"Super Earth" discovered around the nearest single star
Barnard`s Star, one of the nearest stars to the Sun, has a planet. The discovery was announced in the latest issue of Nature. One of the authors of the publication is a Polish astronomer from the University of Warsaw - Dr. Marcin Kiraga.
Barnard`s Star is one of the nearest stars to the Sun. It lies just 6 light-years from the Sun. Only the three-star Alfa Centauri is closer, which makes Barnard`s Star the nearest single star to the Solar System.
An international team of astronomers led by Ignasi Ribas (the Institute of Space Studies of Catalonia and the Institute of Space Sciences, CSIC in Spain) and Guillem Anglada Escudé (Queen Mary University of London) carried out research as part of the campaigns Red Dots and CARMENES. The search for a planet orbiting Barnard`s Star was successful - astronomers found a candidate for a planet with a mass of at least 3.2 Earth masses, making it a "super-Earth". The planet completes one orbit around its star every 233 Earth days. It was given the designation Barnard`s Star b.
The newly discovered planet is closer to its star than the Earth to the Sun (about 0.4 AU), but despite this it`s very cold, with temperatures reaching minus 170 degrees Celsius. The reason are the properties of its parent star, which is a small red dwarf. The planet receives from its star only 2 percent energy that reaches the Earth from the Sun.
The planet was discovered thanks to the so-called radial velocity method. When a planet orbits the star, its gravitational interaction causes the star to make small circles - it periodically approaches us and moves away. As a result of the Doppler effect, this movement affects the light emitted by the star, which can be measured by analysing the line shifts in the star spectrum. When the star approaches, spectral lines move towards the shorter (blue) waves, and when it moves away, the shift is towards the longer (red) waves. It can be compared to the changing tone of an ambulance signal, depending on whether it is approaching or moving away.
In particular, the measurements with the HARPS spectrograph on the VLT telescope of the European Southern Observatory (ESO) allow to measure changes of a star`s radial velocity with 3.5 km/h accuracy, which corresponds to the human walking speed.
Of course, before astronomers confirm that they are dealing with the planet, they must first exclude other possible factors that can potentially affect the observed properties of the star`s light. Scientists have already excluded those factors.
"After a very careful analysis, we are 99 percent confident that the planet is there" - said Ignasi Ribas.
Astronomers searched for a planet around Barnard`s Star before, but without success. The latest result was possible thanks to the combination of measurements from several very precise instruments working on telescopes around the world.
According to Ribas, researchers used observations from seven different instruments, spanning 20 years of measurements, a total of 771 measurements.
The following instruments were used in the research: the HARPS at the ESO`s 3.6-meter telescope; UVES at the VLT telescope belonging to ESO; HARPS-N at Telescopio Nazionale Galileo; HIRES at the Keck 10-meter telescope; PFS at the Magellan 6.5-m telescope; APF atr the 2.4-meter telescope at Lick Observatory and CARMENES at Calar Alto Observatory. Additional observations were made with a 90-cm telescope at the Sierra Nevada Observatory, a 40-cm robotic telescope at the SPACEOBS observatory and the 80-cm Joan Oró Telescope at the Montsec Astronomical Observatory (OAdM).
Among the authors of the publication is a Polish astronomer from the University of Warsaw, Dr. Marcin Kiraga. His participation in the study consisted in the analysis of photometric data (similarly as in the case of the earlier discovery of the planet Proxima b). The data came mainly from the ASAS review carried out by the University of Warsaw Astronomical Observatory. The aim was to determine whether photometric variability (star brightness variation) can be associated with observed changes in radial velocity. No such link was detected.
"In the future, it will be possible to confirm the existence of this planet with astrometric observations, because the expected amplitude of Barnard Star`s location changes associated with the presence of the planet is greater than 13 microseconds of the arc. Perhaps also with direct detection, although the maximum distance of the planet from the star is 0.22 seconds of the arc, and the expected ratio of brightness of the planet to the star in the maximum separation is on the order of one billionth" - explains the Polish astronomer in an interview with PAP.
Among the scientists involved in the project is Dr. Adrian Kaminski, a German astronomer of Polish origin working at the University of Heidelberg. In the consortium CARMENES he was responsible for data analysis, in particular data quality and understanding the measurements made by the spectrograph at the 3.5-meter telescope at the Calar Alto Observatory.
According to Kaminski, over the last years the CARMENES team has invested a lot of time and effort in building an instrument that would be able to find rocky planets near not very large stars (dwarfs).
"The orbit of the planet candidate is located at a distance of 0.4 AU from the star, in the expected position of the so-called a snow line, behind which volatile substances condense to a solid state. It is believed that this area has favourable conditions for the formation of planets and is therefore particularly interesting in the context of understanding the formation processes of planets. Our discovery shows that current high-resolution spectrographs are able to study the occurrence of small planets in this interesting area" - explains Kaminski.
Interestingly, Barnard`s Star has the fastest movement across Earth`s sky. It travels the width of a full moon every 180 years. This is much faster than the apparent motion of any other star. The velocity of Barnard`s Star relative to the Sun is also high, about 500,000 km/h, but it is not the fastest known star in terms of velocity in km/h (and not angular velocity in the sky, as in the case of apparent motion).
The planet around Barnard`s Star is the second nearest known exoplanet. The nearest one is Proxima b, which orbits Proxima Centauri in the Alfa Centauri trio.
The results of the research have been presented in an article in the prestigious weekly "Nature".
PAP - Science in Poland, Krzysztof Czart
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