Howework 3: Explain in an essay of several pages how astronomers are searching for solar systems and even individual planets outside our
own Solar System. Give examples of what astronomers have found so far. Please list the references that you use on a separate
Detection of extra-solar systems:
In 1994 the Hubble Space Telescope imaged disks of gas and dust around young stars
in the Orion nebula (a star-forming region in our Galaxy).
Hubble could see the disks because they were illuminated by the hottest stars in the Orion Nebula,
and some of them were seen in silhouette against the bright nebula.
These disks are thought to be proto-solar systems from which planetesimals are currently
forming, in the same manner as our own Solar system formed 4.5 billion years ago.
These proto-planetary disks are also kown as 'proplyds'.
Detection of extra-solar planets:
The direct detection of planets is very difficult since
the parent star is much, much brighter than its planets.
A number of techniques have been developed in order to
gather indirect evidences for the presence of planets around
nearby stars. The first detection occurred in 1994 using
the pulsar timing method. To date, there are nearly 50 extra-solar
planets discovered, most of them using the Doppler spectroscopy
Next is a list including some of the methods proposed for the
detection of planets.
There are various methods for planet detection:
Pulsar Timing: A pulsar is a neutron star than spins very fast and emits beamed
radiation which can be intercepted by an observer at the right location. The
observer detects radiation in the form of regular pulses with a
period that corresponds to the spin period of the pulsar. If there
is an object in orbit around the pulsar, its presence can be detected by measuring
the periodic variation in the pulse arrival time. The first detection of a planet
around a pulsar was done by Wolszczan in 1994.
Doppler Spectroscopy: As a planet orbits a star, its gravitational pull causes
the star to wobble. This motion can be detected as a Doppler shift of the
stellar spectrum. The star's light blueshifts when it approaches the observer and
redshifts as it recedes from the observer. This periodic velocity shift
of the stellar spectrum was detected for the first time in 1995 in the star 51 Peg.
This method requires a very precise spectrograph since the velocity shifts are of the
order of only a few meters/sec. This measurement is only possible for Jupiter-mass
planets and is made harder as the orbital size of the planet increases.
Some stars pulsate and mimick the Doppler shift caused by a planet, making
it difficult the detection of planets in some types of stars. To date, about 20 planets
have been discovered using the Doppler technique.
Astrometry: The same back and forth movement that produces Doppler shifts also causes
the star's path to appear as a zig-zag rather than a straight line. This motion
can be detected by precise measurements of the star's position. The expected star's
motion amounts to only a few micro-arcsec (1 micro-arcsec is the angle subtended
by an object 2 mm across on the surface of the Moon!) which is a very difficult
measurement, indeed. Most telescopes on the ground produce images no better than
0.5 arcsec, i.e., 500,0000 times worse than required for planet detection. It is
possible to get around this problem by using interferometric techniques. By combining
the light from two or more mirrors with a large baseline, it will be possible
in the near future to reach the required precision. The Keck telescope is being
prepared to measure angles as small as 20 micro-arcsecond. The four mirrors
of the Very Large Telescope in Chile will deliver images of comparable quality. The
Space Interferometry Mission (SIM), in development, will be able to
measure positions with precision as low as 2 micro-arcsec.
To date there are no planet detections using this method.
Transit Photometry: Photometry is the technique that measures the brightness
of the celestial objects. If a planet passes in front of the star along
the line of sight from the observer the star's light will be partially eclipsed.
A planet orbiting a star will produce a periodic dimming which can be detected
with the photometric technique. Giant planets could produce a dimming of up to 1%.
To date, there have been two such detections with the Keck telescope.
Earth-size planets are expected to produce much smaller effects which are
very difficult to detect with ground-based telescopes. The Kepler Search Mission
is planned to detect such transits from high-precision photometry. It is planned
to be launched as early as 2002.
Microlensing: This technique relies on a background star to provide a source
of light. Then, foreground stars act as gravitational lenses when they pass in
front of the background star and cause the source star to brighten. The MACHO
project has yielded the discovery of many faint stars in the halo of our Galaxy, by
monitoring the light of background objects on a nightly basis. It is
believed that planets orbiting such stars could be detected if the time sampling
is increased to a few hours or minutes.