Astronomers Find Third Planet in Beta Pictoris System After Decade-Long Hide-and-Seek

Astronomers have discovered a faint exoplanet, Beta Pictoris d, that hid in telescope archives for 11 years. Independent confirmations from VLT and JWST make the finding unusually robust.

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Astronomers have discovered a third planet in the Beta Pictoris system, a world that evaded detection for about 11 years despite being captured in telescope archives. The planet, designated Beta Pictoris d, orbits a star 63 light-years from Earth and is the faintest exoplanet ever directly imaged from the ground.

Beta Pictoris d is a gas giant with a mass of about 2.4 Jupiter masses, though estimates range from 2 to 4 Jupiter masses. It is roughly 100 times fainter than the system's previously known planet, Beta Pictoris b, and it has the widest orbit of the three known planets. Its average distance from the star is about 26 astronomical units (AU), with an orbital period of roughly 91 years, although some uncertainty remains.

The discovery came about almost by accident. Ben Sutlieff of the University of Edinburgh led a team that spotted the planet using the ERIS instrument on ESO's Very Large Telescope. "This was a serendipitous discovery," Sutlieff said. "We initially wanted to look more at a known planet in the system, Beta Pictoris b, to see how it changed over time." Further analysis of archival images from SPHERE on the VLT and NIRCam on JWST revealed that Beta Pictoris d had been hiding in plain sight for over a decade.

A separate team, led by Aidan Gibbs of the University of California, San Diego, independently confirmed the planet using infrared data from the James Webb Space Telescope. "This discovery adds another piece to an already fascinating planetary system," Gibbs said. "Beta Pictoris has long served as a laboratory for understanding how planetary systems form and evolve, and now we have another planet helping us tell that story." The JWST observations, which included NIRSpec and MIRI instruments, detected methane, carbon monoxide, and water vapor in the planet's atmosphere. Follow-up observations with MIRI also revealed water vapor and methane, and radial velocity measurements helped confirm that Beta Pictoris d is indeed a planet.

The star Beta Pictoris is young, estimated at about 20 million years old, though one source puts it at 23 million. The system is only the second, after HR 8799, to have more than two directly imaged planets. Beta Pictoris b and c each have masses around 10 Jupiter masses, making Beta Pictoris d the lightest of the trio.

Beta Pictoris d has an effective temperature of about 600 K (roughly 327 °C). Its gravity likely explains the sharp inner edge of the system's debris disk. Computer modeling suggests the planet may orbit at about 30 AU, which conflicts with the VLT analysis of 26 AU; the Webb analysis suggests it could be beyond 30 AU.

The planet is notable for being the first discovered using moderate-resolution spectral template matching. It is also one of the lowest-mass exoplanets ever directly imaged from Earth. Jayne Birkby, who was involved in the work, said, "Planet d has apparently been playing hide-and-seek with us for over a decade, and only now can we say 'found you!'"

Two studies describing the findings were published in The Astrophysical Journal Letters on the same day.