At a distance of just over four light-years, Proxima Centauri is the nearest neighbor star of the sun. Together with the nearby double star Alpha Centauri, the Red Dwarf would probably be the first candidate for a space mission to a foreign star. No wonder astronomers are looking for clues to planets in these stars - but for a long time with rather disappointing results. Because the transit method, the search for a regular dimming of the light by a planet passing in front of the star, was unsuccessful. Neither Proxima nor Alpha Centauri researchers were able to detect such brightness fluctuations. But the exoplanet hunters have another method in their hands: the measurement of radial velocity. It takes advantage of the fact that the gravity of a circling planet causes the central star to reel a little. These regular movements cause the light spectrum emitted by this star to shift slightly blue or red, depending on whether the star is just moving toward or away from us.
Exactly these slight fluctuations in the light spectrum have now told astronomers that there must be a planet around Proxima Centauri. Guillem Anglada-Escudé of the Queen Mary University of London and his colleagues used this to evaluate observational data from two telescopes of the European Southern Observatory, including the HARPS spectrograph at the 3.6-meter telescope of La Silla Observatory in Chile. Already in 2013, the researchers found in his data first signs of a tumbling of the star in the clock of 11.2 days. But the validity of these data was not enough to conclude from this safely on a planet with this orbital period. In theory, even stellar outbursts and other disturbances could only fool the existence of such a planet - exactly this was the case with a supposed planet around Alpha Centauri B: He turned out to be a mistake. Only when the astronomers reviewed this data in March 2016 by re-surveying for 54 nights were they sure: Proxima Centauri has a planet.
Super earth in the habitable zone
The newly discovered planet is probably a super-earth with about 1.3 times the mass of the Earth. It orbits its star at a distance of only seven million kilometers, which is only five percent of the distance of the earth to the sun and is therefore very close to the star. In our solar system, such a planet would therefore be an unstable, hot glowing world. But Proxima Centauri is much smaller and less luminous than our Sun: The Red Dwarf has only twelve percent of their mass and 0.17 percent of their luminosity - correspondingly less heat energy he sends out into space. However, this means that the habitable zone is much closer to the star than in our solar system. "With a half-axis of 0.05 astronomical units, Proxima b lies in the center of the habitable zone around its star, " the astronomers report. The temperature on this super-soil could therefore be mild enough to allow liquid water and thus potentially life. Should this be confirmed, Proxima Centauri b would not only be our nearest neighbor among the exoplanets, but also a possible second Earth at halfway distance. It would be the next target for a future first interstellar space mission.
Our neighbor star and his planet (Video: Nature)
However, it is not certain yet whether Proxima Centauri b is really life-friendly. Because with a planet with such a tight orbit, other factors influence whether a planet is life-friendly or not. "The most common arguments against it are a bound rotation, strong stellar magnetic fields, strong flares and a high intensity of UV and X-ray radiation", explain the researchers. For example, because of its close proximity to the star, the planet Proxima Centauri b is likely to be exposed to 400 times higher X-rays than Earth. In addition, the super Earth will probably always apply the same side to the star - much like the Earth's moon. On Proxima Centauri b could prevail on one half of eternal day with correspondingly high temperatures, on the other hand icy night. Together with the X-ray radiation and a very strong magnetic field of the red dwarf these are rather unfavorable conditions for life on Proxima Centauri b.
More information on what Proxima Centauri b looks like may be used in future to provide targeted observations with high-resolution telescopes. However, astronomers estimate that it may take several more decades to know more about the chemical properties and thus the possible existence of life on this planet. However, with its discovery, Proxima Centauri b will in any case become a probable target for a first interstellar space mission sometime in the distant future.
- Guillem Anglada-Escud (Queen Mary, University of London) et al., Nature, doi: 10.1038 / nature19106