James Webb has had to investigate whether he was born “from the top down” or “from the bottom up”

29 Cygni b is a huge celestial object, with a mass equal to 15 times the mass of Jupiter. Apparently it is a planet, but that mass could place it as a star. For example, a brown dwarf. Therefore, a team of astronomers has used the James Webb to analyze its origin, further refining the concept of the formation of stars and planets.

A question of metals. The authors of the study, who it was just publishedhave used the NIRCam camera on the James Webb Space Telescope to take photographs of this planet. This instrument allows high-resolution images and spectroscopy measurements to be taken, with which the composition of the atmospheres of stars and planets can be studied, taking into account how they reflect light.

Thanks to this, it has been seen that 29 Cygni b is very enriched in metals compared to the star around which it is located. Specifically, it has an amount of metals equivalent to 150 Earths. This is compatible with the accretion of a large amount of metal-laden solids into a protoplanetary disk. It is then confirmed that it is a planet, but a planet very unusual.

Planet or star? That’s the question. Planet formation takes place in a bottom-up process. In a disk of gas and dust, known as a protoplanetary disk, dust particles collide to form small fragments of rock and ice, which continue to clump together and grow until they form a planet. It is a process called accretion. The largest ones, in addition, in this process capture gas, which is why they later become gas giants.

On the other hand, stars form from top to bottom. A gas cloud fragments and each fragment collapses under its own gravity, becoming smaller and denser.

From paradox to paradox. This definition could lead us to think that planets are larger than stars. After all, planets go from less to more and stars from more to less. However, that is not true. Stars form when huge clouds of gas collapse, so they are still very massive. So much so that nuclear fusion can occur in them due to the high conditions of pressure and temperature. On the planets, although there is a growth from less to more, it is not so great.

The problem is that with planets as immense as 29 Cygni b there are doubts about the formation from less to more. It would seem that they were also formed by a fragmentation process in protoplanetary disks. As explained by the European Space Agency in a statementis something that “could explain why some very massive objects are found billions of kilometers from their host stars, in regions where the protoplanetary disk should have been too weak for accretion to occur.” That’s just what happens with 29 Cyni b. It has an enormous mass and is 2,400 kilometers from its star.

What James Webb teaches us. The fact that 29 Cygni b is so rich in metals indicates that it must have been formed by an accretion process, in which it accumulated more and more. In fact, heIt is normal for a planet to have many metals in proportion to its starwhich happens in the system in which 29 Cygni b is located. In short, it is shown that much larger planets than we thought can be formed by accretion, without having to resort to a top-down process.

And now what? 29 Cygni b has been the first of the four objects that will be studied by James Webb. All of them have a mass between 1 and 15 times that of Jupiter and are at least 1.5 billion kilometers from their star. This indicates that they are all in that dilemma of being huge planets or another star. Cataloging them into one of the two groups can help us understand much better the process by which the largest planets are formed.

Image | NASA, ESA, CSA, J. Olmsted (STScI)

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