On October 6 1995, the world knew that there were worlds in which the Sun was not the king star. At a congress in Florence, Swiss astronomers Michel Mayor and Didier Queloz, from Geneva Observatory, announced the discovery of the first planet outside the solar system around a star similar to the Sun, winning a silent contest with American colleagues. “Everyone can now look at the sky at night, see the stars and say: ‘there are planets out there?”, said Mayor. There were no images of the celestial companion that circulated the star Pegasus 51, some 40 light-years away (the metric equivalent of a light-year is 9.5 trillion kilometers). There were, rather, indirect evidences of the presence of an object whose gravitational field caused a subtle and periodic variation in the movement of the star with the name of the winged horse and which, every 4.2 days, made a complete trip round its sun.
There was a planet there. In many aspects, but not in all, the new world was reminiscent of the largest solar planet, Jupiter, whose mass is 318 times greater than that of the Earth. Without a solid surface, devoid of water, it was gaseous and gigantic, with about half the weight of Jupiter. However, unlike Jupiter, which is very far from the Sun, it was almost glued to its star. To use the jargon of the astrophysicists, it was a hot Jupiter, with temperatures on its surface in the order of 1,000ºC (the original Jupiter is frozen). In short, the companion of the Pegasus 51 star was a unsuitable place for any form of life.
In the last nine years, with small variations, every planet around stars similar to the Sun – and there were about 130 – was a variation on the same theme. A copy, more or less exact, of the first world revealed by the Swiss duo: a giant ball of gas, dozens or hundreds of times larger than the Earth. A Jupiter-like world, hot or otherwise, according to the distance from its sun. At the end of August, the monotony of only finding heavyweights around stars finally came to an end: three smaller worlds, between 40 and 50 light-years away, were located with the assistance of Earth-based telescopes. The era of the middleweights started.
Once again, captained by Mayor and Queloz, the Europeans were the first to break the good news. On the 25th of that month, they announced the location of a planet with 14 times the mass of the Earth – similar in mass, therefore, to Uranus – in the environs of a star similar to the Sun, Mu Arae, located in the Altar constellation. With the sense of marketing typical of the American scientists, and perhaps some exaggeration, the astronomers from the Old World said that the companion of Mu Arae could be a Super-Earth, a term which appealed to the press. To be an Earth, the planet would have to be a lightweight, even smaller.
Even so, the middleweight on the heels of Mu Arae pleased the audience and threw the opponents of the Swiss against the ropes. Less than one week later, on August 31, two independent teams of researchers from the United States hit back: they presented two planets of a size similar to the one recently discovered by their colleagues (and rivals) on the other side of the Atlantic. For the magnitude of their masses, the worlds were compared with Neptune, which is 17 times heavier than the Earth.
The group led by Barbara McArthur, from the University of Texas, located a celestial body with 18 times the mass of the Earth orbiting 55 Cancri, a star similar to the Sun and belonging to the Cancer constellation. Three giant gaseous worlds had already been detected around 55 Cancri, and the arrival of a fourth, smaller brother made the star the owner of the largest known extra-solar planetary system. The team led by Geoffrey Marcy and Paul Butler – astrophysicists, respectively, from the University of California at Berkeley and the Carnegie Institute of Washington – found a planet with a mass 21 times greater than that of the Earth around a small cold star of the Leo constellation, Gliese 436. “These worlds of the size of Neptune prove that there are not only gaseous giant planets out there”, comments Marcy, the Europeans’ main rival in the hunt for other Earths. “We are beginning to observe planets that are smaller and smaller.”
Besides the similar size, the three planets have another detail in common: they are very close to their stars, more so than Mercury, the first world of our system, is to the Sun. Its orbital period, the time needed to complete one trip around its star, is less than ten days, an indication that they must be very hot planets. The Earth, as we know, takes 365 days, one year, to complete one orbit around the Sun. Mercury, 88 days. Why is it that locating a Super-Earth and two Neptunes is heartening astrophysicists so much? It was not just a question of mass and size, but also of the possible constitution of the recently-discovered stars.
They believe that this trio of new planets are the first to show an even more important characteristic: they may be solid, totally or at least partly solid. “The planet orbiting the Mu Arae star represents the first discovery of rocky world more similar to the Earth”, says the Portuguese astronomer Nuno Santos, from Lisbon Observatory, who is part of the European team. “Up until now, we did not know whether rocky planets were frequent or not. Now we know that they must be. We took the first step towards finding a true Earth.” The main responsibility for the European find lies with Santos, and his signature is in first place, ahead of his more famous colleagues, in the scientific article on the Super-Earth.
The planets discovered by the Americans may also be fundamentally rocky, or, in the case of the world orbiting the cold star Gliese 436, perhaps a mixture of rock and ice. One cannot however completely discard the hypothesis that the three new planets are gaseous in their major part. Like their larger cousins, the extra-solar Jupiters. Anyhow, the researchers are optimistic as to the prospects for locating shortly a planet like ours and, who knows, signs of complex life. “These discoveries show that we are on the way to finding the first extra-solar Earth”, says Barbara McArthur.
“If technology continues to progress, we may, who knows, reach this objective in a few years.” In our system, Mercury, Venus, Earth and Mars – the first four planets – are rocky. The worlds more distant from the Sun – Jupiter, Saturn, Uranus and Neptune – are essentially gaseous, without a solid surface, with rocks only in their core. The most remote of the solar planets, small dense Pluto is a case apart, in terms of its composition. Like a comet, it is made essentially of ice.
Almost all the 130 known extra-solar planets, including the three with medium mass, were discovered in the same way: by the use of the technique of radial velocity, which measures the effect exerted by the gravitational field of one or more planets on the movement of their sun. It is an indirect way of producing evidence that there is a celestial object in orbit around a star. The logic behind this procedure is easy to grasp. The presence of a planet, or any other celestial body, produces, periodically, a tiny variation in the speed of movement of the star. In other words, in the position of its sun. It is as the company of the planet were to make the star dance, going, from time to time, backwards and forwards.
The greater the mass of a planet is, and the nearer it is to its sun, the larger will be the space ballet step carried out by the star. Measuring this disturbance in a sun, the astrophysicists can infer the minimum mass (but not the maximum mass) and the orbit of the planet that surrounds it. Objects with the mass of a Jupiter bring about alterations to the radial velocity of their sun in the order of dozens or hundreds of meters per second. Worlds of the Neptune kind make their star dance a few meters. “The disturbance of the Earth on the radial velocity of the Sun is in the order of 13 centimeters a second”, explains Sylvio Ferraz-Mello, the coordinator of the planetary systems dynamics group at the Astronomy, Geophysics and Atmospheric Sciences Institute of the University of São Paulo (IAG-USP). Zilch.
The alterations in radial velocity are figured out using a piece of equipment called a spectrograph (or spectrometer), which, as the name indicates, spreads the light from the star into the frequencies and wavelengths that make it up. In this way, employing the concepts of the so-called Doppler effect, the astrophysicists have an idea of the influence caused on the orbit of a star by the presence of a planet in the environs. When the star dances closer to its observer, the measured light becomes more blue. If the opposite happens, the red tones predominate.
The successful employment of the radial velocity method for finding planets depends on the access to a spectrograph of the latest generation. The European team, for example, found evidence of its Super-Earth with the assistance of the Harps, a spectrograph capable of measuring variations in the radial velocity of celestial objects in the order of 1 meter per second. Regarded as the most powerful instrument of its kind, the Harps was installed at the end of last year in a 3.6 meter telescope of the ESO (European Southern Observatory), in La Silla, in the north of Chile. The American teams also found their Neptunes with the help of potent spectrographs.
Today, there are still no means capable of detecting worlds like the Earth around other stars. The radial velocity technique favors the discovery of planets with great masses and/or that are very close to their suns. But the limitation should shortly be overcome. The dissemination of the planetary transit method, an alternative way of finding worlds that does not show the same bent as radial velocity, is one of the bets for the next few years. The approach consists of monitoring the brilliance of a star.
From a fixed observation point, in search of periodic decreases in its intensity. This reduction, a small zone of shadow, can be caused by the passage of a celestial object of a certain size – perhaps a planet – between the star and its observer. The passage is the transit, which, in practical terms, causes a micro-eclipse in the star, detectable only by sensitive telescopes. “The transit method is especially powerful if used in conjunction with the radial velocity technique”, explains the Spaniard Roi Alonso, from the Astrophysics Institute of the Canaries. “With it, we can estimate with greater precision the mass of a planet, and have, for the first time, a notion of its size and, consequently, of its density”. Last August, working with the two techniques and data from a small satellite network, Alonso discovered a giant planet, of the Jupiter kind.
The Brazilian astrophysicist Claudio Melo, who has been working with the Europeans at the ESO observatory, in Chile, also recently took part in the detection of a new world by this double approach. He helped to locate a hot Jupiter almost glued to Ogle-TR-111 star. In spite of the discovery, Melo says that it was not easy to arrive at the result. They observed 4,000 stars in a region of space, they found 40 stars with a suspicious dimming of brilliance, and were able to confirm, with the use of radial velocity, only one planet. “The transit method lends itself better for supplying candidate planets, which, at a second moment, have to be ratified or otherwise by other techniques”, Melo ponders. There is almost a consensus in the scientific community that the transit method will shortly find planets far smaller than the current extra-solar Jupiters or Neptunes: the first candidates for being an Earth. Projects in this direction are under way and are going to gain ground in the second half of this decade.
Once again, the Europeans are ahead of the Americans. In June 2006, a small French satellite will be launched, the Corot, with 670 kilos, which during three years will remain in a polar circular orbit around the Earth. Its mission, using the transit method, will be to look for planets around thousands of nearby stars and to study seismic shocks in a dozen others. It was to be a project just of the CNES (the French space agency), but it was short of funds, and the enterprise was opened up to other countries. Austria, Spain, Germany, Belgium and the ESA (the European space agency) became partners in the venture. Brazil also found room in the Corot mission and has become a partner in the venture.
The earth station of the Southern hemisphere that will be receiving data from the satellite is in Natal (the one in the Northern hemisphere is located in Spain). In charge of its assembly is the National Institute for Space Research (Inpe). “Without the Brazilian station, Corot would manage to observe and send data on 40,000 stars”, explains Eduardo Janot Pacheco, from the IAG-USP, the coordinator of the Brazilian participation in the mission. “With our station, this figure will increase to 60,000.” Because of the partnership, the country has now sent to France software engineers to work on satellite programs, and it will have the chance to take part in scientific studies that perhaps may lead to the discovery of the first planet of the size of the Earth.
Nasa will go straight into the race for an extra-solar Earth at the end of 2007, with the launch of the Kepler satellite, which will also use the transit method to hunt down its worlds. It is known that rocky planets, of a similar size to the Earth’s, exist. Some have even been found. Except that around a kind of star that is inhospitable for fostering life: pulsars, also called neutron stars. Pulsars are dense stars, of high rotation, that emit pulses of radiation. They are dead stars.
In 1991, four years before the bombastic announcement of Mayor and Queloz about the observation of a planet around a star similar to the Sun, Alexander Wolszczan, from the Pennsylvania State University, discovered three planets – two with a mass similar to that of the Earth, and a third with the weight of the Moon – around a pulsar in the Virgo constellation, the PSR B1257+12. Strictly speaking, these were the first planets found in a star other than the Sun. The finding is almost ignored, because, as astrophysicists know, the neighborhood of pulsars is not apt for supporting worlds with life. At bottom, the great interest is for stars like the Sun, of medium brilliance, that, according to forecasts, may perhaps house thousands or millions of planets with a mild climate like the Earth.
Historically speaking, man has been facing difficulties in finding planets. To begin with the very nature of this celestial object, which does not favor its location in space. With the exception of a brief period of their youth, planets do not emit light of their own, a characteristic that makes their direct visualization difficult. Logically, a planet can be illuminated by the light of nearby stars, as happens with some worlds of the system, sometimes visible to the naked eye. But, as a general rule, the extra-solar planets are obfuscated by the brilliance of the stars. They become hidden objects, even for the most advanced optical telescopes. For the time being, they are distant faceless worlds. The only face they display is the one that draftsmen lend them in their “artistic representations”, intended to divulge a discovery to the public at large.
Even so, the scientists do not quit trying to catch in a direct way the worlds discovered in the last nine years. Last month, for example, researchers from the ESO observatory made known what may be the first image of an extra-solar planet (see the photo on this page): the smaller dot, in red, is a planet with a mass five times that of Jupiter. Alongside it is a star from the Hydra constellation, the 2M1207, the larger sphere with the lighter brilliance.
This kind of announcement, far from being the first of its kind, is still seen with skepticism by the academic community. Astrophysicists believe that it will only be possible to “take reliable photos” of extra-solar planets in the next decade, when devices with new techniques come into operation, like interferometry, capable of producing this kind of image. But the last planets of the solar system, our celestial neighbors, were discovered little by little, slowly. At the beginning of the 17th century, Galileo Galilei became the first man to scrutinize the skies using the lenses of a telescope.
With the help of this artifact, the Tuscan astronomer and mathematician, whose defense of heliocentrism was to earn him a condemnation in the court of the Holy Inquisition, made countless unprecedented observations. He showed the low brilliance of the stars of the Milky Way, sighted sunspots, descried craters on the Moon, found moons on Jupiter, and distinguished the phases of Venus. Planets, he found none. Accordingly, until the end of the 18th century, humanity reckoned, besides the Earth, five worlds, all in orbit around the Sum and occasionally visible to the naked eye: Mercury, Venus, Mars, Jupiter and Saturn. Officially, these planets do not have any discoverers. Recording them blends into the history of civilization. New worlds were only identified over one century after Galileo, as telescopes became more powerful. And at the pace of one planet per century. Uranus was discovered in 1781; Neptune, in 1846; and Pluto, in 1930.
Besides nurturing the hope of locating new Earths around other stars, the discovery of over a hundred extra-solar Jupiters, and in a way even of the three very new worlds of a medium size, is challenging the most accepted theory about the birth of planets, formulated after the configuration of the solar system. These new worlds seem to be out of place. The majority is located absurdly close to their sun and shows an elliptic, non-circular orbit. Everything different from the gaseous giant planets of the solar system, which are to be found distant from the Sun and show circular orbits. This apparent incoherence led English astrophysicist Martin Beer, from the University of Leicester, to propose recently, in a scientific article, that the solar system may be a “special”, non-typical place in the Universe. If this polemical idea is right, there are no other Earths out there. “Thinking that all planets are formed basically in the same way may be an error”, the Briton speculates. “There may be more than one mechanism for originating these objects.”
According to the most accepted model, planets are formed from a small solid core, a sphere of rock and/or ice, of some 10 kilometers in diameter, called a planetesimal. Rocky cores located at enormous distances from their star manage, by means of their gravitational force, to attract round themselves large quantities of gases coming from the cold sectors of a vast disk of matter existing in the environs of their sun. This is how, distant from their star, giant gaseous planets are always formed, like Jupiter and Saturn.
“Almost the whole of the Universe is made up of hydrogen and helium at very low temperatures”, comments astrophysicist Gustavo Mello, from the Valongo Observatory, from the Federal University of Rio de Janeiro (UFRJ). But the planetesimals closest to their star, also according to the model, are capable of originating only rocky planets. Very hot, they do not manage to attract nor to maintain a covering of gas around them for very long. The result is that they originate smaller and denser bodies, like Mercury, Venus, Earth and Mars (distant Pluto is a case apart). “The formation of a planet is a race against time”, says Gustavo Mello. “The material that originates them, waste from the process of the birth of stars, can easily be dissipated.”
Accordingly, the dominant theory apparently does not serve for explaining the location of almost all the known extra-solar worlds – unless these planets arose from the conventional process at some other point in space, far from their star, and migrated to their current position. Or it may be that the known extra-solar systems are simply not representative of the major part of the worlds existing around stars. Today, the detection methods favor the observation of large planets that are close to their sun. This may have caused a distorted idea of the profile of the worlds present “out there”.
Beer himself does not discard this possibility, advocated in a more marked manner by the Portuguese astrophysicist Nuno Santos, the discoverer of the Super-Earth. “It is too early to defend changes in the theory”, ponders the researcher from the Lisbon Observatory. “We suspect that the worlds found up until now are a small part of the planets existing. And the majority of them should be similar to those of the solar system.” Honoring his overseas origins, Santos defines what drives a discoverer of planets. “At bottom, what we are doing is giving the Universe new worlds, like the Portuguese gave new worlds to the world in the 16th century. Human beings like to explore, and that is what we are doing”, he philosophizes.
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