If on August 15 the list of localized extrasolars reached 648, lThe Kepler probe – launched two years ago – has another 1,285 candidates waiting to be studied. Thanks to one of these works carried out with telescopes in New Zealand and Chile, the existence of a new class of planets was confirmed in May 2011. “They are the so-called wandering planets,” says Pérez. “Worlds that can be both gaseous jupiters and rock spheres, that are not bound by gravity to any stars and that have possibly been ejected from their solar systems at some point in their evolution by collisions or disturbances in their orbit, so that they wander now orphans in immense orbits through the cold and dark interstellar medium. “
Telescopes had spent two years focusing on the center of the Milky Way when that May 2011 international study identified a dozen of these lonely planets at a distance of 10,000 to 20,000 light years from Earth. Some astronomers speculate that since their ejection appears to be a fairly common event in the formation of solar systems, there could be twice as many of these wandering worlds as there are stars in the universe. That was an important discovery, but these new objects are not among the priorities of the US space agency. “Herewe are trying to find life“says Pérez, designated by NASA to supervise the investigations in the Large Binocular Telescope or LBT, in Arizona, and the Keck telescope, in Hawaii, among others.” We look for planets with conditions that support life. The wanderers have nothing to illuminate them, since they are worlds whose surfaces are in equilibrium with the background radiation, that is, close to absolute zero (-273.15 ºC). If they have an atmosphere, it would be frozen “, recalls the astrophysicist.? That is why, from the point of view of astrobiology, they are not so interesting. Although it is true that life appears in extreme conditions, it makes more sense that we focus our efforts initials in the most obvious candidates, and once identified, do you have to characterize them?
Habitable planets
Therefore, NASA is busy hunting for planets that are fit for life. What are the conditions that must be met in candidates for exo-Earth? The demands are so many that it would seem impossible to run into one. Australian astrophysicist Jonti Horner wrote in the February issue ofAstrobiology that the first exo-Earth would be discovered within a couple of years. Once this happens, the true search for life beyond our solar system will begin.
For a planet to have the potential to host the kind of life we know, its parent star must meet certain requirements for size, age, luminosity, and stability. The candidate must also have certain characteristics of mass, orbit, distance to the star, moons, continents, tectonic activity, atmospheric pressure and magnetic field. Likewise, it has to be located at a certain distance from its sun to allow the existence of liquid water on its surface, the so-called habitable zone.
The first characteristic that you must meetthe host star is not to be too massive-its mass in relation to that of the Sun? “The largest stars in the universe have masses hundreds of times greater than those of our sun. They are stars that live fast and die young. When this happens, there is no time for life to establish itself,” says Horner inAstrobiology. “In the case of small stars, let’s say 8% solar mass, it doesn’t work either, because then they wouldn’t be able to sustain a hydrogen fusion in their nucleus.”
Regarding your age,If the sun is too young – a few hundred million years – the opportunity for life in its bosom has probably not yet arisen. Therefore, we must look for stars that are in the main sequence stage, in the prime of their lives, with a minimum of one billion years of age, which is when their luminosity increases. For example, in its youthful stage, the Sun barely had 70% of its present luminosity, and that changes the extension and location of the habitable zone. “For the planets within this area to have had time to host a biosphere, they would need to have spent hundreds of millions of years under an appropriate and stable luminosity,” Pérez recalls. For this, good candidates would be the M dwarf stars, as they are the most common in the cosmos and have the longest main sequences, with a constant luminosity. Likewise, binary systems – two astronomical objects linked by their gravitational force – should not be ruled out in the search, because, so far, they have yielded a quarter of the discovered exoplanets, whose orbits appear to have remained stable for a long time. Until recently, an exosolar system with a world the size of Jovian and beyond the habitable zone was thought to be a good sign, because the planet itself would defend it from collisions with dangerous objects. “But now we know that this is not the case,” writes Horner. “In fact, giant planets can be a double-edged sword, as they make the little ones attract each other.”
Moons and seas
On the other hand, would the habitability of a planet increase if it had a moon? The satellite is not only responsible for the tides, but also stabilizes the inclination of the axis of rotation. For example, Mars has angle variations of up to 60 degrees latitude, which if it were to happen on Earth, the Arctic Circle would pass over Cairo. Thepresence of liquid water, which must also be complemented with the existence of terrestrial continents is another important factor for an extrasolar planet to be habitable. “Would it be an exo-Earth with oceans hundreds of kilometers deep and no land mass?
It has been suggested thatthe presence of the continents also plays an important role in the development of life, since, as the rocks degrade, they provide minerals and metals that would be diluted in the sea “, points out Mario Pérez. Another fundamental characteristic for an aspiring exo-Earth is theatmospheric pressure, which guarantees the presence of liquid water on the surface and prevents it from evaporating. Also important is tectonic activity, which returns the sequestered carbon to the atmosphere, although it has often been suggested that without water there would be no such activity, since the liquid acts as a lubricant for the continental plates.
Furthermore, without a tectonic dynamics, a magnetic field would not appear, and then there would be no atmosphere protected from the ultraviolet radiation sent by the host star, which was what happened to Mars when the Sun was young. “If, when life appeared on Earth, the planet’s atmosphere had been the same as it is today, the planet would have frozen, because then the sun shone less”, sums up the astrophysicist. “And if the Earth today had the same atmosphere as billions of years ago, the greenhouse effect would be so severe that the water in the seas would have vaporized long ago.”
And theoxygen? “It is not, actually, a good detector,” confirms Pérez. “An alien observing Earth today will be able to detect oxygen emissions in the atmosphere, but if it had done so at the beginning of its evolution, it would not have seen a trace of oxygen. It could have encountered methane or carbon dioxide. Depending on the age of the star we observe, the exobiological rates of detection of life are different. So the solution is not unique. “
Like a lens
Nor are exoplanet detection methods unique. “The most popular is radial velocity, which is similar to Doppler. Of the 648 known planets, 429 have been discovered in this way,” explains Pérez. And he adds: “With the transit photometry method, some 139 worlds have been found. It is effective if the planet passes in front of its star, since it causes a small drop in the luminosity of the star. The newer gravitational microlensing is responsible. to find the wandering planets. It works when the celestial body produces a lens effect and increases the brightness of the distant star in the background. The fourth method is that of direct images, a system that has discovered 24 exoplanets “, concludes the expert.
Pérez is convinced that we will find life on an exo-Earth, based on carbon and silicon, or why not, on arsenic. After all, they are the same elements throughout the universe, but rearranged in different ways. The astrophysicist checks his smartphone again. The meter still indicates 648? “But not for much longer”.
Angela Posada-Swafford
