On May 2, a team of astronomers from the Massachusetts Institute of Technology (MIT) announced an unusual finding in the journal Nature. They had detected three Earth-like exoplanets around an ultracold dwarf known as TRAPPIST-1 – in the image on the left – a star slightly larger than Jupiter whose brightness is only 0.05% that of the Sun. they had hunted such objects accompanying such a small star. And these are very close, 40 light years from us. Moreover, one seems to be in the habitable zone of the system, a region in which the necessary conditions exist for water to exist in a liquid state on its surface. Perhaps it could even harbor life.
These extrasolar worlds are part of the more than 3,400 that have been found since 1992, when the existence of the first was confirmed. To find them, astronomers use various techniques that, so to speak, are based on a single fact: lone stars do not move like those with a cohort of planets around them . Until 2014, the most widely used method was radial velocity. In essence, this consists of decomposing the light of the star and studying possible variations in its spectrum induced by the presence of a planet. These discrepancies are very small, but they can be detected with spectrometers such as the HARPS, installed in the 3.6-meter telescope of the La Silla Observatory, in Chile, or the HIRES of the twin Keck telescopes, of ten meters, in Hawaii.
Currently, the strategy most used by experts in planetary sciences who are dedicated to hunting extrasolar worlds is the so-called transit or photometric method. This, precisely, was what the MIT astronomers used to discover the three possible earths that orbit TRAPPIST-1. It consists of observing the small decreases that occur in the brightness of a star when a planet passes in front of the disk . To do this, the orbit that the object follows must be in our line of sight of the star. Thus, if we were to observe the solar system from above, for example, we could never discern the transit of the planet in front of its sun.
Ultracool stars, such as the one in the aforementioned MIT study, are not usually a common target in the search for exoplanets. Rather, it focuses on the largest, hottest and brightest, perfect targets for a tool that, in this matter, has become indispensable in recent years: the Kepler space telescope. This probe, launched by NASA in 2009, was designed to investigate 100,000 stars located about 5,000 light years away. In fact, you are able to observe many of them at once precisely because they are bright; if they were ultra-cold, their low luminosity would only allow them to be studied one by one , something that, of course, scientists do not like very much, since it implies a huge investment in time and resources. It is said among astronomers that searching for planets in ultracool stars is like taking a shot in the dark trying to hit the target.
The fact is that the MIT experts decided to take a risk and, with the patience of a mother and the devotion of a monk, they began to track these faint stars. Now, they introduced a subtle but very important condition in their search: they had to be close to us. And that’s how they found those three Earth-like planets within a cosmic stone’s throw.
A week after this finding was made public, Ellen Stofan, NASA’s chief scientist, announced at the headquarters of this agency in Washington that, based on the data provided by the Kepler telescope, the existence of 1,284 worlds had been verified. more, twice as many as had previously located this same probe. The probability that these objects actually exist is 99%, according to astronomers. What’s more, NASA officials believe that 550 of them could be rocky – like Mercury, Venus, Earth and Mars – and suspect that nine are located in the aforementioned habitable zone . In addition, there are another 1,327 objects pending study, although their presence is not so certain.
The TESS telescope will search for habitable worlds
By now, the Kepler mission can be clearly said to have been a success. Especially because it has allowed to elucidate a question that corroded the minds of scientists: are planetary systems common in the Milky Way or, on the contrary, are they space oddities? “Thanks to Kepler we now know that there are possibly more planets than stars, ” said Paul Hertz, director of NASA’s Astrophysics Division.
The ship will still be active for a while – it has far exceeded its useful life, which in theory was three and a half years, until 2012 – although it already has a replacement. NASA has commissioned the company SpaceX to launch the TESS (Transiting Exoplanet Survey Satellite) probe in 2017, which will use the same photometric method as its predecessor.
One of the drawbacks of Kepler is that little else can be done once it has discovered a planetary system. Most of them are too far away, and many times it is not possible to investigate them in detail. Well, the TESS telescope will allow narrowing down the search. It has been designed to monitor 200,000 stars close to us , especially in search of lands – planets like the one we inhabit – and super-earths – rocky worlds between one and ten times more massive than ours – with orbital periods around two years. In this way, it will provide the targets to which the James Webb space observatory will direct its immense eye, the successor of Hubble, which is scheduled to launch in 2018. The latter will allow, for example, to study the atmospheres of these worlds.
Astronomers investigating extrasolar systems dream of discovering a true extraterrestrial Earth, a rocky planet similar to our own and of a similar size that is in the habitable zone of its star. Now, as the number of known exoplanets increases, a new problem arises: of all of them, on which should we focus our efforts? How do we prioritize?
This is the question that a team of researchers from the Virtual Planetary Laboratory of the University of Washington wanted to answer. Last year, they presented a system that allows exoplanets to be compared and ordered using a parameter that they have dubbed the habitability index for planets in transit or HITE . Actually, it is a tune-up of an old idea.
In 1953, the father of space medicine, the German physiologist Hubertus Strughold, and Harlow Shapley, an American astronomer who succeeded in determining the position of the Sun in our galaxy, independently proposed that the development of life on a planet requires it to exist. liquid water on its surface , which means that it must be at a certain distance from its star.
For this reason, Shapley called this region a belt of liquid water, which corresponds to the aforementioned zone of habitability . But that condition does not guarantee that life will emerge on an exoplanet. “It was a good start, but it makes no distinction about what happens inside the habitable zone,” says astrobiologist Rory Barnes, one of the creators of HITE. Barnes and his collaborators have looked for a way to quantify the possible habitability of a world taking into account many other factors, such as their possible composition and the orbit they follow and, therefore, the amount of energy they receive.
There are stars much more biocompatible than ours
In his opinion, the best candidates to become habitable planets are those that receive between 60% and 90% of the solar radiation that reaches Earth . “Actually, we are near the limit of the habitable zone of the Solar System. If we looked at our planet from another system with current technology, we might conclude that it is too hot to support life, “says Barnes. In fact, if we apply the HITE, there would only be an 82% probability that it would have developed on our planet. “The reliability of our index will increase as we learn more about exoplanets,” says Victoria Meadows, another researcher at the Virtual Planetary Laboratory.
What is clear is that astronomers love to make tables. In 2011, a group of researchers led by astrobiologist Dirk Schulze-Makuch, from Washington State University, presented the Earth Similarity Index (IST), which, as the name suggests, takes into account the resemblance of an exoplanet with ours. The idea is to study its size, density, distance from its star, escape velocity and its surface temperature , among other things. The problem is that of the vast majority of them at the moment we only know their orbital period and, in some cases, their size. We can only speculate about many of the properties that Schulze-Makuch and her colleagues cite, at least for the moment.
Even so, these scientists have tried to derive the index that would correspond to some extrasolar worlds. In this peculiar classification, the reference value would be the Earth, which would be worth 1 . That of Mars would be 0.7; and that of Kepler 438b, an object slightly more massive than our planet located almost 473 light years away, would be 0.88. It is the exoplanet whose existence has been confirmed to have the highest index. The calculation, in any case, is highly speculative. For example, Gliese 581g, about 20 light years away, would have a higher index, 0.89, but we don’t even know if it’s there.
The IST says nothing about the development of life on a planet. To deal with this, Schulze-Makuch together with other astronomers developed a planetary habitability index (IHP). This takes into account whether it has an icy or rocky cover, atmosphere or a magnetic field capable of protecting its surface from harmful cosmic radiation. Also if organic compounds or the necessary solvents are present in it for chemical reactions to take place. In this case, the value for Earth would be 0.96 out of 1. Titan, a moon of Saturn, would follow with 0.64; and Mars, with 0.59. But this account, which can be done very well with objects in the Solar System, slides if we look further . To say that the IHP of exoplanets is speculative is an understatement. Despite everything, Schulze-Makuch and his colleagues estimate that that of the hypothetical Gliese 581g, the best-placed extrasolar world in this ranking, is 0.45. In fact, the Gliese 581 system, chaired by a red dwarf and made up of four or five planets – experts still debate about it – is the one that has the extrasolar worlds with the highest IHP.
The optimism of astrobiologists is contagious. Many believe that with the space telescopes to be launched in the next few years we will be able to fine-tune enough to provide real value to these indices. Furthermore, they may allow the detection of certain biomarkers, such as the presence of chlorophyll, in the atmosphere of exoplanets. It is possible that in a few decades the existence of extraterrestrial life can be confirmed . The future will tell us whether such optimism was founded or not.
Image 1 (ultracold dwarf that presides over the TRAPPIST-1 system): ESO / M. Kornmesser
Image 2 (TESS telescope): NASA
