[LUM#5] Proxima b, the invisible beauty

[LUM Magazine] - Published on March 7, 2017 in Science-Society

It could harbor life. Just a few light-years away, Proxima b revives an old dream: that of discovering other habitable planets outside our solar system. But what exactly is a habitable planet?

It isthe closestexoplanet to Earth. As is the case with most of its counterparts—there are now more than 3,000 known exoplanets—no one has ever observed Proxima b. The distance and lack of brightness of this celestial object, drowned out by the light of its star, make direct observation impossible. A planet invisible to the naked eye, yet one that shines with a special brilliance for astrophysicists...

So close yet unknown

The result of ten years of data compilation and analysis, its discovery last August caused excitement far beyond the astrophysics community. Why such enthusiasm? Because this planet is exceptional for at least two reasons. First, its proximity. Proxima b, aptly named, orbits Proxima Centauri, the closest star to our solar system, located "only" 4 light-years from Earth: a mere hop, skip, and jump on the scale of the universe.

Secondly, its nature, reminiscent of that of Earth: a likely rocky composition and a mass roughly equivalent to that of our planet (Proxima b, a potentially habitable rocky exoplanet, UM website, 2016). "The discovery of extrasolar planets¹ that could potentially support life is nothing new. What is new is the proximity of this planet, which in a few years' time will enable us to observe for the first time the interactions between a planet similar to ours and its star," summarizes Julien Morin, a researcher at the Montpellier Universe and Particles Laboratory and member of the international team behind this historic discovery ( A terrestrial planet candidate in a temperate orbit around Proxima Centauri, in Nature, 2016).

A dream playground for astrophysicists, and a source of hope: "This discovery concerns the system closest to the solar system. If it is home to a planet of this type, then we can reasonably assume that many others exist around us," notes the researcher.

Habitable zone

A habitable planet just a stone's throw away from Earth? Nothing could be less certain. To qualify as a habitable planet, Proxima b would have to meet a series of criteria that are currently unverifiable. As is often the case in astrophysics, the information gathered about Proxima b is based on deductions rather than direct observations. "By studying the star, we were able to measure shifts in the frequencies of the light it emits and thus deduce that a planet was orbiting it. It was also possible to estimate the planet's mass," explains Julien Morin. A mass approximately 1.3 times that of Earth suggests a terrestrial composition, making the presence of life conceivable.

However, it is not enough for a planet to have land for humans to set foot on it. The temperature on its surface must be within a temperate range to allow for the presence of water, and therefore life ( The habitability of Proxima Centauri b, in Astronomy and Astrophysics, 2016).  Scientists have estimated that Proxima b is 7 million kilometers from its star, Proxima Centauri. This is much closer than the distance between Earth and the Sun, but it still places Proxima b within the so-called "habitable zone." Why? Because its star, a red dwarf, is seven times smaller and a thousand times less luminous than our sun. It therefore seems possible that the temperature on the planet's surface could be such that water could flow there in liquid form.

Atmosphere, atmosphere...

One crucial question remains: whether or not there is an atmosphere, that gaseous envelope that insulates the planet from the vacuum of space, regulates its temperature, and makes it possible for clouds and rain to form thanks to the greenhouse effect. "It is currently impossible to know whether or not Proxima b has an atmosphere, which is necessary for water to remain in a liquid state , " explains Julien Morin. The short distance between Proxima b and its star poses a problem here: due to their proximity, the planet is likely bombarded by enormous amounts of X-rays and ultraviolet rays, as red dwarfs experience much more violent eruptions than our sun. Hence the fear that Proxima b's atmosphere—if it ever existed—may have been destroyed by this stellar radiation.

Many questions remain unanswered, but the upcoming launch of a new generation of "super telescopes" could soon shed some light on the matter. The prospect of one day visiting our galactic neighbor seems much more uncertain: with current propulsion technologies, it would take tens of thousands of years to reach this relatively nearby planet. An eternity, unless a technological breakthrough changes the game. This is the dream of Russian billionaire Yuri Milner, who aims to develop postage stamp-sized nanosondes powered by photonic sails and laser beams as part of the Starshot project. Although many technical obstacles still need to be overcome, these probes could one day reach unprecedented speeds of around 20% of the speed of light. Proxima Centauri would then become reachable... in about 20 years! So now more than ever, we can dream.  

1. An exoplanet, or extrasolar planet, is a celestial body similar to a planet that orbits a star other than the Sun.
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Keywords: Space, LUM#5, LUM Magazine, Research, Science