[LUM#5] Proxima b, the invisible beauty

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

It could support life. Just a few interstellar light-years away, Proxima b is reviving an old dream: the dream of discovering other habitable planets beyond our solar system. But what exactly is a habitable planet?

It isthe exoplanet closest 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 the dimness of this celestial object, which is dwarfed 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 so unknown

The result of nearly a decade of data collection and analysis, its discovery last August sparked excitement far beyond the astrophysics community. Why all the excitement? Because this planet is exceptional for at least two reasons. First, because of its proximity. The aptly named Proxima b orbits Proxima Centauri, the star closest to our solar system, located “only” 4 light-years from Earth: a mere stone’s throw on a cosmic scale.

Next, in terms of its nature, which resembles 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 of the terrestrial type that could potentially harbor life is not new. What is new is the proximity of this planet, which will allow us, within a few years, to directly observe for the first time the interactions between a planet resembling our own and its star,” summarizes Julien Morin, a researcher at the Montpellier Universe and Particles Laboratory and a 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 our own solar system. If that system harbors 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 inferences rather than direct observations. “By studying the star, we were able to measure shifts in the frequency of the light it emits and thus deduce that a planet was orbiting it. We were also able to estimate the planet’s mass,” explains Julien Morin. A mass of about 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 solid ground for humans to set foot on it. The temperature on its surface must fall within a temperate range to allow for the presence of water—and thus life ( The Habitability of Proxima Centauri b, in Astronomy and Astrophysics, 2016).  As for Proxima b, scientists have estimated the distance between the planet and its star, Proxima Centauri, to be 7 million kilometers. This distance is much shorter than that between Earth and the Sun, yet it 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 is 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 shields the planet from the vacuum of space, regulates its temperature, and enables the formation of clouds and rain through the greenhouse effect. “It is currently impossible to know whether or not Proxima b has an atmosphere, a necessary condition for maintaining water 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 an enormous amount of X-rays and ultraviolet radiation, as red dwarfs experience eruptions far more violent than our Sun. Hence the fear that Proxima b’s atmosphere—if it ever existed—may have been completely destroyed by this stellar radiation.

Many uncertainties therefore remain, which the upcoming deployment of a new generation of “super-telescopes” could soon help clarify. The prospect of one day visiting our galactic neighbor, however, seems far more uncertain: with current propulsion technologies, it would take tens of thousands of years to reach this planet, which is relatively close by. An eternity, unless a technological breakthrough comes along to change the game. This is the dream of Russian billionaire Yuri Milner, who aims, through the Starshot project, to develop postage-stamp-sized nanosondes propelled by photonic sails and a laser beam. Although many technical hurdles still need to be overcome, these probes could one day reach unprecedented speeds, on the order of 20% of the speed of light. Proxima Centauri would then become reachable… in about twenty 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