Prospecting for Planets – Radial Velocity Searches
The first planet orbiting a star other than our Sun (or 'exoplanet') was discovered in 1992,
orbiting an odd type of star known as a pulsar. It wasn't until three years later that the next
exoplanet was discovered, this time around a star similar to our Sun. Since then the number of
exoplanets we have discovered has snowballed, but we have only just began to detect these
exoplanets directly, by taking images of them orbiting their parent star (using techniques such as
Coronography). Until then, we had to find exoplanets by what are known as indirect methods;
looking for effects the exoplanet has on the light we observe from its host star. This ‘radial velocity’
method was used to find the first few exoplanets, and has since been used to discover many more
around very different types of stars.
Wobbling Stars
All the planets in our Solar System orbit the Sun, because the Sun's gravity stops them
escaping, in the same way that the Earth's gravity always pulls you down, towards the floor. In
fact, everything with mass has what scientists call a gravitational field - even you! However, we
don't have objects being pulled towards us by gravity because the strength of a gravitational
field is proportional to mass – the more massive you are, the greater the gravitational field. We
happen to be next to a very large object (the Earth!) whose gravity dwarfs our own, and so the
dominant attraction any objects feel is towards the Earth, rather than towards us.
Similarly, the Sun is over 300 million times more massive than the Earth, so its gravity is much
stronger and so pulls the Earth round it on an orbit. But the Earth's gravity does have some
effect on the Sun; because the Earth is much smaller, its gravity doesn't make the Sun move on
a huge orbit but it does make move slightly – more of a wobble than an orbit! All of the planets
in our Solar System affect the Sun this way, with Jupiter, being the most massive planet, creating
the biggest wobble. In fact, over 12 years, Jupiter causes the Sun to move in a circle with a
diameter of 740,000 km. To someone looking at our Sun from very far away, it would look like
our Sun was moving very slightly back and forth over time, with a regular pattern to the motion.
Detecting the Wobble
In 1952, a scientist named Otto Struve realised that if an exoplanet existed around another star, it
would cause that star to wobble, in the same way that the planets in the Solar System cause the
Sun to wobble. By looking for signs of these wobbles, we could find worlds orbiting other stars!
It would seem simplest to measure this wobble directly, but this requires very precise
astrometry, (the accurate measurement of the star’s position in the sky), to be able to notice a
change in position of only a few hundred thousand kilometres for a star lying a few million billion
kilometres away. It’s only been in the past couple of years that we have had instruments able to
measure a star’s location precisely enough to see changes this tiny. ESA (the European Space
Agency) are planning to launch a spacecraft, GAIA, in 2012 that will allow astronomers to
measure the positions of more than a billion stars with enough accuracy to also look for
exoplanets. Until then, astronomers have had to use a more cunning method to spot stars
wobbling.
Instead of directly measuring the distance the star has moved due to the planet, astronomers
instead looked for changes in its velocity in our direction. As an orbiting exoplanet causes a
star to wobble slightly towards or away from us, it will cause in turn a slight increase or decrease
in its velocity towards or away from us, which we measure using a technique called Doppler
spectroscopy. We can measure this change in velocity very accurately. For example we could
detect Jupiter if we were looking back towards the Solar System from another star, as it causes
the Sun to move back and forth at a velocity of 12 metres per second. Because this method
finds exoplanets by measuring the star’s motion in our direction it is often referred to as the
Radial Velocity, or RV, method.
Text Box Aside – [ Doppler Spectroscopy
Doppler spectroscopy makes use of a property of waves called ‘Doppler shift’. If something
(normally referred to as the source) emits light in our direction while moving away from us, the
wave making up the light will stretched out. This makes it appear further toward the red end of the
electromagnetic spectrum (of which visible light is just a small part), and so we call it ‘red-shifted’.
In contrast, if the source is moving towards us, the detected light wave will be compressed, and will
be observed further down the light spectrum towards the blue end, and so it is called ‘blue-shifted’.
It is important to note that the original light source is emitting at exactly the same wavelength
whether it is moving towards or away from us; the difference in the light detected is caused by the
wave being stretched or compressed by the our (the observer’s) and the source’s relative motion.
Astronomers can use this effect of light being blue-shifted or red-shifted to measure whether an
object us moving towards or away from us, and from the amount of the shift, calculate how fast
that motion is. ]
What can we learn about Planets found by Radial Velocity?
As well as confirming the presence of an exoplanet, the RV method also tells us about the
dynamics of the planet; how far away it is from its star and how quickly it orbits the star. We
can also estimate an upper limit for how massive the exoplanet is, as a larger planet will cause a
greater shift in the star’s motion. We can only find an upper limit for the mass of an exoplanet
detected by RV as we don't normally know the orientation of the star and the planet relative to
us on the Earth, and this orientation effects the velocity we see the star move at. The RV
method has even found multiple exoplanets orbiting around the same star, by picking out the
part of the star's velocity caused by each planet.
Although astronomers have found hundreds of exoplanets using RV, there is limit in the amount
of information they can learn about the exoplanet. It reveals almost no information about an
exoplanet’s physical characteristics like its atmosphere or composition. In addition, RV methods
are inherently limited by current survey’s duration; for a good detection, we need an exoplanet
to have made at least one orbit of its host. This restricts the planet-star separation that can be
detected, as the further a planet is from a star, the longer it takes for it to complete one orbit.
For example, the Earth takes a year to complete one orbit of the Sun, but Jupiter, which is 5
times further away, takes almost 12 Earth-years to go around once! The RV method is also far
more sensitive to massive planets, as these cause a greater shift in the star’s motion.
Current Radial Velocity Missions
Because measuring a highly accurate velocity for a star does not require a particularly large
telescope there are many teams all around the world who have successfully used the RV method
to find hundreds of exoplanets, including:

HARPS – the High Accuracy Radial Velocity Planet Searcher, based at the La Silla
Observatory in Chile.

Elodie – an instrument at the Haute Provence Observatory in France, where the first
planet discovered by the RV method was found.

The Anglo-Australian Planet Search group, based in Australia

Some RV exoplanets have even been found by amateur astronomers!
Figures
Wobbling Stars:
Fig 1 – In this diagram the orbiting planet is causing its host star to move on a much smaller orbit
around their barycentre, or centre of mass.
Fig 2 – This diagram shows how our Sun’s position changes over time due to the influence of the
planets, relative to its overall size. It is much more complex than a simple circle because all of the
planets tug it in different directions, and by different amounts. The dashed circle shows the path the
Sun would have if it was only orbited by Jupiter. (credit B.W. Jones).
Detecting the Wobble
Fig 3 – We can detect an unseen companion orbiting a star by looking for changes in the star’s
radial velocity relative to us by measuring the light it emits. If the light periodically changes
from being slightly redder to slightly bluer, it means the star is sometimes moving towards us and
sometimes away from us, indicating something – like an exoplanet – is effecting it gravitationally.
Dopper Spectroscopy
Figs 4 & 4b- This shows how Doppler shifting affects light spectra. A spectrum is what you get
when you shine white light through a prism, splitting the light into its different colours. The
wavelength of light at the blue end of the spectrum is very short; wavelength then gets longer as
you go from blue to green to yellow to red. The black lines of the three spectra above are called
absorption lines, this is where certain wavelengths of light have been absorbed by certain elements
in the star. We know at exactly what wavelengths these absorption lines should occur for a
particular element from experiments on Earth. If we observe a source where these lines shifted to
the right (towards the red end of the spectrum), we know the source must be moving away from us,
whereas if these lines are moved to the left (towards the blue end of the spectrum), we know the
source must be moving towards us. (If the lines are sometimes red-shifted and sometimes blueshifted this indicates the object is moving in a circular path, sometimes towards us and sometimes
away from us).
What can we learn about planets found by Radial Velocity?
Fig 5 – This is an example of the sort of data found by RV observations. The vertical axis shows
how the stars velocity in our direction changes, compared to time on the horizontal axis. If the
star has a companion, we expect to see a repetitive change in its velocity, due to the
gravitational influence of the companion. From this, astronomers can work out the planet’s
orbital period (the length of time it takes to complete one orbit of its star) by measuring the time
between two equivalent points on the graph, such as between two peaks. They can also
calculate the mass of the companion, as this is relative to the size, or amplitude, of the peaks or
troughs of the curve.
Anywhere:
Fig 6 – This is an artists impression of the exoplanet 51 Pegasi, the first exoplanet discovered
using the RV method.