This planet is so big that astronomers found it hard to believe that it could form like a normal planet.
Credit: NASA, ESA, CSA, Joseph Olmsted (STScI)
A new study published in Astronomical Journal Letters digs into data collected by the James Webb Space Telescope, possibly providing new insight into the formation of both planets and stars.
The matter comes down to an exoplanet called 29 Cygni b, which resides about 133 light-years from Earth. The first exoplanet discovered near the star 29 Cygni, in the Cygnus constellation. 29 Cygni b intrudes at 15 times the mass of Jupiter and orbits about as far away as Neptune, which is difficult for existing models of planet formation.
Planets are generally thought to form “bottom-up,” meaning they grow from small nuggets of matter by colliding with (and eventually absorbing) more matter. Depending on what the material is and how much, the planet will come out rocky, icy, or gassy.
On the other hand, stars are thought to form when large clouds of gas form into smaller clouds, which can then condense towards the core. In theory, heavy elements and large chunks of matter could tear apart the proto-planetary disk and contract into a planet in the same way.
29 Cygni b is such a heavy planet that it appears to have been formed by accretion of material from the bottom up within the protoplanetary disk, but at a distance where such accretion should be rare due to the low density of material. That makes the star-like condensation theory another tentative explanation for its existence.
Webb’s images show evidence of heavy chemical elements like carbon and oxygen, correcting speculation about how it might have happened.
Credit: NASA, ESA, CSA
That ambiguity made the planet interesting to NASA researchers, who saw it as something that could be born from any process.
“For compact systems, it’s much easier for disc separation to escape from a much larger population than 29 Cygni b,” lead author William Balmer said in a NASA statement. “This is the lowest price you can get. But at the same time, it’s about the highest price you can get for growth.”
The evidence they reviewed seems to favor the idea that it expanded like a normal planet, making it an outsider. They believe this because the spectral analysis shows a higher concentration of metals in the atmosphere than in the host star, which means that it did not arise due to the dimming of a different shoot in the same cloud of gas.
There is also the fact that the planet’s orbit coincides with the spin of its host star, 29 Cygni. This would be expected for a planet formed by accretion, but would only have a 50:50 chance of occurring from the cloud collapse model.
So, most likely, it formed like a normal planet, not a star. It just happens to push the limits of what is possible for that model, in terms of its size and its distance from its star.
The team has three other giant and distant planets it wants to explore in a similar way. Among themselves, these observations should help suggest how much more likely the cloud-collapse model of planet formation is.
