Formerly the planets were points in the sky. As time passed, however, those lights in the sky began to be treated, at least in elite intellectual circles, as objects, and finally as places. This revolution came, in large measure, with the invention of the telescope. Beyond the short human senses hid many worlds to be discovered.
Since the days of Aristarchus of Samos (ca. 310 - ca. 230 B. C. E.) there were suspicions that the Sun and stars had something in common. One of the first to clearly say that the Sun ought to be a star, so close that its light dazzles us and heats the ground in which we step on, was Christiaan Huygens (1629-1695).
Consequently, the other stars are suns, only that they are too distant so we perceive them as mere dots in the sky. But if we travel to them, we would see them as big and hot as our Sun, and the Sun, so small and distant as any other star. Thus Huygens dared to predict that there should be other planetary systems. In his posthumous book “Cosmotheoros” (Adriaan Moetjens publishing house, The Hague, 1698, and Timothy Childe Publishing, London, 1698) he allowed his thought flow further: “What a wonderful and amazing Scheme we have here of the magnificent Vastness of the Universe! So many Suns, so many Earths, and each endowed with many Herbs, Trees and Animals, and adorned with so many Seas and Mountains! And how must our astonishment and admiration might be increased when we consider the prodigious distances and a multitude of Stars?”
Even after the discovery of more planets in our own Solar System (Uranus by William Herschel in 1781, and Neptune by John Couch Adams and Urbain Le Verrier in 1846) and new categories of objects (asteroids, by Giuseppe Piazzi in 1801, and the today-called trans-Neptunian objects, by Clyde Tombaugh in 1930), there were few who dared to search for planets around distant stars. If the light from the star itself comes so weakened after much travel, imagine the dim light reflected by extrasolar planets.
One of the first to try, however, was Piet van de Kamp. This professor at Swarthmore College, working from the Sproul Observatory, in the United States, proposed to himself starting in 1937 to examine the nearest stars, including the one called Barnard’s star. His technique was to photograph each star and carefully measure its position relative to other stars. He hoped that if an object was orbiting, for example, Barnard’s star, the tug that gravity generates between them (and that is the reason why the planets are kept revolving around a central star) would leave some “trail”, in the form of a slight swaying of the star under study. It is similar to that swinging seen in a small tree when someone grabs her or himself to its flexible trunk and begins to circle it childishly, hanging from the plant while doing so.
After decades of observation and thousands of photographs, van de Kamp triumphantly announced in 1963 that he had detected the gravitational tug of an object, about 11 times the mass of Jupiter, orbiting Barnard’s star.
However, in Science there is something important called reproducibility: it is not enough that someone observe something once, anyone should be able to get the result by her or himself if she or he repeat the observing by her or his own means. And while the photographs of van de Kamp were there, no one could see the same phenomenon with other telescopes.
After careful analysis of the case, it was concluded that unfortunately what was happening was due to a defect in the telescope of van de Kamp. He had not discovered any extrasolar planet.
With the development of electronics, however, there were people who felt that with new ultra-sensitive instruments they could detect the swinging that Van de Kamp sought so much among the stars.
In 1992 there was an indication that there could be worlds as imagined by Huygens and many others, when Aleksander Wolszczan and Dale Frail announced that something was interfering with the rotation of a dead star called PSR B1257 +12, a neutron star.
Using the Arecibo Radio Telescope, in this case they detected a slowing in its rotation, measured by the frequency of the natural radio waves emitted by the star, that supposedly must be extremely regular, synchronized with its rotation speed. The phenomenon of the slowing of the rotation is known, as is the case with Earth because it is piggybacking the Moon. The day is becoming slightly longer over the millions of years. The same thing was happening to PSR B1257 +12, by piggybacking, they calculated, objects the size of Earth. However, there was no other indication, let alone pictures.
In 1995, Michel Mayor and Didier Queloz had ready at the Observatoire de Haute-Provence, France, a high-definition spectrometer for measuring fluctuations in the radial velocity of the stars. This complicated phrase means they had built a very sensitive instrument to analyze carefully the color of the stars. It is known that when a star (or any object) is moving towards us, becomes slightly bluer (or less red). When it is receding away, however, becomes slightly more red (or less bluish). This is due to the Doppler effect, well known in the case of sound: the pitch of an approaching vehicle becomes higher, but when it recedes becomes lower. The same applies to light, however here the effect is imperceptible to the naked eye although it can be noted with instruments. The new device, attached to the telescope of 1,93 m in diameter, was so precise that could measure fluctuations in velocity of just 40 km/h.
Armed with this, Mayor and Queloz were able to detect fluctuations in the star 51 Pegasi, because it actually had another object in tow. It was estimated that the companion was about 10 times the size of Jupiter, insufficiently big to be another star (such as small dark stars called brown dwarfs), so it is considered to be a planet. What was remarkable about this discovery was that now the star is one of the so-called main sequence stars, i.e. that shares certain characteristics with our Sun.
This discovery was followed by others, most of them by the team of Paul Butler and Geoffrey Marcy, who also pioneered the technique of radial-velocity perturbation.
NOTABLE EXTRASOLAR PLANETS
In 1999 the discovery of the first planetary system around another star was published. It is the Upsilon Andromedae system, with three planets with masses comparable to Jupiter’s.
That same year, a significant event was the detection of the passage, or transit of an extrasolar planet, HD 209458 b, in front of its star. By the decrease in brightness of the latter (actually an eclipse) the diameter of the planet could be calculated, discovering that is about 15 times larger than the small Earth.
The same transit method also allowed detecting, in 2001, a slight variation in the color of the star during the eclipse, indicating that the planet has a gaseous envelope, that is, an atmosphere. With the Hubble Space Telescope sodium was detected. Later, between 2003 and 2004 were detected hydrogen, carbon and oxygen. Then in 2005, with the Spitzer Space Telescope, observing in the infrared, its light could be directly detected, its temperature being calculated at about 1500 kelvins (above and beyond 1000 degrees C). In 2007, what probably are silicate dust clouds were found.
Meanwhile, Spitzer Telescope data showed that the light coming from the planet HD 189733 b has the characteristics of being reflected by water vapor molecules. And in December 2008 methane was detected in that world, an organic compound. In November 2009 a similar discovery was announced for HD 209458 b.
Also in 2008 it was discovered that the star HD 40307 has a planetary system with three planets of size comparable to Earth’s. The lightest extrasolar planet detected so far is Alpha Centauri B b, with 1,1 times the mass of the Earth, at the star next-door to our Solar System; and the smallest is KIC 12557548 b, with 0,4 times the diameter of our planet, although all these are very close to their respective stars and are therefore too hot. However, with a technique “à la Einstein” called microlensing observation was detected a planet, OGLE-2005-BLG-390L b, which is at a distance from its star equivalent to about three times the distance between Earth and the Sun.
Finally, in November 2008 it was reported that, thanks to the Hubble Space Telescope, for the first time in History direct photographs of these subtle points in space were obtained. It was the case of Fomalhaut b, 25 light-years distant from us. It is a planet about three times the mass of Jupiter, orbiting its star at a distance of about 10 times the distance Saturn orbits our Sun. It takes 872 years to make one complete revolution. Simultaneously, it was announced that the giant Keck and Gemini telescopes in Hawaii directly photographed HR 8799 b, HR 8799 c and HR 8799 d, planets forming a planetary system around a star located in the area of the sky known as Pegasus.
Thus today Homo sapiens have on record much more planets outside the Solar System than within it, being discovered at the fantastic pace of dozens every year. With newer equipment like the Kepler Space Telescope launched in March 2009, it is expected finding planets, one after another, more and more like the Earth.
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Originally published in ABC Color, 1 February 2010. Photograph: This image obtained with the adaptive optics system of the Keck II telescope, the largest in the world, by the NIRC2 camera, in near-infrared light, displays four planets orbiting a star 120 light-years away from us. The planets’ star, called HR 8799, is the central blob. Probably the four planets, called HR 8799 b, HR 8799 c, HR 8799 d and HR 8799 e, are gas giants like Jupiter. Planet b is about 5 times the mass of Jupiter and orbits at about 68 astronomical units (AU, the distance between Earth and the Sun) of the central star. The planets c, d and e are about 7 times the mass of Jupiter and orbit at about 38, 24 and 14,5 AU, respectively. For an idea of scale, Jupiter resides at about five times the Earth-Sun distance, and of course Earth 1 time, or 1 AU. Photo credit: National Research Council of Canada’s Herzberg Institute for Astrophysics, Christian Marois & W. M. Keck Observatory. With permission from the authors.