The cold super-earth around Barnard's star

The cold super-earth around Barnard's star https://danielmarin.naukas.com/files/2018/11/eso1837b-1024x643.jpg

The star of Barnard is one of the most popular stars due to its proximity and its high speed with respect to the solar system. And it will continue to be famous because we now know that it possesses the one that at the moment is the second closest extrasolar planet. Discovered officially by the American astronomer Edward Barnard in 1916, it is only 5.96 light years away, a fact that makes it the fourth star closest to the Sun after Proxima Centauri and the double Alpha Centauri system. It is a red dwarf of spectral type M4, so, even being so close, it is invisible to the naked eye (its magnitude is 9.5). These characteristics have turned the star of Barnard into the object of a multitude of works of science fiction - well, to boat soon, I remember the novels Hyperion by Dan Simmons or Rocheworld Robert Forward-, but also scientific observations in search of planets. In the 1960s the astronomer Peter van de Kamp became famous for the supposed discovery of one or several gaseous giants around him using the astrometry technique using a 61-centimeter refractor telescope.

Recreation of Barnard Star b (or Barnard b) (ESO).

The discovery of van de Kamp proved a mirage and contributed to the scientific community being very skeptical about the search for exoplanets during the 70s and 80s. From 1995, when it was already evident that exoplanets existed around stars of the In the main sequence, several teams of researchers searched for planets around the Barnard star using the radial velocity method, the transit method and direct vision. Unsuccessfully. Recall that the first two methods, the most fruitful by far, favor the detection of very large worlds located near their star. If Barnard's star had planets, they must be far away or very small. But astronomers have not given up their efforts and, finally, the search has paid off.

Movement of Barnard's star measured by van de Kamp, from which he predicted - erroneously - the presence of a planet in the 60s (Wikipedia).

A team led by Spanish Ignasi Ribas, of the Institute of Science of the Space (ICE, CSIC) of Barcelona, has analyzed about twenty years of radial speed measurements -771 observations- taken by seven different instruments, including spectrometers CARMENES (Calar Alto, Spain), HARPS (Chile) and HARPS-N (Canary Islands, Spain). The result? That the team of researchers is convinced that they have detected a super-earth around the Barnard star. Barnard b -o GJ 699 b- is a world with a minimum mass equal to 3.2 times that of our planet and with a period of 233 days. If its orbit is circular this means that it is 60 million kilometers from its star. At this distance, in our Barnard b solar system it would be too hot to be habitable, but since Barnard's star is so small and cold, the temperature of this super-earth should be -170 ºC. That is, it is outside the habitable zone and on the border of the so-called freezing line. Nor can it be ruled out that Barnard b is not a minineptuno, although it is most likely a supertierra with a rocky surface.

The signal of the planet is only 1.2 m / s! (Ribas et al.).

The search for planets around red dwarfs using the radial velocity method is not an easy task because of the high variability of this type of stars, so it is essential to first determine the period of rotation of the star and its luminous noise. background. The Ribas team has concluded that the Barnard star rotates on its axis once every 140 days, although with an error of about ten days. And it is that the presence of Barnard b causes a speed of just 1.2 m / s in its star (!), So it is not surprising that it took so long to be discovered. That is why its discoverers speak of "candidate planet", since it is still not possible to confirm 100% that it is real. The reason is that one of the methods used to filter the radial velocity signal of the planet from the noise coming from the stellar activity has reduced the discovery's statistical significance. But, in any case, the probability that the signal originates from stellar activity is only 0.8%, so we can be sure that the planet exists 99%.

Artistic recreation of the surface of Barnard b (ESO).

Naturally, there is a possibility that there are more planets in the system. The radial velocity of Barnard's star shows a long-term pattern that could reveal a further, more distant exoplanet ... or it could be due to stellar activity. It is possible that Barnard b is a disappointment for some because it is not a habitable world, but it would have been very coincidental that, after Proxima b, nature would have given us another potentially habitable planet so close to the solar system. On the other hand, it is also true that it is the closest world around a star that is not part of any multiple system (Proxima and Alpha Centauri form a triple system).

Our solar system and that of Barnard's star scale (Nature).

The really interesting thing about this and other nearby exoplanets is not that we can travel to them - that is beyond the reach of technology that we will enjoy in our lives - but that we will be able to study them in detail with advanced instruments in the future. In the case of Barnard b, the discoverers have calculated that its brightness is one billionth of that of the star, a proportion of contrast that is outside the current instruments, but which, due to the high angular separation of the planet -220 milliseconds of arco- will allow you to see directly with the new generation telescopes, which will make it possible to study your hypothetical atmosphere. In fact, Barnard b will be about five times easier to see directly than Proxima b. It is also possible that it can be studied using astrometry - the method used by van de Kamp - based on data from the European mission Gaia. In short, Barnard b may not be potentially habitable, but has many ballots to become in a few years in the nearest exoplanet that we can see directly. And, on the other hand, I can not stop thinking that Peter van de Kamp would be very happy to know that, indeed, Barnard's star has planets around him. Do not you think Van de Kamp would be a good name for this world?

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