A New Planet in the Beta Pictoris System

Planet found after more than ten years of hide-and-seek.

To the point

• Discovery, Beta Pictoris d: Astronomers discovered a new, very faint planet called Beta Pictoris d orbiting the star Beta Pictoris by direct imaging, and it is one of the lightest exoplanets imaged from Earth.
• Planet Characteristics, Orbit: Beta Pictoris d is a gas giant about 2.4 times the mass of Jupiter, orbiting much farther from its star than many other known gas giants, with an orbital period of 91 years.
• Multiple Planets, System Comparison: Beta Pictoris is only the second system with more than two directly imaged planets, offering a unique chance to study planet formation and atmospheres in the same environment.

A team of astronomers, including MPIA scientists Elisabeth Matthews and Iain Hammond, 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) and its new instrument ERIS, the team, led by Ben Sutlieff (University of Edinburgh, United Kingdom) and Markus Bonse (European Southern Observatory, Garching near Munich, Germany and Max Planck Institute for Intelligent Systems, Tübingen, Germany), found it had been hiding in other observations collected over more than a decade. An independent team led by Aidan Gibbs (University of California, San Diego, La Jolla, USA) also discovered the same planet using the James Webb Space Telescope (JWST). Both results are complementary.

“While we see Beta Pictoris d’s full light allowing us to derive its fundamental properties such as temperature and planet mass, Webb’s spectra probe the planet’s atmosphere, revealing methane (CH₄), carbon monoxide (CO), and water vapour (H₂O),” explains Matthews, who made significant contributions to the atmospheric interpretation of the results drawn from the VLT observations. The articles are published today in The Astrophysical Journal Letters.

A serendipitous discovery

This discovery was a surprise, because the team aimed at the Beta Pictoris system to look for variability in Beta Pictoris b’s light. However, while analysing their images of the system, the astronomers noticed something else, separated from Beta Pictoris b, that led them down an entirely new path.

To confirm the nature of their detection, the team looked through the ESO data archive, a catalogue of past observations made with ESO facilities. They found the newly discovered planet in multiple images dating back as far as 11 years, including one in which 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 astronomers for over a decade.

Beta Pictoris d, like the two other planets in the system, is a gas giant like Jupiter or Saturn. However, while the other two planets each have roughly ten times the mass of Jupiter, Beta Pictoris d weighs in at just 2.4 Jupiter masses, making it one of the lightest ever imaged from the ground. Given that its size is only 30% larger than Jupiter's, Beta Pictoris d could be much more similar to the gas giants of the Solar System than most orbiting other stars.

However, the new planet has a much wider orbit than any other known gas giant planet in the Solar System or the Beta Pictoris system. Its orbital radius of 26 au (astronomical unit = the mean distance between the Sun and the Earth; 1 au = 149.6 million km) would place Beta Pictoris d right between Uranus and Neptune in the Solar System, resulting in an orbital period of 91 years.

With an estimated surface temperature of about 600 Kelvin (approximately 325 Degrees Celsius), Beta Pictoris d is much hotter than any of the gas planets in the Solar System. With an estimated age of 23 million years, the Beta Pictoris system is much younger. Hence, the planets are still warm from their formation and continue to radiate heat as they contract under their own weight. Eventually, Beta Pictoris d will cool down just like Jupiter and Saturn did. However, compared to many gas giants orbiting other stars, this planet is relatively cold and hence extremely faint compared to its host star.

Leveraging archival data to confirm the detection

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,” says Iain Hammond, who reviewed and analysed data from observations spanning an entire decade. “When we realised where to look for the potential new planet, it turned out it was hiding in the data all along.” This discovery is a textbook example of how an open data policy supports and improves scientific investigations.

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 Ben Sutlieff.

“This new planet is particularly special since it is part of a three-planet system, and in a group of stars with a very well-determined age,” says Elisabeth Matthews. “That provides a rare opportunity to compare atmospheres between planets in a controlled environment, and understand how the star they orbit impacts their formation and evolution. We found that the planet atmosphere looks strikingly similar to 51 Eri b, another planet in the same group of stars.”

Planet found – mystery solved

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) and its first-light instruments MICADO and METIS, as well as the Coronagraph Instrument on board NASA’s Nancy Grace Roman Space Telescope slated to be launched on 30 August this year. MPIA is heavily involved in all of these instruments, aiming to make considerable progress in discovering numerous new planets through direct imaging.

Additional Information

MPIA astronomers involved in this study were Elisabeth C. Matthews and Iain Hammond. The article reporting the results featured in this press release, co-led by B. J. Sutlieff (University of Edinburgh, United Kingdom) and M. J. Bonse (European Southern Observatory, Garching bei München, Germany and Max Planck Institute for Intelligent Systems, Tübingen, Germany), involves over 90 authors from around the world, including Belgium, France, Germany, Ireland, Italy, The Netherlands, Switzerland, The United Kingdom, and Chile.

The James Webb Space Telescope is the world’s leading observatory for space research. It is an international programme led by NASA and its partners ESA and CSA (Canadian Space Agency).

The Very Large Telescope (VLT), operated by the European Southern Observatory (ESO), is located at the Paranal Observatory in Chile's Atacama Desert and consists of four 8.2-meter Unit Telescopes that rank among the world's most advanced optical/infrared astronomical facilities. The European Southern Observatory (ESO) has its headquarters in Garching near Munich, Germany. It operates as an intergovernmental organization comprising 16 member states with Chile serving as the host state and Australia participating as a strategic partner.

This press release was adapted from the original release, which the European Southern Observatory published on the same day.

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