Planetary systems in space

1989/03/01 Arregi Bengoa, Jesus Iturria: Elhuyar aldizkaria

• Astronomia

In addition, we do not totally discard the problem of brown dwarfs that we touched in the previous specimen. On the one hand, because the evolution of the planets and the brown dwarf that moves around larger stars is the same problem. What's more, being these last ones of greater mass, are also easier to detect and this particularity has become evident. The second reason is more theoretical and may be related to the origin of these two types of body, but we speak later.

As is logical, the news mentioned above has been due to the improvement of detection methods. The method traditionally used has been astrometry based on the measurement of the position of stars. The method is based on the influence that the gravity fields of the hypothetical friends of the star would have on their trajectory. The effect is easily understandable: both the planets and the central star move around the baricenter of the system. Therefore, if the only star is not the correct route. Of course, the use of these studies requires working with the nearest stars, since the diversion of the planet on the direct path of the star is very small.

Artistic image of a planetary system in formation.

For example, to highlight another similar to our Solar System that would correspond to ten light years, we should be able to measure the position of the star with an accuracy of one-thousandth of the second arc. The results obtained by this method are not conclusive. The case of Barnard's star is still discussed. This star is the only one closest to the Sun (6 light-years) and P. Although Van de Kamp has tried to analyze it for more than twenty years, there has been no unified opinion on the consequences of his work. There are others, but these also unconfirmed.

Thus, for example, the star Lalande 21185, at 8.2 light-years, could have a planet 30 times greater than the mass of Jupiter, 6 times greater than Epsilon Eridani, at 10.7 light-years, 12.4 light-years, the RD 551688, which is 60 times greater or the planet of Cassiopeiae ten times greater.

However, the results obtained with this method will obtain great precision (maybe of two orders) if this year the satellite Hipparcos is put in orbit as planned. The data sent by this satellite for about four years will be sufficient to answer undoubtedly to the question of the existence of planetary systems.

Meanwhile, what is imposing is another method based on spectroscopy. The idea is: The Doppler effect allows measuring the star's radial velocity over the Earth for a long period of time. Due to the natural movement of the star, if you have no friends, it will go straight and the modification of the radial component is random. If you have a friend, on the contrary, it will be a periodic change (with a period of rotation proper to the star), since when the body is between the Earth and the star, the radial component is smaller than when the star is in the center. The advantage of this technique compared to astrometry is that the speed variation does not depend on the distance to the star. Therefore, it could apply to many more stars (the limit of astrometry, even if the satellite was used, would not be much beyond 20 light years).

The first success obtained by spectrometry was the variation of the speed of the star HD114762 to 90 light-years and the measurement of its period. The first is about 700 m/s, while the planets only affect the Sun 14 m/s. The second is 84 days, very short. These data indicate that the mass of the auxiliary body is at least 10 times greater than that of Jupiter. Since the characteristics of the orbit and its orientation towards Earth are not known, the calculations are made with the radial component of the speed variation, but this will always be less or equal than the total variation. Therefore, we must consider what is given as a minimum value.

As the measurement methods of the Doppler slide have improved, discoveries have also increased. In this sense we must mention two new techniques. First (B. Developed by Campbell), it consists of placing a hydrogen fluoride container in the spectrometer, in order to have a reference line to measure the slip. Since the loss of luminosity is relatively high, this method is only applicable to luminous stars. However, not to be missed, the team of Campbell has studied nineteen stars obtaining the following results: in nine cases no periodic change of speed has been measured, that is to say, the change was random; in another nine a modulated change has been observed, but the period has not been limited, since it is probably longer than the duration of the study, and in these cases masses of invisible bodies have been calculated between one and ten times greater.

Finally, in the case of star 36 of the Osa Mayor or Ursa Major, both magnitudes were delimited, with a speed change of 20 m/s and a period of about 3 years. Body mass is estimated to be 1.6 times higher than that of Jupiter. The minimum velocity that can be measured today by this technique is about 10 m/s, that is, the necessary one to reveal the effect of the planetary system of the Sun on our star.

The result of the second innovation was the CORAVEL spectrometer (Correlation Radial Velocity). This instrument does not measure a single mass of the spectrum, but a part of it (thousands of lines). The light received is compared by computer with the pattern of the star spectrum. The one who has to move so that the pattern is incorporated into the light coming from the star gives us the displacement. This technique, developed initially by astronomers of the Geneva and Marselle observatories, has been designed today with tools that reach an accuracy of 0.2 km/s.

Results are already being obtained and the influence of some bodies has been shown. They are usually of great mass: More than a tenth of the mass of Jupiter. But this is not the only particularity. In most cases, it has been calculated that their orbits should be quite eccentric: Between 0.20 and 0.50. Taking as a reference the largest planet in the Solar System, values would not exceed 0.06. What is the reason for the difference so evident?

This question takes us to the terrain of the brown dwarfs we mentioned at the beginning. If we consider that the border between brown dwarfs and planets is about one-tenth of the mass of Jupiter, most of the previously detected stars would be located in the area of brown dwarfs. If we take into account that their formation process is similar to that of the stars and that the planets are formed again with acresium, the difference between the orbits is due to those different forms of nature or composition. But to end, we must take into account another problem that we still have to mention. The existence around the stars of brown dwarfs or very large planets (say five times bigger than Jupiter) gives no hope that there is a solid planet like Earth.

On the contrary, the presence of Jupiter in the Solar System prohibits the existence of other planets until Tuesday. Therefore, it is very debatable the possibility of coexistence between giant planets or brown dwarfs and terrestrial planets. Consequently, it is also debatable whether the nature of the systems found is similar to ours. Some believe that the latter (because the resolution is still too low) would be in the vicinity of stars where nothing has been detected. It is clear, therefore, that we must still walk the way, but we can say that the steps being taken today are great.