Knowing whether or not we are alone in the universe has been one of the goals of many philosophers and scientists throughout history. Until recently, the only known planets were part of the Solar System. The discovery of extrasolar planets is a fairly recent event. Although the systematic search began in 1988, the first extrasolar or exoplanet planet was detected in 1995.
But observing planets directly is not easy. The existence of extrasolar planets has been deduced in the first instance from indirect tests. However, several projects are underway that will allow observing these planets in the visible or infrared. From there we could obtain some data that allow us to deduce, with reservations, whether these planets house life or not.
Until recently, scientists have not had techniques and instruments capable of detecting extrasolar planets, that is, planetary systems around other stars. But the existence of our planetary system has encouraged the search. Thus, one of the first steps towards the discovery of planets beyond our Solar System occurred in 1983, when a disk was discovered around the Beta Pictoris star. But for a long time this has been the only test available.
The arrival of the Hubble space telescope allowed detailed observations of star-forming regions, such as the one in the Orion constellation. Thus protoplanetary discs were detected around young stars in formation, and it was found that a large part of the stars that were forming had discs that could give rise to planets in the future.
At the beginning of the 1990s, the discovery of planets revolving around pulsars was announced. Pulsars are very compact stars that rotate very quickly, emitting electromagnetic radiation that, if the axis of rotation is properly oriented, can be detected from Earth. Later it was seen that there were errors in the analysis of the data and that these planets did not exist. Then, however, the existence of planets revolving around pulsars has been confirmed.
Finally, in 1995, the discovery of the first extrasolar planet was announced, revolving around a solar-type star, 51 Pegasi. From that moment, the announcements of new extrasolar planets have been happening without pause until reaching the present time. Now many extrasolar planets are known, and the number of acquaintances grows every year.
Given the difficulty of direct observations, the first attempts to search for planets that have resulted have been based on indirect observations. The methods used are based on gravitational disturbances caused by the planets over the stars and on the transit of the planet ahead of the starlight.
Most of the planets orbit their star at a distance much smaller than the Earth-Sun distance. In addition, the observed mass is of the order of Jupiter's mass. This is, in part, a consequence of the detection methods used. The planets of greater mass and that rotate closer to the star are more likely to be detected by the techniques employed.
However, the refinement of these techniques and the use of new ones should allow in the near future to also detect terrestrial planets, that is, planets with a mass equivalent to that of our planet. In the future, thanks to new ground-based telescopes and new space observatories, we will be able to collect light coming directly from the planets to obtain images. From there, with the help of spectroscopy, we can know which are the main components of the atmospheres or surfaces of the planets.
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