A team of astronomers have discovered a third planet orbiting the star Beta Pictoris. The new planet, Beta Pictoris d, is 100 times fainter than Beta Pictoris b — the first planet discovered in the same system — and is among the lightest exoplanets ever to be imaged from the ground. After spotting the planet using the European Southern Observatory's Very Large Telescope (ESO's VLT), the team found it had been hiding in archive observations spanning more than a decade.
"This was a serendipitous discovery," says Ben Sutlieff, co-lead of the study published today in The Astrophysical Journal Letters and astronomer at the University of Edinburgh, United Kingdom. "We initially wanted to look more at a known planet in the system, Beta Pictoris b, to see how it changed over time," he adds. However, when the team went to analyse their images of the system, they noticed something else, separated from Beta Pictoris b, that led them down an entirely new path.
"'There's something else there, did you see it?'" Markus Bonse, ESO astronomer in Germany and the other co-lead of the study, recalls saying when looking at the data. To confirm the nature of their detection, the team looked through the ESO archive, a catalogue of past observations made with ESO facilities. They found a new planet, Beta Pictoris d, in multiple images dating back as far as 11 years ago, including one where it was only just visible against the glare of its larger neighbour Beta Pictoris b. "Planet d, it seems, has been playing a game of hide-and-seek with us for over a decade and only now can we say 'found you!'" says Jayne Birkby, co-author of the study and astronomer at the University of Oxford, United Kingdom.
The newly discovered planet, like the two others in the system, is a gas giant like Jupiter or Saturn. However, Beta Pictoris d has a much wider orbit than the planets Beta Pictoris b and Beta Pictoris c. Moreover, while the first two planets are each around ten times the mass of Jupiter, the new planet is only 2.4 times more massive than Jupiter, making it one of the lightest ever imaged from the ground. The planet is also relatively cold and, hence, extremely faint relative to its host star.
Direct imaging, where the light from an object is captured as in a photograph, only works for planets bright enough to show up next to their much brighter host stars. Taking a direct image of a planet as faint as Beta Pictoris d, therefore, represents a significant achievement. "The new planet is 100 times fainter than Beta Pictoris b, the famous planet in the same system, making it the faintest exoplanet ever imaged directly from Earth," explains Bonse [1] .
This first clear detection of Beta Pictoris d, which is 63 light-years away from us, was made with the ERIS instrument on the VLT by Sutlieff, Bonse and their team. An independent team led by Aidan Gibbs at the University of California, US, also discovered the same planet using the James Webb Space Telescope (JWST), a facility of the US, European and Canadian space agencies. Their results are also published today in The Astrophysical Journal Letters.
To confirm a planet's discovery from a detection, astronomers usually have to make follow-up observations. However, this system had been extensively studied, with several images stored in the ESO and JWST science archives. "To our joy, out it popped in previous SPHERE observations," says Birkby, referring to another VLT instrument previously used to observe the Beta Pictoris system. The planet was also spotted in archival observations from NIRCam, a JWST instrument. Now that the team knew where to look for the potential new planet, "it turns out it was hiding in the data all along!" says Birkby. Co-author Valentin Christiaens, researcher at CEA Paris-Saclay, France, adds: "The detections in the archival SPHERE data are not only very exciting on their own, but also because they suggest a number of treasures are still hidden in the archives of VLT instruments!"
Beta Pictoris is now the second system, after HR 8799 , where more than two planets have been directly imaged. "Systems with multiple directly imaged exoplanets are the 'holy grails' of discoveries, because they can teach us a lot about what different exoplanets are like in the same formation environment," says Sutlieff [2] . Beta Pictoris d also clears up a mystery in its planetary system, as it has exactly the right mass and position to explain the particular shape of the surrounding debris disc, made of the leftovers of planet formation.
The discovery of Beta Pictoris d in this way encourages further direct imaging of planetary systems where faint planets may have been hiding in plain sight, including with ESO's upcoming Extremely Large Telescope (ELT). "Planets seem to have friends," says Beth Biller, also a co-author of the paper and astronomer at the University of Edinburgh, "many of the famous directly imaged exoplanet systems seem to have multiple giant planets in the same system, and likely there are even more lower mass planets hiding in these systems that might be revealed with instruments on the ELT."
Notes
[1] Beta Pictoris d is the faintest exoplanet ever imaged from Earth when corrected for the distance to the system — faintest in absolute magnitude (owing to its size and temperature only) not in apparent magnitude (where distance also contributes to faintness).
[2] Beta Pic is part of a group of stars all with the same age, and some of them have planets too. Beta Pic d seems to be almost a twin of one of these planets, 51 Eri b, meaning astronomers can use them both to anchor their models of how planets evolve and grow over time.
More information
This research was presented in a paper to appear in The Astrophysical Journal Letters ( https://doi.org/10.3847/2041-8213/ae80a0 ).
This paper, co-led by B. J. Sutlieff and M. J. Bonse, involves over 90 authors from around the world, including Belgium, France, Germany, Ireland, Italy, the Netherlands, Switzerland, the United Kingdom and Chile.
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