How to find an extrasolar planet
How to find an extrasolar planet
There are three main detection techniques that can be used to find extrasolar planets. All of them rely on detecting a planet's effect on its parent star, in order to infer the planet's existence. The three techniques are simple in principle but difficult in practice because exquisite precision is needed to register the planet's effect on the much larger star. This is additionally hampered from Earth because the atmosphere distorts our view of the stars, limiting the accuracy of the observations. Space missions can overcome this problem.
Eventually, astronomers hope to be able to isolate the light being reflected by planets. Using techniques known as direct detection methods, culminating in actual pictures being taken of the surfaces of extrasolar planets.
The vast majority of planetary detections so far have been achieved using the radial velocity technique from ground-based telescopes. The method requires the light from a star to be split into a spectrum, rather like water droplets in the atmosphere splitting sunlight into a rainbow.
When the spectrum is magnified, straight black lines can be seen superimposed upon the colours. These spectral lines correspond to the wavelengths of light that have been absorbed by chemicals on the surface of the star from which the light originated.
Studying these lines can show which stars have large planets around them because, as the planet orbits the star, it pulls on it with its gravitational field, forcing the star into a small orbit, or wobble. In fact, it makes it look as if the star is pirouetting around a point in space. This means that the star will sometimes be spinning towards Earth and at other times spinning away. When the star moves towards Earth, it squashes the wavelengths of the spectral lines in the light it emits. When the star travels away from Earth, the opposite happens, and the wavelengths are stretched. So, astronomers look for wobbling stars, as these must be the ones with planets in orbit around them.
This technique is limited, however, because it will never be able to detect Earth-sized worlds. With the best spectroscopes, astronomers can confidently detect motions of about 15 metres per second. However, Earth only forces the Sun to move at 0.1 metres per second. Even if a spectroscope could be made to detect this, the boiling of the star's gaseous surface would mask the effect of the planet.
A technique related to the radial velocity detection is to precisely measure the position of a star, so that any wobbling can be directly detected. Such observations are known as astrometry. From Earth's surface they too are restricted by the atmosphere.
ESA's Gaia mission is designed to be the most precise astrometric satellite ever created and will survey 1 billion stars after its launch, early next decade. Among a great many other scientific goals, Gaia is expected to find between 10 000 and 50 000 gas giant planets beyond our Solar System.
Once again, however, the wobbling motion caused by an Earth-sized planet will be too small to be detectable, even by Gaia.
A more promising method for detecting small worlds is to look for the drop in brightness they cause when they pass in front of their parent star. Such a celestial alignment is known as a transit. From Earth, both Mercury and Venus occasionally pass across the front of the Sun. When they do, they look like tiny black dots passing across the bright surface.
Obviously, such transits block a tiny fraction of the light. If a distant star were transited by the equivalent of Jupiter, it would cause 1% of the starlight to be lost from view. One gas giant planet, found by the radial velocity method, has also been detected using the transit method from a ground-based telescope. Star HD 209458 was discovered to possess a 51 Pegasi b-like planet (a large planet orbiting its parent star in a tight circular orbit, also known as 'hot Jupiters') and subsequently seen to dim at precisely the time that the planet was predicted to pass in front of the star.
Not every planetary system will be so aligned and a large number of stars need to be observed in order to stand a chance of catching one with the right orientation. ESA's Eddington mission is designed to survey about half a million stars, in the hopes of detecting thousands of planets, including Earth-sized worlds and smaller.
An Earth-sized planet will typically block less than one hundredth of a percent of the starlight but Eddington's sophisticated modern detectors will register the change.
ESA's Gaia mission, which uses the astrometric technique as its main method of detecting planets, will be able to double check its planet findings by using the transit technique.
The holy grail of this research is to obtain pictures of planets around other stars, so that astronomers can analyse the chemical composition and physical state of these distant worlds. This is an extremely challenging task. At visible wavelengths, a star like the Sun will outshine a planet like the Earth by a billion times. This is because planets do not emit visible light; they simply reflect some of the star's light. If, however, astronomers move to longer wavelengths, such as the mid-infrared, then the contrast between the star and the planet drops to a million, because the amount of infrared given out by the star goes down whilst the planet itself, begins to emit.
This mid-infrared radiation, however, is most easily viewed from space. At the Earth's surface, the signals can be swamped by the infrared waves our own planet gives out. Also, because planets and stars are usually close together, the Earth's atmosphere smears their light into a single fuzzy blob. So, astronomers have devised a number of techniques to work around these constraints and make direct detections collecting both visible and infrared light.
Orbiting its parent star, a planet's motion will alter the wavelength of light it reflects from the star. When the planet is moving towards Earth, the wavelengths of light are squashed. On the other side of its orbit, the planet moves away from Earth and so the light is stretched to longer wavelengths. This behaviour is known as the Doppler effect.
The star is also pulled by the planet (see radial velocity technique) creating a smaller Doppler shift in its own light but always in the opposite direction to that of the planet's light. If the intensity of light coming from the star and planet are plotted on a graph, they combine into a shape called the Spectral Energy Distribution. As the planet orbits and the star wobbles, the shape of the graph changes.
In principle, computer analysis would be able to separate which parts of the signal were caused by the star's light, leaving just the planet's signal for analysis. However, 4-metre diameter telescopes do not collect enough light to make this technique work and no one has yet tried it with an 8-metre telescope.
When light is reflected by a planet it is not just the wavelength that can be changed. Being a wave, each light ray oscillates in a particular direction. Light rays emitted by a star are said to be unpolarized because the oscillation direction of each ray is random. When light rays bounce off the planet, however, the oscillations are forced into a preferred direction because of the way the light interacts with the atoms and molecules in the planet's atmosphere. Light rays lined up like this are said to be polarized.
Astronomical devices known as polarimeters are capable of detecting just polarized light and rejecting the unpolarized beams. New, highly accurate polarimeters are currently being made, in the hope that they will be able to pick out the faint polarized light beams coming from the extrasolar planets.
The light from individual telescopes can be combined to simulate collection by a much larger telescope. This technique is called interferometry and was pioneered using radio telescopes. It is now being applied to optical and infrared telescopes.
This method relies on the wave nature of light. A wave has peaks and troughs. Usually when combining light in an interferometer, the peaks are lined up with one another, boosting the signal. In nulling interferometry, however, the peaks are lined up with the troughs so they cancel each other out and the star disappears. Planets in orbit around the star show up, however, because they are offset from the central star and their light takes different paths through the telescope system.
Within five years, a team of astronomers from ESA and ESO will be using a nulling interferometer. The team will use the four 8-metre telescopes of ESO's Very Large Telescope and will combine the beams with GENIE (Ground-based European Nulling Interferometer Experiment). GENIE may be able to take pictures of 'hot' Jupiters but will be hampered by our planet's atmosphere.
Other ground based nulling interferometers are also being developed around the world to perfect the technique. Eventually, sophisticated flotillas of spacecraft, such as ESA's Darwin mission, will use nulling interferometry to isolate the light from Earth-like extrasolar planets.
All of the previously described methods have made direct detections by isolating the planet's light from that of the star. A direct image would be one on which the star's light is totally separated from the planet. A team of British astronomers believe that by using 8-metre telescopes, they may be able to see planets in orbit around white dwarf stars.
When the Sun dies, it will first swell into a red giant star, engulfing the planets out to Mars. It will then shed its outer layers into space and the remains will shrink into a white dwarf star. The gas giants will remain in orbit and may even spiral outwards from the star, making them easier to see. White dwarfs are dim so the contrast between a gas giant and the star falls to a factor of a thousand. That may be low enough to bring them within view of 8-metre telescopes.
Observations are now being scheduled.
Eventually, to take images of the surfaces of extrasolar planets will require much larger telescopes in space, working together as interferometers.