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SMEI: A CURRENTLY OPERATIONAL ALL SKY EXTRASOLAR PLANET
TRANSIT SEARCH MISSION
Steve Spreckley & Ian Stevens
We are currently working with data from a spaceborne all sky photometric monitor to try and detect planetary transits around nearby stars. Using an
instrument with an effective cadence of 100 minutes and a total mission lifetime of 3 years (2 of which have already been completed) we are able to build up
lightcurves of over 34,000 stars with I brighter than 8th mag. Early results suggest that we will be able to achieve the required photometric accuracy to detect
transits for over 70% of these lightcurves, i.e., for 23,000 stars.
The Instrument
Unique data set
We are using the Solar Mass Ejection Imager, a space
based all sky photometric satellite in an 800km Sun
Synchronous polar orbit to obtain high precision
photometry of over 34,000 stars with I < 8. The instrument,
designed to observe CMEs from the Sun, consists of three
o
o
CCD devices that each view a field that is 60 x 3 and are
arranged on the Coriolis spacecraft (shown below left, with
the cameras circled in red) in such a way that they
o
o
essentially view an arc of the sky 170 x 3 in total which
sweeps over almost the entire sky in a single orbit. The
imaging system has a cadence of four seconds but for a
given star we combine the images from a single orbit to
produce a system with an effective exposure time of 16 to
20 seconds and a cadence of 100 minutes.
A number of key features of the SMEI mission offer us a number of unique
opportunities in terms of both stellar photometry and in terms of planetary
transit detection:
• 100% sky coverage
• 3 year mission lifetime
• Near continuous coverage of a single star for over 200 days per year
• A broad CCD response peaking in the R band, ideal for observing G, K, M stars
These features enable us to produce very long timescale lightcurves in which
we can monitor known stellar phenomena, and eclipsing binary systems, but
also find new long period phenomena, new eclipsing binaries, and very
importantly it offers us an opportunity to find transiting extrasolar planets with
moderate orbital periods.
Left: The Coriolis spacecraft
with
the
SMEI
cameras
highlighted. Right: A series of
consecutive SMEI frames taken
every 4 seconds, showing stars
traverse the CCD. Below: An all
sky image produced by SMEI.
Moderate period transiting planets
There are an increasing number
of unanswered questions in the
field of extrasolar planets, one of
which is whether there exists a
period valley in the 10 – 100 day
period regime (e.g. Udry et al.,
2003).
Depending on how a
period distribution plot is binned
one can argue that the paucity of
planets in this region is a real
phenomenon, or is just due to a
convenient choice of bin size.
Another question lies in where the
transition in the apparent high
mass planet regime in long period
orbits to the low mass planets in
short period orbits takes place.
With SMEI we may be able to detect transiting planets in the 10 – 100
day period regime, which is crucial to answering these questions.
Searching for periodic signals in SMEI data
The main goal of the project is to find transiting planet signatures, but a large
number of eclipsing binary systems, and other periodic signals in lightcurves will
also be found. Preliminary lightcurves from SMEI include those of the Lambda
Tau and Algol eclipsing binary systems. We aim to produce a pipeline that
automatically characterises different types of periodic phenomena, and as well as
studying these, we can then remove long period trends from lightcurves that may
contain transits.
Lambda Tau
The phase folded lightcurves of the
eclipsing binary systems Lambda Tau
(above left) and Algol (above right)
clearly display the primary dip in both
cases and also the secondary dip is
visible in both but is more clear in the
Lambda Tau lightcurve . A view of a
portion of the Algol light curve over a
single eclipse (right) reveals that over a
short timescale the photometry is very
stable. This suggests that with the use
of relative photometry, and with
improved photometry techniques overall,
the point to point noise in our light curves
can be reduced significantly.
Using an L-statistic test (Davies, 1990)
to search for periodic signals in the
lightcure of Lambda Tau, provides a
confident detection of the correct binary
period (3.95 days) of the system (right).
This is one method that may be used in
the automated characterisation of
lightcurves before we search them for
transiting planet signals.
Algol
Our preliminary lightcurves are encouraging, and suggest we could be able to detect transit
like features in around 23,000 lightcurves. Early lightcurve production has focussed on the
very brightest stars in the sky e.g. Canopus. In the preliminary data the Canopus lightcurve
exhibits a noise level of  = 0.006 mags, well below the required noise threshold for Hot
Jupiter transiting planets to be found. We estimate that in order for the signal of a transiting
Hot Jupiter to be detected we require a signal to noise ratio (where the signal is the transit
depth) greater than 0.75. A Hot Jupiter planet causes a dip of typically 0.01 mags and
therefore the noise level needs to be lower than ~0.013 magnitudes.
Canopus
Based on the success of box fitting methods for
searching for planetary transit signals we decided to
try such a method in our search. In order to test the
effectiveness of such period searching algorithms
for this project we initially used synthetic data
consisting of white Gaussian noise with a transit
signal implanted within it and we varied the period
and duration of the transit signal in different
lightcurves. However, now the first lightcurves are
being produced we are implanting transit signatures
into them and analyzing the outcome. For example
we implanted a 0.01 magnitude variation at a period
of 3 days in the Canopus lightcurve and used an
algorithm based on the method described in Kovacs
(2002) to search for periodic signals. The resulting
power spectrum is shown left, and there is a clear
detection peak at the correct period, therefore we
will use a box fitting method when we perform the
full scale analysis of the lightcurves.
Future Work includes:
• Characterize star PSF as a function of colour and magnitude
• Devise a more effective background removal system
• Fully automate data reduction pipeline
References
Davies S.R., 1990, MNRAS, 244, 93
Kovacs G., Zucker S., and Mazeh T., 2002, A&A, 391, 369
Udry S., Mayor M., and Santos N.C., 2003, A&A, 407, 369