Located 63 light-years from Earth, a giant planet has just been discovered around the star Beta Pictoris. Named Beta Pictoris d, it had eluded astronomers for more than eleven years. Its very faint signal was already present in archived images. Two teams finally identified it independently.
Beta Pictoris is a very young star, about 23 million years old. By comparison, the Sun is 4.6 billion years old. Its environment is therefore still close to that of a planetary system in formation. A vast disk of dust and debris still surrounds the star, with several giant planets already known.
This illustration shows the Beta Pictoris system with, on the right, the giant exoplanet discovered Beta Pictoris d. Its orbit is the widest of the three known exoplanets in the system.
Illustration: NASA, ESA, CSA, STScI, Ralf Crawford (STScI)
The newcomer is the third planet observed in this system. Beta Pictoris b and c had been detected earlier. Only the HR 8799 system already had three directly imaged planets. These multiple systems allow comparisons of several worlds formed around the same star, under the same initial conditions.
Beta Pictoris d is a gas giant, like Jupiter or Saturn. Its mass is estimated at about 2.4 times that of Jupiter. The other two giants in the system would each be close to ten Jupiter masses. The new planet is therefore the least massive of the three, while remaining far from being a rocky world.
It orbits at about 30 astronomical units from its star. This distance corresponds approximately to the region occupied by Neptune in the Solar System. Its year would last about 91 Earth years. The planet, however, circulates inside the edge of the debris disk, where the remnants of planetary formation are still visible.
Its detection is remarkable because Beta Pictoris d is very faint. It appears about 100 times less bright than Beta Pictoris b. The Very Large Telescope of the European Southern Observatory observed it directly from the ground. According to ESO, it is the faintest exoplanet ever directly imaged from Earth.
Another team found the planet with the James Webb Space Telescope. It distinguished the chemical signature of its atmosphere: it contains carbon monoxide, water vapor and methane. These signatures confirmed that it is indeed a giant gas planet.
Archival data then allowed the planet to be tracked over more than a decade. It was barely visible in some VLT and Webb images. The discovery shows the value of reexamining old observations with new tools; unknown exoplanets could still be hiding in already collected data.