How to find an extrasolar planet?

Radial Velocity
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 on 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. How? As the planet orbits the star, it pulls on it with its gravitational field, forcing the star into a small orbit, or wobble. It makes it look as if the star is pirouetting around a point in space. 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. Astronomers therefore look for wobbling stars, since 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.

Astrometry
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 thousand million 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.

Transits
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.

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.

Direct detection and imaging
The ultimate aim 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 thousand million times. Why? 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, the contrast between the star and the planet drops to a million, because the amount of infrared given out by the star goes down while the planet itself begins to emit.

This mid-infrared radiation, however, is most easily viewed from space. At 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, Earth's atmosphere smears their light into a single fuzzy blob. Astronomers have therefore devised techniques to work around these constraints and make direct detections collecting both visible and infrared light.

Doppler isolation
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, four-metre diameter telescopes do not collect enough light to make this technique work and no one has yet tried it with an eight-metre telescope.

Polarimetry
When a planet reflects light, 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 unpolarised 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 polarised.

Astronomical devices known as polarimeters are capable of detecting just polarised light and rejecting the unpolarised beams. New, highly accurate polarimeters are currently being made, in the hope that they will be able to pick out the faint polarised light beams coming from the extrasolar planets.

Interferometria de Nulling
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.

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