How do supergiant exoplanets form? James Webb Space Telescope finds a clue

Using the James Webb Space Telescope (JWST), astronomers have investigated an alien planet that could help define the line dividing planets and stars.

The curious exoplanet is 29 Cygni b, a gas giant with around 15 times the mass of Jupiter that lies 133 light-years away from Earth.

Most planets are thought to form via a "bottom-up" process that sees tiny clumps of rock and ice coming together to gradually grow a world. However, bottom-up processes struggle to account for the formation of planets with as much mass as 29 Cygni b.

Such giants are thought to form instead via a top-down process — the direct collapse of dense patches of gas and dust in the protoplanetary disks that swirl around infant stars. That's the same way that stars themselves form, from dense patches in much larger clouds of interstellar gas and dust.

Now, JWST has collected multiple lines of evidence that suggest there is a way that huge planets such as 29 Cygni b could form via bottom-up processes, just like their more diminutive counterparts.

29 Cygni b sits on the dividing line of formation processes. Though its large mass suggests a top-down process, its wide orbit — an average distance from its star of 1.5 billion miles (2.4 billion kilometers), similar to that of Uranus in our own solar system — hints at a bottom-up formation mechanism.

The team directly imaged 29 Cygni b using JWST's Near-Infrared Camera (NIRCam), as part of a program that will image four exoplanets, all of which orbit their stars within around 9.3 billion miles (15 billion km) and have masses between one and 15 times that of Jupiter. The planets are all also relatively young and are still hot from their formation, with temperatures ranging from 990 to 1,830 degrees Fahrenheit (530 to 1,000 degrees Celsius), meaning they should all have similar atmospheric chemistry, too.

The researchers hunted for light being absorbed by carbon dioxide and carbon monoxide, which allowed them to measure the proportions of elements heavier than helium, which astronomers call "metals," in 29 Cygni b's atmosphere.

This revealed that, not only is the exoplanet around 150 times richer in metals than Earth, but it is also much more metal-rich than its parent star. This indicates that, as it was forming, the gas giant gathered a wealth of metal-enriched clumps of material from its natal protoplanetary disk.

The team also determined that the orientation of 29 Cygni b's orbit is aligned with the rotation of its parent star, which indicates it did indeed form within a protoplanetary disk.

As the program continues to investigate similar planets, it will discover if other such worlds also greedily grabbed metal-rich matter during their formation. This could finally help scientists understand how the most massive planets in the Milky Way were born, be it like stars or like smaller planets.

The team's research was published on Tuesday (April 14) in the Astrophysical Journal Letters.