A giant baby planet is still pulling in leftover building material from its surrounding cloud. That's the scene the Hubble Space Telescope captured when it aimed its cameras at an object catalogued as SR 12 c. This companion planet orbits a young star at a wide distance — farther out than Neptune is from the Sun. In its spectrum, stretched out like a rainbow across wavelengths, there's a distinctive step — the Balmer jump. It arises when hot gas falling onto the planet abruptly changes its transparency at a specific frequency of light. It's like water pouring into a bathtub that suddenly starts glowing in ultraviolet, and from that glow you can measure exactly how much water is coming in.
The measurements showed that the planet emits radiation from the infalling gas with a power roughly 60,000 times smaller than the Sun's total luminosity — a tiny but noticeable nightlight. From this energy, scientists calculated the accretion rate: about eight trillionths of a solar mass per year. For comparison, that's like a layer of gas a thousandth of a millimeter thick falling onto the planet each year. At this rate, over the remaining growth time, the planet won't gain even a percent of its current mass: it's already on the home stretch of formation. Interestingly, over a month of observations, the accretion brightness didn't flicker. But compared to data from five years ago, the H-alpha line (another hydrogen indicator) faded by 90%. It seems the gas faucet is slowly being turned off.
Combining observations from ultraviolet to submillimeter radio waves, astronomers have built a nearly complete picture of the young planet's emission — from 0.2 to 880 micrometers. This is one of the most detailed "passports" of a forming giant. Such data help us understand how diverse the birth scenarios of giant planets can be: some quickly gain mass and "quiet down," while others, like SR 12 c, take a long time to finish their leftovers. Ultimately, it's a story about how ordered worlds emerge from the chaos of dust and gas, and each grows at its own pace, like a tree that sometimes shoots up vigorously and sometimes pauses to gather strength.
The Balmer jump in the spectrum of SR 12 c lies in the near ultraviolet, right at the edge of visible light. If our eyes were just a bit more sensitive to ultraviolet, we would see this jump as a sharp change in color in the glow of the gas falling onto the planet.