For about twenty years, almost 3500 exo-planets have been detected around stars other than our sun and until now, these discoveries had "forgotten" Proxima Centauri, the nearest star to our sun.
The news reaches the editorial staffs worldwide on 24 August 2016: Proxima has a planet that can support life !
"Awesome , fantastic, Christmas time !... when we go there? "
Wow, calm down people:
Guillem Anglada-Escudé (from London Queen-Mary University) is moreover careful.: "We can only calculate probabilities."
And a recent article on the NASA website said:
"(...) Some involve a completely dry planet, while others permit the presence of a significant amount of water in its composition. .(...)"
(Source: https://exoplanets.nasa.gov/news/1395/s ... habitable/ )
It may be noted in the article a lot of "if", and that the use of the conditional is common (just like this topic by the way ). That is to say that we have no absolute certainty for now about the viability of this planet, as long that further observations will not be made.
But what do we know exactly?
Proxima b rotates around its star and appears likely rocky, like our Earth, its mass being 1.3 greater than our planet. Finally, it is 7 million kms from Proxima Centauri, so 20 times closer than Earth is to the sun. But its star, a simple red dwarf, is much less brilliant than ours. Therefore, Proxima b appears within the habitable zone, and its temperature would allow the presence of liquid water on its surface.
The research teams, French for the greater part, studied the probability of habitability of this planet, so the possibility that it contains water in its liquid form on its surface. Studies show that if Proxima b is actually located in the habitable zone of Proxima Centauri, where oceans may exist on the surface, this has not always been the case.
Indeed, early in its history, this star was much brighter and therefore, the water could not exist in a liquid state on the surface of the planet, water being sprayed in a thick atmosphere exposed to the energy radiations of its star. The researchers studied the characteristics of this radiations, the planet now receives 250 times more X-rays and 15 times more extreme UV than Earth.
This strong irradiation was able to break the water molecules and highly heat the upper atmosphere of the planet to generate a significant loss of water and gas into space. Nevertheless , the planet could now possess a thick atmosphere and oceans.
All depends on the amount of water and gas which it inherited when it was formed.
Many simulations were conducted to explore the amounts of water and carbon dioxide present, and demonstrated that according to the rotation period and the amount of greenhouse gas, liquid water could be present on the entire surface of the planet or only in the most sunny regions, but If the planet turns out poor in water, it is likely that this one condenses and accumulates in the coldest regions. However, the atmosphere and the greenhouse effect could allow the melting of ice water and the creation of lakes and rivers in these areas.
But everything also depends on the rotation of the planet, which could constantly present a face at night because blocked by gravitational forces, a typical characteristic of planets orbiting in the habitable zones of red dwarves (see below).
Furthermore, the "habitable" zone of a star does not necessarily mean that a planet located within this area is hospitable to humans: Mars is considered part of the said zone but I strongly discourage anyone to go there without a pressurized suit.
Other factors could be taken into account: First of all, here is a schematic representation of the Alpha Centauri system:
*Note: Picture taken from http://physiquereussite.fr *
Yes Alpha Centauri is a sytem containing 3 stars:
Alpha Centauri A – A yellow star, similar to our and slightly bigger
Alpha Centauri B – An orange dwarf
Orange dwarvess presenting interresting particularities:
1) They have a very long life which give ample time for complex life to develop.
2) They are much less effusive than red dwarves.
3) A planet in the habitable zone of an orange dwarf will not be locked in its orbit.
And finally :
Alpha Centauri C or Proxima – The one that interests us and therefore a red dwarf.
1) They constitute 70 to 80 % of stars in our galaxy
2) The habitable zone is much closer to the star and when a planet orbiting a star too much closely, the gravitational attraction pull of this one can force the planet to adopt a synchronous rotation with it. This means that it always presents the same face to its star, like our moon does to Earth. They have thus a permanent day and night side.This exposure scenario for permanent light could lead to a kind of world split in two. Its "night" side would be covered with a shell of ice, while its "day" side host a desert constantly exposed to the heat of its star.
Here's an artist's view:
*Note: Picture taken from http://www.gurumed.org*
So, life and its evolution could be very difficult around this kind of stars.
And nevertheless, the discovery of Proxima b would make this last information lie. Indeed, who tells us that the two other stars does not influence the behavior of this planet?
But in all cases, and although it is an excellent candidate of a planet convenient to liquid water and may harbor life, Proxima b can not be considered as an Earth twin, contrarily to what some people says. The history of Proxima b and its star was radically different from the history of the Earth and the sun. The formation of Proxima b, its irradiation by its star, the tidal forces it is subjected and which affect its internal structure and its rotation, have no equivalent in the history of our planet. But we'll know more when E-ELT (http://www.eso.org/public/unitedkingdom ... -elt/?lang), in construction in Chili, and SPIRou (http://spirou.irap.omp.eu/) will be activated, the SPIRou beeing planned for 2017.
http://www.ice.cat/personal/iribas/Prox ... ity_II.pdf
Who made the discovery itself:
- ESO (European Southern Observatory : http://www.eso.org/public/unitedkingdom/?lang)
- Michel Mayor and Didier Queloz from Geneva University
- Geneva University
- Grenada Astrophysics Institute
- CNRS/INSU (French National Scientific Research Center)
- Bordeaux Astrophysics Laboratory
- Paris Dynamic Meteorology Laboratory
- Marseille Astrophysics Laboratory