The star Beta Pictoris lies just 63 light-years away and is a mere 20 million years old. That youth makes it a gift to planetary science: it offers a rare view of what a planetary system looks like in its earliest days. Now an international team has pinned down a third planet there — completing a system that until now was known to hold only two worlds.
The new gas giant is called Beta Pictoris d. Unlike its two siblings, each roughly eight to ten times the mass of Jupiter, it weighs only about 2.4 Jupiter masses — making it one of the lightest exoplanets ever imaged directly from the ground. It is correspondingly faint: its light is about a hundred times weaker than that of the inner planet Beta Pictoris b.
A discovery by accident
Planet d turned up almost by chance. The team, led by Ben Sutlieff of the University of Edinburgh and Markus Bonse of the European Southern Observatory (ESO), had aimed the Very Large Telescope in Chile at the already-known planet b to track how it changes. But a second point of light, farther out, appeared in the images. "There's something else there — did you see it?" Bonse recalls saying.
To rule out a background star or galaxy lurking behind the system, the researchers combed through older observations. They found the same speck in archive images from the VLT and the James Webb Space Telescope, some dating back as far as eleven years. Its slow drift over that time matched an orbit around the star precisely. "Planet d has been playing hide-and-seek with us for over a decade," said co-author Jayne Birkby of the University of Oxford. "Only now can we say: found you."
With the discovery, reported in The Astrophysical Journal Letters, Beta Pictoris becomes only the second known system in which at least three planets can be directly imaged. Its wide, cold orbit may also help solve a long-standing puzzle: why a broad zone of the star's dust disk is so conspicuously swept clean.