Astronomers using NASA's James Webb Space Telescope have discovered a giant exoplanet, named Beta Pictoris d, hidden within the well-studied Beta Pictoris planetary system in the Milky Way, as per information on the space agency's website. The discovery makes Beta Pictoris only the second known planetary system to contain at least three directly imaged planets and marks the first time a directly imaged exoplanet has been discovered primarily through moderate-resolution spectroscopy rather than conventional imaging.

The findings, published in The Astrophysical Journal Letters, reveal that the planet was identified through the unique chemical signature of its atmosphere, a technique researchers say could reshape the search for planets around other stars.

"This discovery adds another piece to an already fascinating planetary system," said lead author Aidan Gibbs, a postdoctoral researcher at the University of California, San Diego. He said Beta Pictoris has long served as a laboratory for understanding how planetary systems form and evolve.

Hidden planet found by atmospheric signature

Located 63 light-years from Earth and about 23 million years old, the Beta Pictoris system was previously known to host two giant planets, Beta Pictoris b and Beta Pictoris c. The newly discovered Beta Pictoris d is estimated to be at least twice the mass of Jupiter, making it the smallest of the three known planets in the system.

Researchers believe the planet orbits its star at a distance of about 30 astronomical units, comparable to Neptune's position in the Solar System. Although it has the widest orbit among the three known planets, it still lies within the inner edge of the system's debris disk.

Unexpected discovery during Webb observations

The discovery came while scientists were studying the atmosphere of Beta Pictoris b using Webb's Near-Infrared Spectrograph (NIRSpec). The team was not searching for another planet when an unexpected signal appeared in the spectroscopic data.

"We weren't looking for a new planet," Gibbs said. "We were trying to understand one we already knew existed. Then, this telltale signal appeared in the data where we didn't expect it."

Instead of detecting a bright object, researchers identified a distinctive pattern of carbon monoxide absorption lines, indicating the presence of a giant planet's atmosphere.

Spectroscopy confirms planetary nature

The spectroscopic data also enabled scientists to determine the object's motion. Its speed, position and alignment with the debris disk confirmed that it was orbiting Beta Pictoris rather than being a background star or a brown dwarf.

Jean-Baptiste Ruffio, principal investigator of the initial Webb observations, said researchers were cautious because bright features in images can often be caused by instrumental effects or structures within the debris disk.

He said simultaneous imaging and spectroscopy allowed the team to quickly confirm that the signal belonged to a planet.

Follow-up observations using Webb's Mid-Infrared Instrument (MIRI) detected water vapour and methane in the atmosphere, further confirming the planet's identity while providing additional details about its atmospheric composition.

Dusty debris disk concealed the planet

Beta Pictoris d remained undetected for years because it is embedded within one of the brightest debris disks known to astronomers. The dust scattered starlight, making it difficult for conventional imaging methods to separate planets from surrounding material.

Researchers said Webb's spectroscopic technique effectively filtered out the dust by isolating the molecular signatures unique to a planetary atmosphere.

Scientists also noted that astronomers had previously predicted the existence of a planet in this region to explain the sharply defined inner edge and other unusual structures within the debris disk.

New technique could transform exoplanet searches

Researchers said the discovery demonstrates a powerful new approach to finding exoplanets in environments where traditional imaging techniques struggle.

Unlike conventional methods, spectroscopy enabled scientists to identify the planet and begin analysing its temperature, chemistry and motion from the first observation.

A separate imaging study led by researchers from the University of Edinburgh and the European Southern Observatory independently confirmed the existence of Beta Pictoris d using observations from the European Southern Observatory's Very Large Telescope and Webb's Near-Infrared Camera (NIRCam).

The research team plans to continue studying Webb's observations to refine estimates of the planet's temperature, atmospheric composition and orbit, offering further insight into one of astronomy's best-known planetary systems.