from AereeAndBen'sAstronomyLab Website
 

In recent years,

over fifty extrasolar planets

have been detected via various methods

which we'll discuss in turn.
 

What is an Extrasolar Planet?

It refers to a planet that resides outside our solar system. Now, this might seem pretty obvious, but some of the extra-solar planets that have been detected have been extremely large; planets that have been many times the mass of Jupiter have been detected. When you get that large, they really cease to be planets and are rather low-mass brown dwarfs, or 'failed stars' that are too small to undergo hydrogen fusion and emit their own light. Generally speaking, planets cannot be more than ten times the size of Jupiter or else they are too large. As always though, the debate does not end there since we aren't really sure how brown dwarfs are even formed.

The main problem posed with detecting extra-solar planets is that compared to the brightness of the stars they are orbiting, they are extremely dim; planets cannot give off their own light, and so we can only 'see' planets by light that is reflected off them from their stars. With traditional astronomy, it is at the moment impossible to detect these planets since their light is far outshone by the stars they orbit around. Therefore, it has been necessary to use other techniques.


Various Detection Techniques

I. Astrometrics
Instead of trying to look at the planet directly, so far all our efforts have been concentrated on looking at stars to see if we can infer the presence of a planet by the behavior of the star. By this, we mean that we look for signs in the movement of stars that suggest that a planet might be responsible for.

 

To do this, we have to determine how a star should be moving in the first place - if we don't know this, then we don't know if what we're seeing are signs of a planet, or the star's normal movement. Astrometry is the branch of astronomy that can determine the 'normal' movement of a star based on its location in reference to other nearby stars.

 

II. Wobble-Detection
Once we know how a particular star 'should' be moving, we can actually look at it and find out how it is moving in real life. If a star has a large planet orbiting around it, the gravity of that planet will influence the movement of the star - the influence will be very small, but detectable. This influence will result in the star 'wobbling' around the path it 'should' be moving on - which is why we call it 'wobble-detection' (believe it or not, we didn't make this term up - it's regularly used to describe the technique in scientific journals).

 

As yet, only large planets can be wobble-detected since only they exert a large enough gravity to make a large, detectable wobble in the path of their stars. As astrometric and astronomy techniques improve, it is possible that smaller and smaller wobbles - and thus planets - can be detected in this way.

III. Radial Velocity
Radial velocity relies on the very foundation of modern astronomy - redshift. The way redshift works is that when light from an object (generally from a star) appears to be shifted to the red part of the spectrum (hence, redshift), we can tell that the object is moving away from us. When the object is moving towards us, the light is shifted towards the blue part of the spectrum - it is blueshifted. By carefully examining the light emitted by the star (examining the 'spectrographic shift' of the star) we can work out whether the star is moving away from us or towards us.

As the star moves away from the observer - the Earth - due to the influence of the gravity of the planet that is orbiting around it, the light from the star is redshifted.   Conversely, as the star moves towards the observer, its light is blueshifted. This regular change in spectrographic shift of the light from the star would indicate to scientists that the star had a planet.

When a planet orbits around a star, it will slightly alter the radial velocity of that star - in other words, it will also create a wobble in the movement of the star. This time however, we don't look at the movement of the star, but we look at the light from the star. If there is a planet orbiting around a star, the star will appear to be moving towards us, then away from us, then towards us and so on, because it is wobbling along its path. If the star had no planets, it would move smoothly - there would be no changes in its radial velocity.

 

This means that by looking at the spectrographic shift of the star to see if it is redshifting, then blueshifting, then redshifting (and so on) we can determine if it has any planets orbiting around it. This technique is also called the Doppler technique (related to the Doppler effect, which causes sounds to change pitch as their source moves towards, then away from your position). Again, this change in the spectrographic profile of a star is very hard to detect and only very large planets, or planets very close to the star, have been detected using radial velocity.

 

So far, the vast majority of extra-solar planets have been discovered using radial velocity. Among others, the AFOE (Advanced Fiber Optic Echelle) project at Harvard and the Observatoire de Haute-Provence have used radial velocity to detect extra-solar planets.

IV. Transit Photometry
On the rare occasions when the plane of a star and its planet are exactly in line with the direction we are looking, we can detect a planet by transit photometry. When a planet passes in front of a star, the star's brightness will drop slightly. As the planet orbits around the star, this drop in brightness will occur with every orbit of the planet. This regular dimming allows us to detect the planet.

If you are viewing a solar system from the edge on (i.e. parallel to the plane of the system) then as a planet passes in front of the star, the star's brightness will momentarily drop.

Although it might seem very unlikely that we'd be looking in just the right direction for transit photometry to work, scientists from the NASA Ames center used the Vulcan photometer at the Lick Observatory to detect a planet orbiting the star HD209458 in December 1999 with transit photometry. The scientists were able to narrow the odds against them by monitoring roughly 6000 stars every night.

V. Direct Imaging
Direct imaging means detecting planets by looking for them directly. As mentioned earlier, this is extremely difficult to do with the telescopes we have now since the planets are so dim. However, with the development of increasingly large and sophisticated telescopes that use techniques such as adaptive optics, it will be possible to at first detect large planets, then progressively smaller planets. Projects such as the NASA Terrestrial Planet Finder and the ESA Darwin space telescopes will, along with other techniques, use direct imaging.

VI. Coronography
All stars emit a corona of light around them, and this can be detected using telescopes. If a planet was orbiting a star, it would block out part of the star's corona - and so you could detect a planet by studying the corona of a star. The French COROT telescope aims to do just that.

Hubble Telescope Unveils First 'Extrasolar Planet'?
Article by Nicolle Charbonneau (May 28, 1998)

click image to enlarge   [Figure] TMR-1C is the first direct evidence of a planet beyond our solar system

While peering into a dark, dusty region of space in the constellation of Taurus, a team of astronomers in Pasadena, California made an extraordinary discovery: the first direct evidence of what may be a planet beyond our solar system. The small group of researchers were using the Hubble Space Telescope to study a young binary star system -- two stars very close together and orbiting each other -- when they stumbled across what astronomers are calling "a landmark in our quest to understand our origins," a discovery of "historic significance", and "a watershed event."

 

For Dr. Susan Terebey and her team from the Extrasolar Research Corporation, the excitement of this discovery was overwhelming. "It all fell into place," she told reporters at a press conference on Thursday, describing the moment of discovery.

"That was really an indescribable experience. Our team has just been walking on air."

The object, with the rather unglamorous name of TMR-1C, has a mass two to three times that of Jupiter and is located about 450 light-years from Earth. It appears that the planet originally was in orbit around one of the binary stars, but because of the instability of the star system, the planet eventually gained enough momentum that it was thrown out of its orbit.

[Figures from left to right]

1) Dr. Susan Terebey heads up the team at Extrasolar Research Corporation, which discovered the object while using the Hubble Space Telescope;

2) Terebey and her team speculate that the object was ejected from its orbit around the binary stars between 500-1000 years ago;

3) Because of dust and gases in that region of space, the binary star system and its planet are visible only with an infrared telescope;

4) The large number of binary stars in our galaxy could mean the discovery of other planets in the near future

As it traveled away from its binary parents, it left a long filament of matter, which Terebey calls a "lightpipe", because it acts like a tunnel for light to travel through space. Terebey and her team used Hubble's infrared telescope to peer through the gas and dust clouds that darken that region of space. NASA released the images Thurday at 1:00 pm ET, revealing a planet that is 10,000 times less bright than our sun, but bright enough to indicate that it's a very young object. In fact, Terebey and her team speculate that the object may be only 200,000-300,000 years old; that would make it incredibly young, since gas planets usually form over a period of 11 million years.

 

The object's young age has scientists speculating about a new method of planetary formation called the one-step method, in which planets to form in hundreds of thousands, rather than millions of years. And since the vast majority of stars in the vicinity of Earth are binary star systems, this discovery has also heightened speculation about the discovery of other planets in the next five to twenty years. While scientists at NASA are reluctant to make any guesses, they admit that this is a step towards finding extra-solar terrestrial planets like Earth which may have developed in conjunction with a gas giant.

 

The next step for scientists involves confirming that it is a planet and not a star shining from light-years beyond the binary system, or a brown dwarf star--a rare celestial object that forms like a star but doesn't shine like one. This work will be done with the HST, Hawaii's Keck Observatory, and eventually with the Next Generation Space Telescope and the Space Inferometry Mission, which will provide much higher resolution pictures.