Astronomy exoplanets_header

Published on June 3rd, 2011 | by Carl Mundy


Exoplanet detection; Searching for our second home

In this article, Carl discusses a number of methods used to detect planets orbiting other stars in the Milky Way galaxy, just how many we have found and how common bodies just like our own ball of rock really are…

In almost two decades, it has been shown that the model of our solar system has been duplicated many times over. We have discovered hot Jupiters, massive rocky planets and even planets the size of our own. Planets have even been observed to orbit at a distance where liquid water can exist on an Earth-type planet, the habitable zone (HZ). In those two decades, over 550 exoplanets have been found to orbit stars in our galaxy alone. With the Kepler mission announcing that it has discovered over 1200 candidate exoplanets, that number may just about triple in length.

The methods used to detect these planets are wide and varied. Below is a short introduction to some of the most widely used methods and some examples of the planets each method has discovered.

Radial Velocity Method

If a star is being orbited by a planet, or planets, the gravitational interactions between them will cause the star to ‘wobble’ around in it’s own small orbit. This means that at some point the star will be moving towards us and then, on the other side of it’s orbit, away from us. As the stars speed relative to an observer here on Earth changes, the frequency of light observed also changes. These changes can be detected by looking at the light given off by the star over a period of time. This method is sometimes called Doppler Spectroscopy because this change in frequency is called the Doppler effect. It is analogous to the sound of an ambulance as it approaches you and then drives off, with the sound you hear appearing to change as it does so.

Transit Method

Some exoplanets’ orbits cause them to pass between us here on Earth and their parent star. If this happens then it will temporarily block some of the light from the star. This change in brightness can be detected here on Earth.

This method is currently being used by the Kepler spacecraft to monitor the brightness of nearly 150,000 stars in our Milky Way galaxy. The shape and size of the light curve produced can provide information on the size and distance of the orbiting body. Whilst this method potentially allows for the atmospheres of exoplanets to be studied, it suffers from several disadvantages including the very low probability that a planet’s orbit is aligned this way relative to us here on Earth.

Pulsar Timing

Planets can also be detected around a special type of star called a pulsar. A pulsar is a neutron star – a small and extremely dense remnant of a star that has exploded as a supernova. As a pulsar spins, it emits radiation from its magnetic poles; most commonly radio waves but also x-rays, and as long as that radiation is pointed towards an observer on Earth, it can be detected. Pulsars’ naturally rotate and these rotations are regular – some pulsars have been found to be as precise as an atomic clock.

If a pulsar has a planet orbiting it, it will disrupt the pulsar’s orbit. Like with the radial velocity method, this method relies on the change in the emissions of the star – light in the former case and the radio waves in this case. In this case, because the radio wave pulses are so regular, any change can be measured. In this method though, the timing between the pulses are measured rather than the change in wavelength due to the star’s movement. As the pulsar completes its own small orbit, it moves towards and away an observer. When it is furthest away, the pulses are furthest apart whilst when the pulsar is nearest the observer the pulses are closest to one another. Studying the timings between pulses can provide details on the orbit of the star which can then be used to determine properties about the orbiting planet.

Einstein Cross – Four images of the same distant quasar appear around a foreground galaxy due to strong gravitational lensing.

Like the transit method, for this method to be accurate, it requires the pulses of emitted radiation to be directed at the observer. When this is the case though, this method is so sensitive that it can detect planets that are as small as one tenth of Earth’s mass. The first two extra-solar planets ever discovered were discovered around a pulsar but because pulsars are relatively rare, the amount of extra-solar planets discovered around pulsars will be small in comparison to the total found.

Although planets are found around pulsars, life as we know it could not stand the extreme levels of radiation emitted by them.

Gravitational Lensing

Massive objects such as galaxies (and even our own Sun to some degree!) can deflect light from other objects behind them such that we observe their image in the night sky. One famous example is the Einstein Cross, a quasar that is lensed by a foreground galaxy such that the same object appears four times around the galaxy. What’s more impressive though is that if the lensing object is in fact a star and it has an orbiting planet whose orbit means it interferes with the lensed light, this interaction can be measured!

Much like the transit method requires alignments to be just right, gravitational lensing also requires the positions of the stars and observer to be just right. The probability of these events is very low and the chance of follow up observations even smaller, although events have occurred and exoplanets detected this way!

Direct Imaging

Until only recently, it was not possible to achieve a direct observation of a planet orbiting a far away star due to the vast distances and small sizes of the objects involved. As planets are extremely faint sources of light compared with their parent stars, it is hard to distinguish them. With improved techniques as well as vast leaps in technology it has become possible to actually look at one of these planets. Although this technique is limited to observing hot, Jupiter-size exoplanets, it still allows us to gain information on the number of systems in our galaxy.

Binary Minima Timing

When a planet orbits a binary star system – two stars that obit each other – and the two stars pass in front of each other in their orbits, it is called an eclipsing binary system. As the two stars orbit each other, the observed brightness of the system fluctuates giving maxima and minima points of brightness. Like the pulses of radiation pulsars emit, these changes in brightness can be used to detect if there is a mass – a planet – that disturbs the timing of these dips in the system’s brightness.

If a planet is within this type of system, the orbit will be distorted and the timings between the maxima and minima will change periodically. The distortions in the stars’ orbits can be used to determine parameters of the planet’s orbit. At the moment, only relatively close binary star systems can be studied in this way but it is the most reliable method yet of discovering extra-solar planets in a double star system.

Future Homes

With missions such as Kepler keeping tabs on literally hundreds of thousands of stars, the list of exoplanets is only going to get longer and longer. Around 10% of Sun-like stars have been found to have planetary companions. With around 50 billion stars in the Milky Way alone, that adds up to a long list of potential homes. In only two decades it has been shown our solar system is not as unique as we thought it might be and that the chance of finding Earth’s twin is much higher than we could ever have imagined. It is inevitable that sooner or later, humans will leave planet Earth and we may just end up visiting some of the planets we have already observed.

Further Your Knowledge

Check out some of the topics below to top up your knowledge on the subject of exoplanets and their observation!

  • Polarimetry
  • Which method can tell us the most about an exoplanet?
  • What is the smallest exoplanet detected to date?

Header image courtesy vintagedept.

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About the Author

Astronomy PhD student from the UK with a passion for astronomy and science outreach projects. Involved with weekly science-based radio programme The Science Show on University Radio Nottingham (URN).

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