PHYS 400
SPECIAL PROBLEMS IN PHYSICS
CURRENT
AND FUTURE SEARCHES FOR EXTRASOLAR PLANETS
ATAKAN ERDEN
71689-4
PHYSICS-4
SUPERVISOR: Asist. Prof. Dr.
SέNAN KAAN YERLέ
APRIL 14, 2004
1 INTRODUCTION
..1
2.2 Doppler Spectroscopy (Radial
Velocity)
.............................................................3
2.4 Microlensing
...........................................................................................................4
4.CONCLUSION
.7.
References
.7
The planets around main squence stars generated much more interest,and one astronomer even called it hysteria. Perhaps this because the detection of planets around main squence stars made the possibility of finding other Earths seem a lot closer. Should technology advance sufficiently to find other Earths which it surely will the interest will rise even further, and questions such as whether these extrasolar planets can support life will be everyones tongue. Indeed , many will ask if these new worlds are already inhabited.
In this project I prepared planet detection methods and summarized results of them.
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 a rare phenomenon.
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 (gray descending line for stars out to 10 pc) and a 0.4 MJ planet in a 4 year orbit (dark-blue
descending line for stars out to 500 pc).
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.
Figure1:Detection Limits
for Planets Around Solar-Like StarsThe limiting
sensitivities for a solar-like star are shown for:
· Photometry with Kepler (the white region above and to the left of the light-blue lines), COROT (above and to the left of the lavender line); and ground-based photometry (above the solid green line)
· Doppler spectroscopy at 3 m/s (above and to the left of the red line); Planets detected using this method: the first 49 are shown as filled diamonds. (The latest data can be found at Extrasolar Planets Encyclopedia) To date (Nov 2001) about 70 have been detected.
· Astrometry with SIM at 2΅as (above and to the right of the gray and dark-blue lines)
Solar system objects: Mercury, Venus ,
Earth, Mars, Jupiter, Saturn, Uranus and Neptune are shown as solid blue dots 
.
The limits to the maximum orbits are
related to the length of time needed to observe one or more complete orbits to
see the periodic phenomena repeat its signature or by the lifetime of a space
mission. Photometry
is the only practical method for finding Earth-size planets in the continuously
habitable zone. This unique search space is shaded
green in the figure.
In final project I will concentrate on the
technology currently being used or developed that will eventuallly lead to the
detection of Earths. Estimates of the timescale of this endeavour suggest that
by the 2010s we may possess the capability to find other Earth-sized planets
and begin to ansver 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
