For centuries it has been speculated on the existence of planets orbiting other stars, just as the planets of the solar system orbit around the sun. Only in the 1990s has been confirmed the existence of these so-called extrasolar planets, or exoplanets.

Until now, mid-2006, about 200 extrasolar planets have been detected. The observation of these planets is very difficult, and only became possible thanks to the advance technology of astronomical observations, which allowed us to obtain data with greater accuracy. We currently have six different methods of detecting extrasolar planets.

The most difficult of all methods is the direct observation. It would be nice to be able to point a telescope at a star and to directly observe the planets orbiting it, but it is extremely difficult. Remember that a planet is much smaller than a star and, brightless, is usually overshadowed by the star’s brightness. This method works to observe very large planets (greater than Jupiter) and are quite a distance from the star, as is the case of 2M1207b, orbiting the brown dwarf 2M1207, in the constellation Hydra (see image below).

Other methods are based on measuring the change in position of the star caused by planetary motion around it. We say that a planet orbits the star, but, in fact, star and planet revolve around a common point called center of mass. Since, in general, the mass of the star is much larger than the mass of any planet, the center of mass of the group is very close to the center of the star. So while the planet runs its motion around the star, although we can not observe the planet itself, we try to measure the displacement it causes to the star, causing it to spin around the center of mass of the star-planet system.

This shift can be obtained by measuring variations in the position of the star, through direct measurements of its position in the sky. These variations are very subtle, and can only be obtained when a very massive planet is responsible for the wobble of the star. The field of astronomy that measures the position of stars in the sky called astrometry.

Depending on the position of the planetary orbit, we can measure the displacement of the star through the Doppler effect (see article “Doppler Effect”). If the displacement around the center of mass causes the star to arrive and get away from us, we will see its light bluer when it is approaching, and redder when it is moving away.

In the particular cases of pulsars – neutron stars that emit X-rays with great regularity – we can measure the change in position caused by the presence of a planet still a third way: by small variations in the time interval between pulses. With this method, was made the first confirmation of the existence of extrasolar planets in 1992, the pulsar PSR 1257 +12, in the constellation Virgo.

A technique that is not related to variation in the position of the star is to observe variations in the brightness of the star caused by the passage of a planet in front of it. If a planet revolving around its star in an orbit positioned so that eventually it passes exactly between the line of sight that connects Earth to the star, it is possible to observe a small decrease in brightness of the star. When the planet passes in front of the star, the brightness decreases and increases again when it clears the path of light. The time between changes directly provides the period of revolution of the planet (the Earth’s revolution around the sun takes one year, approximately 365 days and 6 hours).

These methods work well, as we saw, for the detection of large planets and massive. However, we can use the so-called gravitational microlensing to detect planets with masses similar to Earth. This method uses the distortion of space caused by any body with mass, causing a shift in light from an object located behind him. So when a planet orbiting a star is positioned in such a way to change the trajectory of light rays coming from another more distant star, the microlensing happens. Knowledge of the effect of gravitational lensing and microlensing is given through the development of General Relativity of Albert Einstein.

Another method for detecting extrasolar planets is the study of dusty disks that surround some stars. Many planetary systems have a significant amount of dust distributed to form a disk around the star. This dust is material left by comets and also remains of collisions between asteroids, asteroids and planets or even between planets. Here in the solar system, that dust causes the meteor showers and the so called zodiacal light. Noting that the dust disk, we can see the density variations and distortions, caused by gravitational interaction with possible planets.

The discovery of extrasolar planets brought up again the scientific discussion about life beyond Earth. The race now is to find a planet that can support life as we know on Earth. It means seek an Earth-like planet orbiting a Sun-like star, and with a moon similar to our Moon. The Moon has an important role in the existence of life on our planet, because it is what keeps a certain stability of Earths rotation axis.

Opinions are divided in astronomy. Some astronomers believe it will be almost impossible to find another place in the universe that can support developed life like Earth. Others think it is just a matter of time until we find many of these places. After all, today we estimate that our galaxy has about 400 billion stars. If many of these stars have planets, the number of planets in the Milky Way is very large. Not to mention the planets orbiting stars in other galaxies, because, as we have seen, it is ,already ,hard enough to find planets in nearby stars. One thing is certain: the nearly 200 extrasolar planets known today (mid 2006) are very few against the number of planets that are not yet seeing.

And whose side are you on? Your intuition says that the development of life is exclusive of the Earth, or that life is common in the universe? And an ethical question arises: if we find a civilization technologically developed, should we contact it? We may interfere in its evolutionary trajectory? As always happened in science, the more we know, more questions arise.

The brown dwarf 2M1207 and its planet 2M1207b (the planet is the smallest body)