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for twenty years, more than 3500 planets, large and small, soft or rocky, hot or iced, hell or paradise potential were detected around stars other than our sun. This exoplanet revolution, today almost commonplace, had previously missed out on Proxima Centauri, yet the closest star to us, distant 4.2 light years “only”. This “red dwarf” pocket sun less bright than ours, was also a companion planet, announced an international consortium of researchers in Nature and other scientific journals.
More exciting still, this extrasolar planet, called Proxima b, is a mass comparable to Earth and travel in an orbit “tempered” , the researchers write, that is ie compatible with the presence in its liquid water surface – which could make another arch of life. All this to our interstellar door!
For several days the news was “percolated” on social networks. In Germany, the Spiegel was partially spilled the beans, raising many questions about the exact magnitude of this discovery. Tuesday, August 23, Nature held a telephone press conference with three of the authors of the article.
“We can just calculate probabilities “
Cutting edge with enthusiasm and sometimes excessive speculation often required when similar exercises organized by NASA, European researchers have stuck to the facts: ” for this detection, we used a technique called radial velocity “, first indicated Amado Pedro (Institute of astrophysics of Andalusia in Granada). This involves measuring the color variation of the star caused by the motion of the planet around it.
As the siren of a fire truck, more acute when it approaches, more serious as it moves away, these movements altered periodically perception of the light wavelength emitted by Proxima Centauri. There are several years “Harps measurements performed by an instrument of the European Southern Observatory (ESO) installed at La Silla, Chile, had delivered clues, but could be other explanations” , said the researcher.
” It there had been unfounded ads and we had to be careful , continues his colleague Guillem Anglada-Escudé (Queen Mary University of London). You had to check the frequency of the signal, with an intensive campaign measures on sixty days, with thirty minutes of daily observation. When combined the two datasets, the statistical representativeness is skyrocketing, and it was determined a period of revolution around the red dwarf of 11.2 days. “
It was possible to calculate the minimum mass of the exoplanet, about 1.3 that of Earth. “One can deduce the distance to its star, Complete Ansgar Reiners (Astrophysics Institute of Göttingen) and the temperature on its surface if it has no atmosphere – 30 ° C. “ Chilling! “Rest assured, if the Earth had no atmosphere, it also would have an average surface temperature of – 20 ° C” , immediately adds the researcher. It is indeed this protective atmosphere that captures solar radiation through the greenhouse effect. On Earth, it is still moderate; Venus, closer to the Sun, it is packed to become hellish.
What is, Proxima it b? “We know nothing of the atmosphere and the presence or absence of water , Ansgar Reiners slice. We just can calculate probabilities. “ As it is probably rocky, not gas, ” there is a nonzero probability that this planet has an atmosphere, “. If c’était the case “surface temperature could range from – 30 ° C to + 30 ° C, depending on whether one is on the side or dark side clear” .
for this is one of the supposed characteristics of Proxima b twenty times closer to its star than the Earth is to the Sun, it would have been strictly controlled him over time due to forces . ‘attraction Two options are available: either it has the red dwarf always the same face, like the Moon from the Earth, or it has entered into “orbital resonance 3: 2 “, as Mercury, which rotates three itself into two orbits around the sun.
both scenarios were explored by a French team to try to determine what types of climates can Proxima rule b. They are described in the journal Astronomy and Astrophysics . “We used terrestrial climate models, first adapted to Mars or Venus, and now for exoplanets” , says Martin Turbet which a thesis on the subject under the direction of François Forget the dynamic meteorology laboratory (CNRS-UPMC).
If Proxima b originally had a vast reservoir of water, comparable to terrestrial oceans, “there is a good chance that it remains of the liquid water on its surface “. If this tank was more limited, this water may have migrated to the polar cold spots. Everything depends on the presence or absence of an atmosphere capable of generating a greenhouse that would allow some of that ice to melt, in which case, failing oceans, Proxima b could be traversed by rivers and dotted with lakes . Otherwise, “Water will be formed gigantic glaciers at the poles and it will not be habitable” , says Martin Turbet.
the scaffolding of assumptions is endless: other articles examine the possibility that during its formation, b Proxima is willing to water as ice. Which depends on its initial position relative to its star, whose brightness determines a “line of ice” below which the latter is vaporized and can aggregate to form the planets.
it will be difficult to ensure that Proxima directly b still has an atmosphere, if it has ever been with. One detection method is to observe changes in the light spectrum of the star when its reach through the putative atmosphere of his companion. But to do this would require a transit situation, the Earth, Proxima b and its star are aligned. “The radius of the star is so low that the geometrical probability of such transit is only 1.5%” , laments the astronomer Jean Schneider, who maintains the the most comprehensive catalog of exoplanets.
If all conditions are met, she has life had a chance to develop it? One of the characteristics of red dwarfs is to produce very strong radiation, which is harmful to the maintenance of an atmosphere, but also to the emergence of life. It is unclear whether Proxima b has a protective magnetic field. But it is estimated that X-rays are 250 times stronger on the surface of Proxima b than on Earth, and extreme ultraviolet fifteen times more virulent, which is in principle not good for assembling the building blocks of life. “There are too many uncertainties in the habitability models currently” , Xavier Dumusque tranche (Geneva Observatory), which, in 2012, had also announced in Nature the discovery of an exoplanet around Alpha Centauri B, one of the stars of Proxima companions.
A mirage?
Since this finding has been questioned, although Xavier Dumusque estimated that only new observations will have the heart net. Proxima b could she, too, be only a mirage? “I think it’s a solid detection, which will remain” , says the American astronomer Artie Hatzes, which issued doubts about Alpha Centauri B, and is one of the “reviewers” section Nature , where he posts a comment on the discovery. “I personally do not think that this planet is habitable very , he said. But you never know. “
Researchers already imagine what clues it will try to observe with future instruments like the European giant telescope E-ELT 39 meters in diameter, built in Chile, or Hubble’s successor, the James Webb space telescope to be launched in 2018. it is first touch Proxima b with those eyes. Because, to get there, we must be patient: the light itself takes more than four years to make the trip – against eight minutes from our sun. Probes with current propulsion technologies could hardly get there before 20 000 to 80 000 years, calculated the website Universe Today
Unless the Russian billionaire Yuri Milner succeeds his bet. In April he bet $ 100 million (€ 88 million) on the success of “Breakthrough Starshot” a solar sail propelled by laser project supported by astrophysicist Stephen Hawking and Mark Zuckerberg, founder of Facebook. If all goes as he wishes, the first Alpha system overview images Centauri and Proxima b might be received in 2061 …
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