Sounds like a movie name: “Blue stars have giant planets too.” But that’s what an international group of astronomers found when they discovered a gas giant world orbiting b Centauri, a binary star, which, in sum, has 6 to 10 times more mass than the Sun. It is the first exoplanet of its type found in a star with more than 3 solar masses.
The double star b Centauri is 325 light-years from Earth, in the constellation Centaur (not to be confused with Beta Centauri, the second brightest in that constellation). It is known that it is composed of two stars that orbit each other closely, and the largest of them is of type B, a blue star.
The colors indicate temperature, which in turn, for stars in the “adult” phase of their life cycle (astronomers say they are in the “main sequence”), also indicates their size. They are sorted by letters, in the sequence MKGFABO. The smallest are the M stars, also known as red dwarfs. The Sun is a type G, yellow dwarf star. The main star of the Centauri b system is blue, type B. (And yes, it is also called a dwarf – every star that is in the main sequence, regardless of size or mass, is, for astronomers, a dwarf. is in the main sequence. Which is beside the point now.)
To date, searches for exoplanets have failed to find gas giant planets like Jupiter around stars with very high mass, up to 3 times solar. It was thought that these large stars, because they emit so much radiation, ended up dissipating the disk of gas and dust around them too quickly to allow the formation of worlds with large gaseous envelopes, as is the case of Jupiter and Saturn.
The new finding, published in the British journal Nature, comes to shake this conviction. The team led by Markus Janson, from Stockholm University, used the Sphere instrument from the VLT (Very Large Telescope) in Chile to observe the surroundings of b Centauri. Capable of blocking the light from the central stars, the instrument allows direct observation of possible planets around – something that would otherwise be impossible, due to the powerful brightness of the stars.
In 2019, the group found a star that appeared to be an exoplanet. But it could also be a background star. The only way to know was to wait and perform further observation later, to see if it was gravitationally trapped ab Centauri AB (capital letters correspond to the two central stars) or if it was a background star only incidentally close in the telescope’s field of view. . The new observation was made in 2021 and confirmed the link. It was really b Centauri (AB)b (the lowercase b at the end is the planet), or, more simply, b Centauri b (I know, this nomenclature, although quite logical, to scientists’ taste, makes you cry).
The group was even digging through data collected in 2000 by the telescope in La Silla, Chile, and the said whose was already there — but at the time it was impossible to distinguish it from noise in the image.
With a reasonable arc of motion of 21 years for the planet, it was possible to estimate (albeit somewhat crudely) essential parameters of the orbit. It was concluded that that giant world circles the two parent stars at about 560 astronomical units from them (1 AU is the Earth-Sun distance, some 150 million km). The brightness, contrasted with the age (it must be a still young planet, some 15 million years old), indicates that it must be something like 11 times the mass of Jupiter. It may seem colossal, but what astronomers count most at this time is the mass ratio between star and planet and, in this case, the difference there (planet with between 0.1% and 0.17% of the star’s mass) it is similar to the Jupiter-Sun ratio. In other words, something quite normal, judging by what we know.
What is really out of step is this planet being at a distance. The finding is difficult to reconcile with more traditional models of planetary formation. Its orbital parameters suggest an origin more or less where it is, but a traditional accretion process (which would involve gradually gathering rocks and gas from the star’s protoplanetary disk) would not work for that distance (in addition to being hampered by radiation stellar, which would quickly dissipate the gas). This causes researchers to favor an alternative model, in which a part of the disk alone collapsed by gravity to form the planet, a scheme that allows for faster and more distant formation. Despite being a more natural explanation, it is still not possible to believe that this is what happened.
In any case, the result might suggest that we were misled by our own observational biases. By not finding giant planets around high-mass stars with the most traditional search methods (detection of planetary transits, in which the planet passes in front of the star and temporarily reduces its brightness, or radial velocity variations, in which the The planet’s gravity produces a subtle and observable wobble in the star), which favors planets with shorter orbits, we jump to the conclusion that they would not exist.
The evolution of direct imaging techniques, provided by instruments such as the Sphere (which began operating in 2014), brings the opposite bias, making it much easier to discover planets in long orbits. Since then, giant planets have started to swarm at distances from their stars that are much larger than those that Jupiter, Saturn, Uranus and Neptune keep from the Sun.
In the end, advancing the study of exoplanets is a brutal exercise in humility and a thorough demonstration of the Copernican principle. Not only is the Earth just one planet among the eight in the Solar System, as our planetary family is just one more, among so many and so varied that exist in the Universe. And there’s nothing about her that makes her particularly special. There are ultra-compact systems, much more squeezed than ours, and there are others that are quite loose, as b Centauri seems to be, besides, of course, many others that are similar to ours in architecture. The richness of nature is expressed in its enchanting diversity, paradoxically produced by uniform basic laws, applicable throughout the Universe.
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