September 4, 2016
On August 24, the European Southern Observatory ESO and the Pale Red Dot project announced the discovery of a planet in the habitable zone of the star Proxima Centauri - the results of years of work . Already two weeks earlier the SPIEGEL had spread an unofficial report about an "Earth 2.0" around this star . Many media remained with the original representation that the planet is "Earth-like" and "potentially habitable". But is this really the case? Without doubt, Proxima b, as the planet is named, is in many respects one of the most significant exoplanet discoveries. But is it "Earth-like" in the meaning of "like here"? We will see. Let us first look at the known facts.
Proxima Centauri is the closest known star to the Sun. It is a small, dim Red Dwarf, only 4.24 light years away from the Sun. It is seperated by 0.2 light years from the binary system Alpha Centauri A and B. The astronomers do not yet agree as to whether Alpha and Proxima Centauri are connected with each other through the gravitational force to form a very wide triple system. If this were the case, Proxima Centauri would circle the Alpha Centauri binary once every 500,000 years.
Proxima is a Red Dwarf. It has only 12.3% the mass of our Sun and releases 0.14 - 0.15 promille of its energy. In visible light, Proxima is even darker - the highest energy output is in the range of infrared light at 1.2 micrometers. This is due to the surface temperature of 2,700 ° C which is low for a star. The star is so small and dim that it can not be seen with the naked eye despite its proximity to Earth. Thus, Proxima was not discovered until 1915. You need a telescope with at least an 8 centimeter mirror to see this dim star. With the Very Large Telescope in Chile, the diameter of Proxima has been narrowed down to approx. 200,000 kilometers.
The small size and mass result in some fundamental differences to our Sun. In the Sun, the energy released in the core by hydrogen fusion is predominantly transmitted to the surface by radiation through the outer layers and finally released into space. It is only in the core itself that mixing of the material, called convection, occures. On the other hand, Proxima is very different: the star is completely convective, the material is circulated throughout the entire stellar volume. These material flows are associated with strong magnetic fields which can be discharged at the surface of the stars and cause sun spots and sunbursts. Such eruptions can cover up to 88% of Proxima's surface and can sometimes exceed the diameter of the star itself. During such eruptions, Proxima's brightness more than doubles within a matter of a few minutes. X-rays are also emitted during these eruptions.
This has very dramatic effects on the newly discovered planet and its "Earth-likeness". But let us first look at the basic facts which the astronomer team has published .
Proxima b was discovered using the method of radial velocity measurement. In doing so, one measures the displacements of the light spectrum which result from the movements of the star. The movements, in turn, are the result of the attraction of the star by its planet - the larger the mass of the planet and the closer it is to the star, the greater the influence of its gravitational force. This can be tricky: In small and "Earth-like" planets the measured radial velocities of the star can be within the magnitude of events on the stellar surface. With very complicated methods using high-precision spectrographs, these background motions must be recognized and removed from the measurements in order to finally be able to isolate the movements of the star due to the planetary influence. The team at Pale Red Dot managed this with an impressive sensitivity of 5 km per hour - that is, they could still measure a movement of Proxima that corresponds to the walking speed of a person. This is the real technical sensation behind the discovery!
As viewed from Earth, the measured radial velocity is greatest when the observer looks directly at the edge of the orbit of the planet, the planet on its orbit is once directly before and once just behind the star (see Figure 1). If, however, the orbit is inclined relative to the point of view of Earth, the radial velocity with the same planet will be lower because only a part of the motion of the star is in the direction of the observer. This point is important! The radial velocity is the smallest when Earth based observers look directly at the plane of the orbit.
This means that without further methodological approaches, the radial velocity of the star can initially be determined only by the smallest possible mass of the planet, namely the best case of looking directly at the edge of the orbit. For all other slopes, the mass of the planet must be larger.
Figure 1: Illustration of the basic principle of the method of radial velocity measurement. Shown are three different inclinations of the orbit for the same planet. In example a) the orbit of the planet is exactly in line with the viewing direction of the observer. The planet - shown here at the two extreme points of its orbit - pulls with its gravity at the center star and lets it move easily with the speed component VStern. The telescope can fully grasp this velocity as Va, because it takes place as an approach and retreat directly in the direction of the observer. In example b), the orbit of the planet is inclined by 45 ° to the viewing direction of the observer. The movement of the star in the plane of the orbit of its planet is the same as in the example a), but only a small part of the motion component takes place in the direction of the observer (Vb). In example c), the planet's orbit is inclined by 90 ° to the viewing direction of the observer. Again, the movement of the star in the plane of the orbit is still the same, but due to the inclination of the orbit, no velocity component is in the direction of the observer.
Only in the example a) one can determine the mass of the planet from the motion data alone. In examples b) and c) one would underestimate them; At b) the planet appears easier and at c) it would not be discovered with this method. Since, in most cases, the inclination of the orbit is not known, this method gives only the minimum mass of the planet for the situation in example a).
For Proxima b, the Pale Red Dot group has now indicated a lower mass limit of approximately 1.3 Earth masses - indicated for the most favorable position of the orbit. This means that the planet can not be lighter, but it can be much more massive. The Nature article gives a probability of 1.5 percent for a transit event. This means the orbit would probably be inclined and the planet actually be more massive. It is important to keep this in mind.
Previous investigations  could show that planets with a lower mass limit of 2-3 Earth masses are not to be expected in Proxima's habitable zone. Again, inclined orbits would result in higher upper limits. This is something to start with. We can say that the planet is, in the best case, 1.3 Earth masses, but its mass can also be several Earth masses in the case of an inclined orbit. With 1.3 earth masses, its composition would resemble the terrestrial planets of our solar system, leaving a very wide range of possibilities from Mercury, Venus, Earth and Mars. In the case of a larger mass, the planet would be a Super-Earth or a very light gas planet. To answer this question we need to know the diameter of the planet in order to determine its density. This is yet to be done.
The changes in the radial velocity indicate one revolution in just over 11 days. This corresponds to a distance to the star of 0.05 astronomical units - 1/20 of the distance of Earth from the Sun or slightly more than 7 million kilometers (see also figure 2). The habitable zone Proximas is given as 0,042 - 0,082 astronomical units. These values depend on the model used. But even a million kilometers more or less would not make such a difference. The planet is certainly safe within this zone. Its average solar radiation is 65% of Earth's value. Mars is also considered to be within the habitable zone and receives 43% of the earth's solar radiation. Excluding atmospheric effects, the Nature article gives an equilibrium temperature of -39 ° C (Earth: - 15 ° C).
Figure 2: Comparison of the mean distance from the central star of Mercury and Proxima b.
In any case, Proxima b is within the range in which the tidal forces from the star would have halted the planet's rotation within 4 billion years. This means that one hemisphere will always face the star. There is one hemisphere with perpetual daylight and one hemisphere with eternal night. The consequences for the possible climatic conditions on the surface are extremely complex and will be dealt with separately in another article.
Does Proxima b have an atmosphere?
For the above-mentioned question about the "Earth similarity" of the planet, it would be interesting to know whether Proxima b has a significant atmosphere or not. The answer is: We do not know yet. For our own solar system it is now assumed by means of isotopic investigations that the terrestrial planets have largely obtained their supply of gases and water from comet material after the planets had already been largely developed. An inflow of water in meteorites is also possible. An accumulation of volatile compounds during the phase of planetary formation is also possible, but a large part of this original material should have been lost during the transition of the sun from the phase of the protostar to the main sequence since this was associated with very violent solar activity. Earth is once again a special case, because the Moon is thought to have emerged from a collision with another protoplanet. In the course of this event, the planet very likely lost water and gases. Models for the planetary development do not rule out the fact that planets within the later habitable zone can also accumulate gases and water during their formation.
For Proxima Centauri, two characteristics are important when compared with the Sun: the status of the star as a Red Dwarf and as a variable "flare" star. Red dwarfs show a very much extended phase of high activity of up to one billion years before they enter the main sequence. Even if the planet Proxima b is now within the habitable zone, it was previously located in a much hotter zone. This should have very strong negative effects on its ability to accumulate gases during this important phase. This prolonged active phase at the beginning of their development is the most important reason why planets of Red Dwarfs are thought to be predominantly dry and airless deserts. This is an important limitation.
Any atmosphere still left from this phase would have been faced with the second eroding force: the strong sunbursts and the variable brightness of Proxima. As we have seen above, Proxima is one of the classic examples of a "flare star" and the fluctuations in brightness are significant. The charged particles released during these eruptions act on the gas molecules of the upper atmosphere and carry them off. Earth is largely protected from this effect by its magnetic field . Proxima b would need a particularly strong magnetic field to hold its atmosphere against the force of the solar wind. This is not impossible. Model calculations for planets of Red Dwarfs have shown that a strong magnetic field can remain and protect the atmosphere as long as the planet's iron core remains in motion. This is still possible with a fixed rotation. However, this is a very special condition which we do not know yet.
The last factor is the inflow of comet material. In our solar system, this became relevant after the planets had already been formed and gas planets like Jupiter directed objects, especially from the outer solar system, onto new orbits, most of which brought them down to a planet. The Oort Cloud as well is a reservoir for comets. The Oort Cloud of the Sun has never been observed directly, but its existence can be indirectly inferred from the orbits of long-periodic comets. Similar clouds are likely to exist around other stars, even if direct evidence is still missing. Proxima Centauri could act as a disturbing force on an Oort Cloud surrounding Alpha Centauri A and B and direct comets towards it. These would then occasionally plunge into the planets, delivering gases and water. For example, Proxima b could maintain an atmosphere and its own water with the help of comet material and a strong magnetic field despite the eroding influence of solar storms. These are questions, however, which can only be answered by later missions and, above all, by much more powerful telescopes.
Proxima b is the closest extrasolar planet to us and will continue to be for the next 30,000 years. Only then the star Ross 248 will come closer to Earth than Proxima. Proxima b is thus a significant discovery, which will provide many opportunities for scientific research in the coming years and decades.
Figure 3: View of the planet Proxima b from a low orbit (artistic representation).
The chosen view as a desert planet with a thin atmosphere corresponds to model calculations
based on a likely scenario. See the article for more details.