The James Webb Space Telescope has observed two large planets in different stages of infancy, orbiting a young sun-like star. One planet boasts an atmosphere rich with dusty clouds, while the other is encircled by a disk of material, illustrating the complex nature of how planetary systems develop.
These two gas giant planets, both more massive than our solar system’s largest planet, Jupiter, were directly imaged by Webb in a planetary system located approximately 310 light-years from Earth within the Milky Way galaxy, in the direction of the constellation Musca. A light-year represents the distance light travels in a year, equivalent to 5.9 trillion miles (9.5 trillion km).
Astronomers have identified over 5,900 planets beyond our solar system, known as exoplanets, since the 1990s. Less than 2% of these have been directly imaged, making this discovery of two exoplanets in their early developmental stages particularly rare.
The formation of a planetary system commences with a vast cloud of gas and dust, termed a molecular cloud, which collapses under its own gravity to form a central star. The remaining material, orbiting the star in what is called a protoplanetary disk, then coalesces into planets.
This particular planetary system was observed by Webb at a very early point in its developmental history. The central star, named YSES-1, has roughly the same mass as our sun. The two newly observed planets orbit at a considerable distance from YSES-1, with each likely taking thousands of years to complete a single orbit.
While our sun is approximately 4.5 billion years old, YSES-1 is a mere 16 million years old, making it a veritable cosmic newborn. Researchers were surprised to discover that the two neonatal planets appeared to be at differing stages of development.
The innermost of the two planets boasts a mass about 14 times greater than Jupiter and orbits its star at a distance 160 times greater than Earth’s orbit around the sun, and over five times farther than our solar system’s outermost planet, Neptune. This planet is surrounded by a disk of small-grained dust, a state that might indicate a very early stage of formation, ongoing coalescence, or perhaps the aftermath of a collision or a moon in the process of forming. Webb detected water and carbon monoxide within its atmosphere.
The outermost planet has a mass approximately six times that of Jupiter and orbits the star at 320 times the Earth-to-sun distance. Its atmosphere is heavily laden with silicate clouds, distinguishing it from our solar system’s gas giants. Webb also identified methane, water, carbon monoxide, and carbon dioxide in its atmosphere. Unlike its companion, it possesses no surrounding disk of material.
The perplexing combination of traits exhibited by these two planets within the same system underscores “the complex landscape that is planet formation and shows how much we truly don’t know about how planetary systems came to be, including our own,” explained astrophysicist Kielan Hoch of the Space Telescope Science Institute in Baltimore, who spearheaded the study published this week in the journal Nature.
“Theoretically, the planets should be forming around the same time, as planet formation happens fairly quickly, within about one million years,” Hoch commented.
A significant enigma, Hoch added, is the precise location where these planets formed, given that their orbital distance from the host star is greater than would be expected if they had originated within the protoplanetary disk.
“Furthermore, why one planet still retains material around it and one has distinct silicate clouds remains a big question. Do we expect all giant planets to form the same way and look the same if they formed in the same environment? These are questions we have been investigating for ages to place the formation of our own solar system into context,” Hoch elaborated.
In addition to amassing a wealth of discoveries about the early universe since becoming operational in 2022, Webb has made a profound contribution to the study of exoplanets through its observations at near- and mid-infrared wavelengths.
“Webb is revealing all sorts of atmospheric physics and chemistry happening in exoplanets that we didn’t know before, and is currently challenging every atmospheric model we used pre-Webb,” Hoch concluded.
