describe how astronomers obtain evidence for the existence of
exoplanets (including astrometry, transit observations and use of
Doppler-shifts)
discuss the difficulties associated with the detection of individual planets
If every star we see in the sky is a sun like ours do they
also have planets in orbit around them? Even the closest stars to our own are so
far away that they appear as just a point of light to us, even with the most
powerful telescopes. How might one possibly detect a much smaller non-light
emitting object in orbit around them?
A planet going around a star other than our Sun is called
an extra-solar planet or exoplanet. Astronomers have considered their
possibility for a long time but, until very recently, evidence of their
existence has been impossible to obtain. It is only with modern, incredibly
sensitive instruments that we can obtain evidence for their existence.
Evidence for Exoplanets
Astometry
Astometry is the very precise measurement of the position
of stars. Imagine we were to look at the position of a star and noticed that
it varied as shown below. What does this mean?
Could it be that a large
Jupiter like planet is in orbit around the star and what we are seeing is
the gravitational influence of this object?
Transit Observations
Below is a light curve for a particular star.
Why does the apparent brightness of the star vary as shown. Perhaps a large object transits the
star at regular intervals, e.g. a planet.
Doppler
Shifts
The position of a star may wobble due
to the gravitational influence of a large planet. This may mean, depending
on the plane of the motion, that the star may be moving towards or away from
us periodically.
This relative motion to us would produce a blue or red
shift in the radiation that the star emits which could be detectable.
There are a number of other methods too. Wiki has
a comprehensive list.
At time of
writing nearly 400 exoplanets have officially been discovered. This suggests
that there are many more.
In March 2009 the Kepler Space Telescope was launched by
NASA. Its mission is to measure the light curves of over 100,000 stars to search
for evidence or earth like extra solar planets.
