Extra-solar planets
Planets don't generate visible light of their own, although they emit electromagnetic radiation as a balk body does; however, they reflect light from their parent stars, and for this reason we can see Mars, Venus or Jupiter in the night sky. But planets are much less bright, and visible, than their parent stars. They might reflect a fraction of the light that comes upn them; this light is usually a millionth or less of the light emitted by the parent star. The direct detection of planets outside the solar system with current technology is just not possible. However, in the last few years there has been a spate of announcements by astronomers who found planets orbiting stars tens of parsecs away. How is it possible? Two main techniques have been used that rely on the effects that planets have on parent stars. Before describing them, keep in mind that in both cases these are indirect methods and no images of planets have been obtained, although plans are being drawn up to build an interfereometer is space to obtain infrared images of planets outside the Solar System.
The first technique, that it will be called here astronometry for brevity, measures very accurately the wobble of a star as is tugged by a planet. A planet is attracted towards its parent star as much as the star is attracted to the planet. The fact that the planet is seen to move, and not the star, has to do with the fact that, although the force of the star on the planet is equal and opposite to the force that the planet exerts on the star - Newton's Third Law, the associated accelerations are not, since their are inversely proportional to the body's mass. Thus, a planet accelerates much more than a star; but also the star accelerates and moves, but much less.
This can be summarized by this relation (Newton's Second Law):
where F is the force, m the mass and a the acceleration.
The motion of the star, under the influence of the planet's attraction, is what atronomers try to measure. Careful observations are required to discern this motion from all other spurious effects, such as distortion of light due to atmospheric effects and change in the size of the star due to dynamic instabilities.
Example of dynamic instability
The second technique consists in the use of the
Doppler
effect. If the orbital plane of the planet-star system is seen
edge on from the Earth, then there will be a Doppler shift in the
light coming from the star as the star is tugged frowards and
backwards due to the rotation of the planet. In such a case, what is
probed is the change in the radial velocity of the star. The
two techniques are complementary. The first is most sensitive to
planets in large orbits, since in this case the effect on the position
of the star is the greatest. The second technique is most sensitive to
planets near stars, since this is when the change in velocity of the
star is the largest. In both cases, these techniques can be used only
on rather large planets, Jupiter-size or Jovian planets, since a
planet like the Earth is not massive enough to cause a detectable
perturbation in the position of a star of the mass of the
Sun.
Although life cannot be sustained on a gaseous planet such as
Jupiter, it is reasonable to think that once a Jupiter-like planet is
found, smaller planets could be present too, as they are in our Solar
System.
The detection of these Earth-size planets poses enormous technical challenges. There is a NASA sponsored study to check the feasibility to build a large infrared telescope to try to detect presence of gases on extra-solar planets; the composition of the atmosphere of these planets could give away the presence of living organisms.
It is interesting to note that the first positively identified extrasolar planet is one orbiting a pulsar (rotating neutron star). Wolszczan and Frail made this discovery rather accidentally in 1991 while they were studying pulsars. Actually they detected three planets rotating around the pulsar PSR B1257+12. As discussed in the section Pulsars , pulsars are the cinders left of a star that died in a supernova explosion. Because a star of that size loses most of its mass at the end of its cycle, if the star had any planets before the collapse, it would lose them during the explosion and the planets will wander away from the star. Thus, the fact that after a supernova explosion there are still planets orbiting what is left of the star can be explained by proposing that the planets formed after the supernova explosion. Because the environment around pulsars, which are neutron stars, is so harsh (due to intense ultraviolet radiation and winds of energetic particles), no life could possibly be found there.
The most recent discoveries of planets orbiting stars outside the Solar System.
are summarized in this Table.
Also take a look to the
Extrasolar planets Encyclopedia (External Link).