Posted by: Kash Farooq | September 20, 2011

Finding Exoplanets with Doppler Spectroscopy

The European Southern Observatory operates three world-class facilities in the clear skies of the Atacama Desert region of Chile. They recently announced the discovery of 50 planets orbiting stars near to our Solar System. This is the biggest number of planets ever to be announced at one time.

The discoveries were made using an instrument called HARPS – the High Accuracy Radial Velocity Planet Searcher – which is permanently stationed in the 3.6 metre telescope at La Silla in the Andes Mountains.

HARPS Spectrograph and the 3.6 m Telescope

Montage of the HARPS spectrograph and the 3.6m telescope at La Silla. The upper left shows the dome of the telescope, while the upper right illustrates the telescope itself. The HARPS spectrograph is shown in the lower image.

HARPS detects planets using the radial velocity method. As a planet orbits a star, just as the star’s gravity affects the planet, the planet’s gravity also tugs on the star. This makes the star wobble. By splitting the light from the star into a spectrum, we can measure the size of the wobble as the planet orbits. From the size of this wobble we can calculate the mass of the planet. We don’t actually see the planet; we just detect its presence from this wobble. The formal name for this technique is Doppler spectroscopy.

Using this information, 16 of the 50 planets have been classified as super-Earths. A super-Earth is defined by its mass and it is generally agreed that any planet between 1 and 10 times the mass of Earth is referred to as a super-Earth. Planets above 10 Earth masses are termed giant planets.

One of the super-Earths, which has designation HD 85512 b, was announced to have a mass of just 3.6 times the mass of the Earth. For comparison, Neptune is 17 times the mass of the Earth.

So 3.6 times the mass of Earth is quite low.

The location of this planet is also favourable. It is located at the edge of the habitable zone – this is a narrow zone around a star in which water may be present in liquid form if the conditions are right. From our current sample size of 1, we know that water is essential for life.

This is all very exciting news.

But now it’s time for me to add a little skepticism.

The radial velocity method of planet detection can only provide us with a minimum mass. It all depends on the angle of a planet’s orbit from our line of sight. Different angles will produce different amounts of measurable wobble from our point of view, and hence give different mass calculations. The actual mass of this planet could be 2-3 times more than the announced minimum.

Planet orbiting a star along our line of sight.

A planet orbiting a star and from our line-of-sight, the orbit is edge on; Doppler spectroscopy will give an exact value of the planet’s mass.

Planet orbiting a star at 90 degrees from our line of sight

A planet orbiting a star and from our line-of-sight, the orbit is at 90 degrees; Doppler spectroscopy method will not detect the planet at all.

Any angle between the two examples above will give different mass measurements. The lowest mass measurement, if we could detect the wobble at all at such an angle, would be at an angle of 89° to our line of sight. The most accurate would be if the orbit was edge on to our line of sight.

So, as you can see, because we have no idea about the angle of the orbit, 3.6 times the mass of Earth is a minimum measurement. If we are viewing this star-planet system at an angle of, say, 50°, the true mass of the planet will be more than 3.6 times.

Apart from its minimum mass and location, we don’t know anything else about this planet. We don’t know its radius. We don’t know if it has an atmosphere. To determine these sorts of properties we need to use a technique known as the transit method, with which we detect a planet passing in front of its star.

We don’t even know if this planet has a rocky surface. Again, from our sample size of 1, there are no planets in the Solar System that are rocky and have a mass 3.6 times that of Earth. In fact, there are no rocky planets in the Solar System with a mass more than 1 times that of Earth! Basically, there is only Earth! We don’t actually know yet where the boundary lies between rocky planets and gas giants.

It is possible that this planet is huge, with a mass of, say, 5 times that of Earth. This would mean it had a low density, and hence must be gaseous rather than rocky.

As the tools available to astronomers have improved, more and more exoplanets have been discovered. The rate of discovery is accelerating. UK scientists are also heavily involved in the Kepler mission, which uses the transit technique of planet detection.

Artist's concept of Kepler

Kepler spacecraft was designed to discover Earth-like planets orbiting other stars.

The Kepler mission recently announced 1235 candidate planets. This included 68 candidates of Earth-like size and 54 candidates in the habitable zone of their star.

Kepler candidates as of Feb 2011

Kepler candidates planets as of February 2011

I’m pretty sure it won’t be long before astronomers will detect a planet similar to Earth – in terms of mass, size and atmosphere. In the meantime there will be many false alarms. There is a European Space Agency proposal to build a much larger version of Kepler, called Plato. It is currently at an advanced stage of planning and selection. If approved, it will launch in 2018 and even more planets, and hopefully confirmed Earth-like planets, will be discovered.

Then we can really start speculating about life elsewhere in the Universe.


I recorded a version of this report for episode 102 of the Pod Delusion. Many thanks to Dr Matt Burleigh for all his help.

All images from Wikimedia Commons and



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