Author: Prof. Rory Barnes, University of Washington
The discovery of Proxima b is the biggest event egzoplanetarne since the discovery of exoplanets. Proxima b is slightly larger than Earth and is located in the ecosphere nearest stellar neighbor sun. The planet may be the best opportunity for humanity to search for life among the stars. Are Proxima b created, but life? Is it live? The answer to these questions is not currently possible, because we know very little about the planet. However, we can extrapolate the data from the worlds of our solar system and apply the theoretical models of the evolution of galactic, stellar and planetary to create realistic scenarios history Proxima b. The possibilities are varied and depend on the phenomena usually studied by scientists working in areas considered to be separate but integrated perspective – perspective astrobiologiczna – can provide a realistic assessment of the possibility of the emergence and survival of life on the nearest we ezgoplanecie.
As an astrobiologist and astronomer at the University of Washington and a member of NASA’s Virtual Plenatary Lab years researching the possibility of life on planets orbiting red dwarfs. My research include the development of computer models to simulate the evolution of the planets and their atmospheres, and how over time change the stars and how they differ from each other planetary orbits. The discovery of Proxima b is very exciting for me, but that is about the size of Earth and is located in the ecosphere is only the first two conditions necessary to ensure that the planet could support life, and the list is much longer for planets orbiting red dwarfs, than stars like our sun. If Proxima b really is in the ecosphere, which is the liquid water, and perhaps it is settled, it had to go a completely different evolutionary path than Earth. This difference is frustrating because the first interpretations will be a challenge for us, and at the same time fascinating, because it gives the opportunity to see how they evolve in the universe of Earth-sized planets. Regardless of whether Proxima b is a barren wasteland, or life on it blooms, we begin just unprecedented phase of discovery, which may provide the answer to the long-asked question: Are we alone?
To assess the possibility of life on Proxima b we have to start from the only inhabited planet as we know it is from Earth. Life on Earth is present in an extremely wide variety of environments, such as in thermal springs, and in most ocean depths, at microscopic tubules in sea ice, or in the deepest levels of the earth’s crust. No matter how extreme the environment, the whole life on Earth requires three basic components: energy, nutrients and water in a liquid state. The first two components are present in the universe very well as water molecule. A limiting factor in terms of astrophysics is that the water must be in liquid form. Ekosfera is a map of the area where water could exist on the surface of the rocky, Earth-like planets, hence its status as the first condition that a planet must meet to ensure that there life on it. Life also needs enough time for this to occur and evolve, and on Earth it is resistant to disasters as trivial or as traumatic as a storm. Diversity and persistence of life formed on Earth, astrobiologists are encouraged to imagine that life could exist not only egzoplanatach similar to Earth, but also in strange, exotic worlds.
So what are we to make of Proxima b? It is at least as massive as Earth, and may be several times more massive. Its “year” lasts a little over 11 days, and its orbit may be circular or significantly delayed. Its native star has only 12% of the mass of the Sun, 0.1% of its brightness, and we know about it, it shines. It may be linked gravitationally with the stars Alpha Centauri A and B, which are within the 15-000 astronomical units. All three contain far more heavy elements than the Sun, however, we know very little about the composition of Proxima b, or about how it came. The new data indicate the presence of another planet in this system, which flow around the star takes nearly 200 days, but its existence can not be confirmed yet. These facts we know and based on them we must deduce whether Proxima b life.
Proxima b was discovered by measuring angular velocity, which do not involve direct calculations of its mass, and only provide information on the minimum weight . So the first question that we want to answer is: Is the mass of the planet is small enough to be a rocky planet like Earth? If the planet is much bigger, it may be more like Neptune with a thick gas layer. We do not know where is the dividing line between the rocky exoplanets and gas, but the models of planet formation and analysis of the planets discovered by the Kepler mission suggests that the dividing line is between the 5 and 10 times the mass of Earth. Only about 5% of possible orbits Proxima b puts it over 5 masses of the Earth, so it is very likely that the planet belongs to the rock.
Another question we must ask ourselves is whether the planet was formed together with water. Water is composed of hydrogen and oxygen, the first and the third element of the most popular in our galaxy, so we should expect it everywhere. However, in a small distance from the star, which is where there is Proxima b, water is heated to a gaseous state during the formation of the planet, so it is difficult for the water to stop. The planets formed at greater distances can collect more water, so if Proxima b formed farther from the star, and later moved to its current orbit, there is a higher probability that is rich in water. At the moment we do not know how the planet was formed, but it can seem to be three scenarios: 1) the planet was formed where it now is, mainly from the local material; 2) The planet formed farther from the star, when the disk of gas and dust from which a planetary system still existed, and the forces of the drive led the planet to its current orbit; or 3) the planet was created in a different place, but kind of instability covering the whole system poprzesuwała planet, and b Proxima was on its present orbit. The first scenario is the one by which created the Earth and Venus, so Proxima b may, but need not have significant water resources, if established in this way. The result of the second scenario are the planet rich in water, because there is a greater likelihood that water will be able to found frozen in a huge distance from the star, so the resulting planet could easily be picked up. The third scenario is not conclusive, because the planet could move from the inner orbit, where established without water, or was farther from the star, and has water, but we can not be sure.
Now, let’s guidance coming from the stars . Computer models of the evolution of our galaxy suggest that stars enriched with heavy elements, such as Proxima which can not occur locally (25 000 light-years from the galactic center), because there is a sufficiently large number of available heavy particles. However, closer to the center of the galaxy, where star formation is more vigorous and lasts a long time, the formation of stars, such as Proxima possible. A recent study by Dr. Sarah Loebman and his team showed that a star located in our galactic neighborhood of the composition such as Proxima, have created at least of 10 000 light-years closer to the galactic center. Apparently, Proxima Centauri wandered the galaxy, and its history could play an important role in the evolution of Proxima b.
The orbit of Proxima orbiting Alpha Centauri A and B, assuming that are linked together by gravity, it is large compared to other multiple systems. Moreover, it is so large that the impact of the stars A and B on the Proxima is weak, but it is the interaction of the Milky Way largely shaped orbit the star. The whole mass of the Milky Way means that the orbit of Proxima constantly changing both the shape and orientation. Proxima is also susceptible to gravitational encounters with the shifting nearby stars, which may affect its orbit. Simulations made recently by prof. Nate Kaiba showed that these two effects often lead to close encounters of stars with multiple systems that interfere with their planetary systems. Disorders are often so strong that it can cause the ejection of planets from the system and completely change the orbits of the other planets. New simulations, conducted by Russell Deitrick show that this scenario raises serious concerns also for Proxima; there is a high probability that in the past Proxima flew so close to Alpha Centauri A and B, that the planetary system fell apart, throwing siblings Proxima b space. If the disturbance occurred, Proxima b could not arise where we see it today, because its orbit would it hurt.
Even if Proxima is not currently associated with Alpha Centauri A and B, it seems that They move with them and it is very likely that these stars formed from the same disk of gas and dust. If you created together, they should have a similar composition and be almost the same age. The reference to the age of Alpha Centauri A and B is so important that it is difficult to tell the age of low-mass stars, such as Proxima Centauri. Astronomers can estimate the age of Alpha Centauri A through astrosejsmologii, the doctrine of “earthquakes stars.” The stars bigger than the sun pulsate with a large enough frequency that can be observed brightness variations, and their detailed monitoring can reveal the age of the star. Research conducted recently by Dr. Michael Bazota proved that Alpha Centauri A is between 3.5 and 6 billion years. This broader scope than we would like, but Proxima is certainly enough age that could host life, and Proxima b can even be the same age as the Earth!
Now look at the guidance supplied by the solar system Proxima Centauri. The vast majority of the energy needed for life on Earth comes from our sun, and small stars, such as Proxima, can produce energy even for billions of years. Their size is almost the minimum size of the star, so that Proxima b able to receive as much solar energy as Earth, must be about 25 times closer to its star than the Earth is the sun. This distance is Ekosfera. Proxima is much darker than the sun, but it remains a series of thermonuclear reactions, so, if all other factors remain the same, the existence of life seems to be more likely in larger distances from the star. Close orbits create many obstacles, which did not have to overcome life on Earth. These include a long period of formation of the star, the short and energy stings energy in the form of UV radiation and X-strong magnetic field, the larger spots star, more coronal mass ejections or tidal forces resulting from gravity, which causes the properties of rotational change, and the oceans (if any), and the rocks are subject to warming by friction.
the history of the evolution of the brightness of Proxima was slow and complicated. All models of stellar evolution predict that the first billion years of Proxima slowly losing its brightness until the state in which there is today, which implies that for about the first quarter of a billion years the surface of Proxima b was too hot to achieve conditions similar to Earth . As recently showed Rodrigo Luger, if our present Earth found itself in such a situation, he would become the world like Venus, with raging greenhouse effect that could destroy the entire planet located on the water. Such drying is possible because the molecular bonds between hydrogen and oxygen in water can be destroyed in the upper part of the atmosphere by radiation coming from the star, and the hydrogen, being the lightest element, can escape gravity of the planet. Without hydrogen water can not exist, and the planet unsuitable for life. Escape or avoid the uncontrollable global warming was the biggest challenge in the chance Proxima b for the creation of life.
When a star loses its brightness, the process of destruction of water molecules is stopped, so it is dry completely avoidable. If some water is, the atmosphere may also contain high amounts of oxygen remaining after destruction of steam. Large amounts of water and oxygen may seem like a good recipe for life, but almost certainly it is not. Oxygen is one of the most reactive elements, and its presence in the atmosphere of a young Proxima b may be prebiotic would stop the development of particles that create the conditions require a small amount of oxygen. Life on Earth was created when oxygen was absent, photosynthesis and ultimately created a large enough amount thereof to become an important component of our atmosphere. Note that the destruction of only a certain amount of water leads to a rather surprising possibility that the planet could have oceans and atmosphere rich in oxygen, but not been able to sustain life!
Another intriguing possibility is that Proxima b began as a a planet like Neptune, but the initial brightness of the star and its flares destroyed the atmosphere rich in hydrogen, to discover hospitable to life Proxima located below. Such a world studied Rodrigo Luger, I and other scientists and we agreed that this is the real way to avoid dry completely. First of all, the atmosphere of hydrogen protects the water. If you created Proxima b 0.1-1% by weight would be a shell of hydrogen, the planet would lose hydrogen, but not water, potentially becoming open to the world of life, when her star has reached its current brightness.
The wide range of possible paths evolutionary poses daunting challenges for the future terrestrial and space telescopes that will search for life in the atmosphere of Proxima b. Fortunately, my colleagues in the Virtual Planetary Lab, prof. Victoria Meadows, Giada Arney and Edward Schwieterman working on techniques to distinguish the possible states of the atmosphere Proxima b, do not worry for now the existence of a life on it, or not. Almost all the elements of the atmosphere are visible in the spectrum, so our knowledge of the possible histories of the planet, we can start to create instruments and plan observations that indicate important differences. For example, when a sufficiently high pressure oxygen molecules may momentarily bind to each other and create an observable property of the spectrum. Most importantly, the pressure required by the observer, so that it can be detected is large enough to allow distinction between the planet with too much oxygen and the amount of the corresponding life. To learn more about the planet and the system, we can create a library including possible spectra by means of which quantitatively we will be able to determine how likely is the existence of life on Proxima b.
The initial brightness of the star around which orbits the planet, it is the biggest obstacle for life, but other issues are also important. One of the obstacles that were initially thought to be to prevent life on planets orbiting red dwarfs was the possibility of synchronous rotation, meaning that one of the hemispheres throughout the life cycle facing the star. This happens with our Moon, the same tidal forces are raising the oceans made the Moon shows only one half of your Earth. Due to the fact that Proxima b orbits so close to its star, it may be in this situation, and it depends on the shape of its orbit. For decades, astronomers believed that the planet in synchronous rotation would be deprived of life, because they thought that the atmosphere above the eternally dark hemisphere freezes and falls to the surface. Currently, this is considered very possible, because the winds in the atmosphere transmit energy on the planet and support on the side of the dark heat sufficient to ensure that the atmosphere is not freezing. So if it comes to the stability of atmospheric rotation synchronous is not an obstacle for the existence of life on the planet.
Although the rotation synchronous is not very dangerous for life, it is possible that tidal forces provide large amounts of energy into the atmosphere and the Earth’s interior . This energy is often called the “tidal heating” and results from the deformation of the planet due to changes in the gravitational force of the star, which acts on the planet across its diameter. For example, if the planet has an elliptical orbit, being closer to the star feels more gravity than when it is away from it. This difference will cause the shape of the planet change, and such deformation can cause friction between the inner layers of the planet, producing heat. In extreme cases, tidal heating may initiate unrestrained greenhouse effect like the one that dried up Venus, regardless of the light from the star. It is unlikely that Proxima b was in this state, but the tidal heating may still be there very strong, resulting in a continuous volcanic eruptions, such as Jupiter’s moon, Io, and / or raising huge ocean waves. Based on the information we have, we are not able to determine the size of the heating tide, but we must be aware of it and explore its implications.
A brief, high-energy bursts of Proxima Centauri, called flares are considered to be the impediment to the emergence of life on the surface a planet orbiting a red dwarf. Flares and eruptions of small areas on the surface of the stars can cause short (a few hours or a few days) increase in brightness. The problem is that the bursts emit gusts of positively charged protons, and as shown by prof. Antigona Segura and his team, they reduce the ozone layer, which can protect life from harmful, high-energy UV radiation. Flares on Proxima occur much more frequently than the Sun, and because Proxima b is much closer to its star than the Earth is likely to be subjected to repeated bombing. If the atmosphere could develop a solid shield that protects against these eruptions, such as the strong magnetic field, then the flares could not have life meaning. The same would be the case if there were a few meters force under water. Then flares should not be fatal to life on Proxima b.
Worried about flares naturally leads to the question of whether the planet ever has to protect her magnetic field like Earth. Over the years, many scientists thought that the emergence of such magnetic fields is not possible for planets similar to Proxima b, because the rotation is synchronous to impossible. It was believed that the magnetic field is generated by the electric current running in the core of the planet, and the movement of charged particles required to produce this current is caused by the rotation of the planet. Slowly rotating planet may not be able to transport charged particles fast enough to generate a magnetic field that can push flares and make the planet’s ecosphere red dwarfs will be able to maintain the atmosphere. However, more recent studies show that magnetic fields of the planets are not really maintained by convection, that is, the process by which hot material inside the kernel rises, cools and sinks again. The rotational movement helps, but Dr. Peter Driscoll and I recently calculated that convection is quite sufficient to maintain a strong magnetic field on the planet with the rotation synchronous and tidal heating for billions of years. It is therefore entirely possible that Proxima b has a strong magnetic field and can fend off flares.
Is Proxima b and therefore able to sustain life? The short answer is: “It’s complicated.” We have not made many observations, but what we know allows for a staggering number of possibilities. Do Proxima b traveled half the galaxy? It does survived instability covering the whole planetary system, which threw her siblings into space and changed its orbit? How he coped with the initial high brightness of its star? What is it made? Do began as a planet like Neptune, and then conformed to the Earth? We are constantly bombarded with the flares and coronal mass ejections? It is tidally heated to a state resembling (or worse than) the condition of Io? These are important questions that must be answered to determine the potential ability of Proxima b to sustain life and determine whether our nearest galactic neighbor is a forbidding wasteland inhabited planet or a future home for mankind.
The last question is not so rhetorical as it might seem. As life needs an energy source, it is possible, in the long run – and I mean dłuuugą run – to planets such as Proxima b become ideal places to live. Our sun will burn as little as 4 billion years, and Proxima Centauri will there be for the next four trillion years. Moreover, if the “planet c” are slightly affect the orbit b, tidal heating would provide an infinitely modest amount of energy to the interior of b, providing energy to maintain a stable atmosphere. If humanity is to survive longer than the sun, we have to leave the solar system and travel to other stars. If Proxima b is able to sustain life, can be the perfect place to move. Perhaps it is discovered future home for mankind! But to know this for sure, we have to do a lot more observation, carry a lot more computer simulations and, with luck, to send a probe to perform the first direct exoplanet reconnaissance. Life support or not, Proxima b offers new insights on how planets and life fit into our universe.
Thanks for Victoria Meadows, Edward Schwietermana, Giada Arney and Peter Kelley.
Editor’s Note. this article is based on popularizing scientific report “The habitability of Proxima b I: Evolutionary scenarios,” http://adsabs.harvard.edu/abs/ 2016arXiv160806919B, which was submitted to the Journal Astrobiology 25 August. Rating presumed location Proxima b in the ecosphere is crucial to interpreting the meaning of detection Proxima b, the design of future observations, and even to give the new shapes instruments and space missions. At the time of the discovery announcement, the band Pale Red Dot made contact with two groups of experts to rate them these early assumptions about the location of Proxima b in the ecosphere. Professor Rory Barnes chaired by one of these groups. The results of the second working group (led by I. M. Ribas and Turbat) are summarized here: http://proximacentauri.info and are technically explained in two articles. Further studies are certainly in progress.
About the author. Rory Barnes, a professor of astronomy and astrobiology, the University of Washington in Seattle. Doctorate in astronomy from the University of Washington in 2004. After working as a postdoctoral the Lunar Planetary Laboratory at the University of Arizona in Tucson he returned to the University of Washington and NASA’s Virtual Planetary Lab in 2009, joining the staff of the University of Warsaw in 2013. Bada exoplanet through computer models. At first dealt with the dynamics of orbits, but expanded his research, which now include the role of the Milky Way, the evolution of stars, the effects of weathering, and thermal and magnetic evolution of the interior terrestrial planets.
This article is part of a social campaign project Pale Red Dot and the original is available on the project website at: http://www.palereddot.org
The translation into Polish: Pulse of the Cosmos / Eve Stokłosa