The
purpose of this project is to give information about current and future
searches for extrasolar planet. Firstly I would like to talk about ‘How the
planet is detected ’
Figure1: Planet Detection Methods
The first widely accepted detection of
extrasolar planets was made by Wolszczan (1994). Earth-mass and even smaller
planets orbiting a pulsar were detected by measuring the periodic variation in
the pulse arrival time. The planets detected are orbiting a pulsar, a
"dead" star, rather than a dwarf (main-sequence) star. What is
heartening about the detection is that the planets were probably formed after
the supernova that resulted in the pulsar. Thereby demonstrating that planet formation
is probably a common rather than rare phenomena.
2.2 Doppler Spectroscopy (Radial Velocity)
Doppler spectroscopy is used to detect the periodic velocity
shift of the stellar spectrum caused by an orbiting giant planet. (This method
is also referred to as the radial velocity method.) From ground-based
observatories, spectroscopists can measure Doppler shifts greater than 3 m/sec
due to the reflex motion of the star This corresponds to a minimum detectable
mass of 33Me / sini for a planet at 1 AU from a one solar-mass
(1 Mo) star, where i is the inclination of the orbital pole
to the line-of-sight (LOS). This method can be used for main-sequence stars of
spectral types mid-F through M. Stars hotter and more massive than mid F rotate
faster, pulsate, are generally more active and have less spectral structure,
thus making to more difficult to measure their Doppler shift. The minimum
detectable planet mass increases as the square root of the planet's orbital
size.
Astrometry is used to look for the periodic
wobble that a planet induces in the position of its parent star. The minimum
detectable planet mass gets smaller in inverse proportion to the planet's
distance from the star. For a space-based astrometric instrument, such as the
planned Space Interferometry Mission (SIM), that could measure an angle as
small as 2 micro-arcsec, a minimum planet of mass of 6.6Me could be detected in a 1-year orbit around a 1 Mo star that is 10 pc from the Earth and a 0.4 MJ planet in a 4-year orbit.
From the ground, the Keck
telescope is being equipped to measure angles as small as 20 micro-arc
seconds, leading to a minimum detectable mass in a 1 AU orbit of 66Me for a solar-mass star at 10 pc.
The
limitations to this method are the distance to the star and variations in the
position of the photometric center due to star spots. There are only 33
non-binary solar-like (F, G and K) main-sequence stars within 10 pc of the
Earth. The furthest planet from its star that can be detected is limited by the
time needed to observe at least one orbital period. There are no planet
detections that have been confirmed using this method.
2.4 Microlensing
This method uses stars in our
Galactic bulge as sources of light rays which are bent by the gravitational
fields of the “lens” stars in the foreground, between us and the Galactic
bulge. This gives a “microlensing light
curve” that rises and falls. Planets that orbit these “lens” stars can be
detected when the light rays from one of the lensed images pass close to a
planet orbiting the lens star. The
gravitational field of the planet distorts the light curve: the deviation is
typically about 10%, and duration is a few hours to a day (compared to 1-2
months for the lensing due to the star).
Unique advantage: Strength of
signal is nearly independent of planetary mass! Microlensing signals of low-mass planets have shorter duration
and lower detection probability compared to high-mass planets, but not a weaker
signal. So microlensing surveys with
frequent observations of large number
of stars should be able to detect terrestrial planets with good confidence.
The big challenge is that
microlensing events are rare, so have to monitor millions of stars, and even of those that lens, only about 2% of
earth-mass planets orbiting these stars will be in right position to be
detected (if all the stars have earth-mass planets). Also need very good angular resolution and fairly accurate (~1%) photometry. Several other problems, but these are being
addressed.
GEST (Galactic Exoplanet
Survey Telescope)—1.5m space telescope with large field of view. Will survey about 100 million stars. Could detect planets down to Mars mass,
should find ~100 Earth-mass planets at 1AU (if all stars have such
planets). “Free-floating” planets will
also be detected! (Only method that can do that.) Will also be able to detect
~50,000 giant planets by transits.
Sensitive to planets at nearly all distances from star, unlike other
methods.
Photometry measures the periodic
dimming of the star caused by a planet passing in front of the star along the
line of sight from the observer. Stellar variability on the time scale of a
transit limits the detectable size to about half that of Earth for a 1 AU orbit
about a 1 Mo star or Mars size planets in Mercury-like orbits with
four years of observing. Mercury-size planets can even be detected in the
habitable-zone of K and M stars. Planets with orbital periods greater than two
years are not readily detectable, since their chance of being properly aligned
along the line of sight to the star becomes very small.
Giant outer planets that produce a
transit signal of 1% ( 120 times that of an Earth, i.e., a SNR >1000) but
have orbital periods greater than 2 years can be followed up with Doppler
spectroscopy or ground-based photometry.
Giant planets in inner orbits can also be detectable independent of the orbit alignment, based on the periodic modulation of their reflected light. For the 10% of these that have transits, the transit depth can be combined with the mass found from Doppler data to determine the density of the planet as has been done for the case of HD209458b and see if these inner giants are "inflated".
Doppler spectroscopy and astrometry (SIM) measurements can be used to search for any giant planets that might also be in the systems discovered using photometry. Since the orbital inclination must be close to 90° (sin i=1.) to cause transits, there is very little uncertainty in the mass of any giant planet detected.
3. SEARCHES FOR EXTRASOLAR PLANETS
Some of
the searches and their methods for extrasolar planet as in Table1. The mission,
status and especially expected results of them will be presented in final
project.
|
|
PT |
DS
|
AST |
MICR |
PH |
GROUND |
SPACE |
|
|
|
|
X |
|
|
|
X |
|
ongoing |
(several planets found) |
|
|
|
|
|
|
X |
X |
|
under construction |
|
|
|
|
X |
|
|
|
X |
|
ongoing |
(several planets found) |
|
|
|
|
|
|
X |
|
X |
launch June 2006 |
|
|
|
|
|
|
|
X |
|
X |
Project |
|
|
|
|
|
|
X |
X |
|
X |
project |
|
|
|
|
|
|
|
X |
|
X |
launch october 2007) |
|
|
|
|
|
X |
|
|
X |
|
under construction |
|
|
|
|
|
|
|
X |
X |
|
under construction |
|
|
|
|
X |
|
|
|
X |
|
ongoing |
|
|
|
|
|
|
X |
|
X |
|
ongoing |
(some planet candidates found) |
|
|
|
|
X |
|
|
X |
|
ongoing |
|
|
|
|
|
|
|
X |
|
X |
project |
|
|
|
X |
|
|
|
|
X |
|
ongoing |
(several planets found) |
|
|
|
|
X |
|
|
|
X |
project (launch 2009) |
|
|
|
|
|
|
|
X |
|
X |
project |
|
|
|
|
|
X |
|
X |
X |
|
under construction |
|
|
|
|
|
|
|
X |
X |
|
ongoing |
|
Table1: Current and Future Searches for Extrasolar Planets
4.CONCLUSION
In final project I will concentrate on the technology currently being used or developed that will eventually lead to the detection of Earths. Estimates of the timescale of this endeavor suggest that by the 2010s we may possess the capability to find other Earth-sized planets and begin to answer the question of whether or not they are suitable abodes of life.
5.REFERENCES
[1] Jean Schneider, Extrasolar Planets Searches, http://www.obspm.fr/encycl/searches.html
[2] Stuart Clarck, Extrasolar Planets, 1998
[3] European Space Agency, http://www.esa.int/export/esaSC/120382_index_0_m.html
Updated: 14 October 2003
[4] Paris Observatory, http://www.obspm.fr/encycl/corot.html,
updated: 27 November 2003
[5] Kepler Mission, NASA,
http://www.kepler.arc.nasa.gov/, updated: 5 January 2004
