To detect a planetary transit as short as
2 hours out of a year requires measuring the brightness of the
stars continuously. You can't blink! That means that you would
need to set up dedicated telescopes in many places around the
globe, so that there would be at least one of them on the night
side of the Earth at all times. In addition, extra ones would
be needed because of the inevitable bad weather. This would end
up being a very expensive operation, even if the stars didn't
twinkle. To detect Earth-size planets, space is a better place
to be, since it is more efficient and less expensive.
Don't the stars vary more than the change caused by a transit?
Yes, the stars
do vary in brightness all the time. In fact it is almost impossible
to make a perfectly constant source of light. Fortunately, the
stars we are most interested in are stars like our Sun. These
are the most commonly seen dwarf stars and vary less than the
change in brightness caused by an Earth-size planetary transit
over the same time intervals as a transit duration.
Our Sun varies over many time scales: There
are Maunder minimums, which do not occur for many centuries or
longer and have caused "mini ice ages" even as recently
as during the 17th century. There is an eleven year "solar
cycle" of minimum and maximum activity. The largest short
term variations are caused by "sun spots" that appear
and fade, and rise and set as the Sun rotates with a period of
four weeks.
Planetary transits have durations of a few
hours to less than a day. The measured solar variability on this
time scale is 1 part 100,000 (10 ppm) as compare to an Earth-size
transit of 1 part in 12,000 (80 ppm). Even then most of the variability
is in the UV, which is excluded from the measurements by the
Kepler Mission .
Why not use the Hubble Space Telescope?
There are three basic reasons why the HST
could not be used to look for planets in the way described here:
- The field of view (FOV) of the HST is too
small to observe a large number of bright stars. The FOV of the
HST is about the size of a grain of salt held at arms length.
There is almost never more than one bright star in the HST FOV
at any one time. However, the FOV of the Kepler Mission photometer
is about the size of your open hand held at arms length. Or another
way of looking at it is, that it is about equal to the size of
three "dips" of the Big Dipper.
- The brightnesses have to be measured continuously,
not just once in a while, since one does not know when to expect
a transit to happen. The HST is for the use of the entire astronomical
community to address thousands of questions and would not be
dedicated to just one question requiring continuous use for four
years.
- The HST does not have a specially designed
photometer with the precision required for the measurements needed
to detect Earth-size transits.
- The HST has been used by Ron Gilliland to
look for transits of giant planets with periods of only a few
days in the globular cluster 47 Tuc, a region of very high star
density. No transits were detected.
What are the new developments that now make this mission possible?
Two recent research results have enabled the
practicality of the Kepler Mission:
- The demonstration that charge coupled devices
(CCDs) have the needed photometric performance to make the measurements.
All sources of noise (photon shot noise, stellar variability
(see above), CCD noise and pointing jitter) when combined together
must be less than one part in 50,000 (20 ppm); four times less
than the effect of an Earth-size transit. The required CCD performance
with all the known noise sources has been achieved in recent
laboratory measurements along with the detection of Earth-size
transit signals (Koch, et al. 2000). Thus, CCDs can be used to
simultaneously measure tens of thousands of stars at one time.
- Until recently, no one knew what the variations
in stellar brightness were on the time scale of a fraction of
a day. This information is now available for one star, our Sun.
These data indicate that on the time scale of a transit, the
variability is typically ten times less than the effect being
measured. Fortunately, our Sun is one of the more common stellar
types, and we expect other solar-like stars to behave in a similar
fashion.
How do CCDs (charge coupled devices) work?
CCDs are at the heart of each "HandyCam"
TV camera and special purpose CCDs are what we use for Kepler.
When light strikes a piece of silicon, it
releases electrons that are free to move about the silicon material.
These electrons form a charge or a current which is measured
to determine the amount of light that has fallen on to the silicon.
In a CCD, the silicon region is divided electrically
into small individual picture elements or pixels with about four
hundred elements per cm in each direction, like a very finely
divided sheet of graph paper. The free electrons are kept from
moving around by permanent channel stops (the vertical lines
in the figure) and externally applied voltages (the horizontal
lines in the figure). Each pixel can then be thought of as an
individual bucket or well that collects electrons.
As shown in the animation, first the CCD is exposed
to light from a telescope or camera lens. Overtime this produces
an image made up of electrons in the CCD.
To readout an image that has been captured
with the CCD requires shifting the information out of the pixels.
First, the columns of pixels are all shifted down one row. The
bottom row of pixels is shifted into a readout register. Each
pixel in the readout register is shifted out to an amplifier
and the number of electrons in each pixel are recorded. This
produces a series of 1's and 0's that represent the image. This
is repeated over and over until all the pixels have been read.
The stream of 1's and 0's is then digitally processed to reproduce
the image that is later displayed.
In the Kepler Mission the 1's and 0's
are recorded onboard the spacecraft and sent to the ground, where
the data are processed to look for changes in the brightness
of each star that may be caused by a planetary transit.
Couldn't the mission be done with a smaller photometer and
cut the cost?
A representation of the scientific performance versus
project cost is shown in the figure. A well conceived project
is at A with maximum possible science per dollar available.
Many times, those who fund a program perceive the project to
be at B, where costs can be cut without much loss in science;
well the science team tries to believe that they are at C, where
more science can be achieved at little extra cost. Good clever
scientists and engineers might be able to get to point D, but
this is unusual. Project managers worth their weight in gold
are those who can push toward E, keeping the performance,
but saving on cost Any project headed from A to B, A to C or
A to F is doomed to be canceled or should be canceled.
For the Kepler Mission to work, a 100,000
main-sequence stars must be monitored to a differential photometric
precision of 1:50,000 every 6.5 hours. Substantially fewer stars
and the results may turn out to be ambiguous. The necessary precision
requires recording ten billion photons from each star every 6.5
hours. Thus, a smaller photometer would mean either fewer stars
at the required precision or poorer precision for most of the
stars and thereby the inability to detect Earth-size planets.
Also, the photometer would need to be much smaller before other
costs, such as the launch vehicle, would begin to drop significantly.
Our project manager has worked hard at both
increasing the performance by increasing the downlink data rate
to permit monitoring 100,000 stars (originally we planned to
monitored only 5000 stars) and in reducing the cost by changing
the orbit to an Earth-trailing heliocentric orbit and thereby
eliminating an expensive propulsion stage needed to get to an
L2 halo orbit. This also allowed us to use a smaller and less
costly launch vehicle. In essence, we have already pushed the
cost-performance curve in both the D and E directions.
Are there other photometry missions?
There are three photometry mission that are
already under development. However, they are considerably less
capable than the Kepler Mission , since their primary
science mission is to measure the properties of stars. These
mission are COROT, MONS and MOST. COROT has 1/10 the collecting area
for photons, 1/20th the field of view of the sky and stares at
a given star field for 1/10 the amount of time that the Kepler
Mission stares. MONS and MOST are even smaller missions and
less capable.
Another mission proposed to ESA, Eddington, is somewhat closer to the Kepler
Mission in capability. It has almost the same collecting
area and hence should achieve similar noise performance for the
same star brightness. However, it only has a field of view of
1/15 of the Kepler Mission and only views the field for
a maximum of 3 years rather the Kepler's 4 years, hence its effective
SNR is about sqrt(3/4) of Kepler for an equivalent transit
of a similar stellar brightness.
Can the data from this mission support other research programs?
Obtaining astrophysical information about
each star is a natural byproduct of detecting planetary transits.
The following are some of the potential uses for these data:
|
Phenomena
Stellar rotation rates
p-mode oscillations
.
Characteristics of solar-type stars
Frequency of Maunder minimum
Stellar activity
.
Cataclysmic variables
Eclipsing binaries
Active Galactic Nuclei variability
|
Information Obtained
Variation in rates with stellar type
Window on stellar interior:
mass, age, metallicity of stars
Determine what is a "normal" star
Earth climate implications
Star spot cycles, white light flaring,
Paleoclimatology
Pre-outburst activity, mass transfer
Detection of high-mass ratio binaries
"Engine" size in BL Lac, quasars
and blazars
|
How does the Kepler Mission contribute to the Origins
missions SIM and TPF?
The
Kepler Mission contributes in several ways to both the
Space Interferometry Mission (SIM) and the Terrestrial Planet Finder (TPF) mission:
- The Kepler Mission determines the
frequency of terrestrial and larger planets in a larger volume
of our Galaxy then available to either SIM or TPF and thereby
determines the expected number of planets that either SIM or
TPF might observe. If terrestrial planets are rare in the extended
solar neighborhood, then the capabilities of both these missions
will need to be increased.
- From the distribution of planets among the
different stellar types observed, SIM and TPF will know which
types of stars in our immediate solar neighborhood are most likely
to have planets.
- Although neither of these missions will be
able to detect terrestrial planets as far away as the Kepler
Mission can, the Kepler Mission does identify planetary
systems already known to have terrestrial planets and SIM and
TPF can observe these systems to determine if there are other
larger planets which would not have been seen transiting there
parent stars, thereby providing a more complete picture of the
composition of planetary systems having known terrestrial planets.
- The systems found by the Kepler Mission
to have terrestrial planets can be examined in the infrared to
measure the amount of zodiacal light within each system. If large
amounts of zodiacal light are common, then it may be difficult
for TPF to image any planets.
- Results from the Kepler Mission provide
the sustained impetus to fund the much more ambitious TPF as
stated by the National
Academy of Sciences (NAS) decadal survey report Astronomy and Astrophysics in the New Millennium
(p. 7), which calls for building the TPF space mission "predicated
on the assumption ... that, prior to the start of TPF, ground-
and space-based searches will confirm the expectation that terrestrial
planets are common around solar-type stars.".
Go to The Next Topic
Return to Kepler Mission Home Page
Curator: David Koch
