Astronomers spot a rare planet orbiting twin stars with a 300-year-long year

The discovery of the planet HD 143811 AB b arose not from new telescope data but from a thorough analysis of archival observations by Jason Wang, assistant professor of physics and astronomy, and his team at Northwestern University.

They revisited data collected over eight years by the Gemini South telescope using its Gemini Planet Imager (GPI), which is designed to minimise starlight interference to detect faint planets. Initially, they had low expectations for new findings, having recorded only one new planet from over 500 stars during GPI’s operational span.

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However, upon reviewing data from 2016 to 2019 and comparing it with follow-up observations from the W M Keck Observatory, researchers noticed a faint object’s consistent motion that corresponded with its host star, a key indicator of a planet. The team confirmed the object’s light signature was planetary, validating their findings alongside an independent conclusion from researchers at the University of Exeter, highlighting the challenges in detecting such distant worlds.

A giant, young, and unusually situated planet

Further analysis revealed the planet is a gas giant roughly six times Jupiter’s size, still retaining some of the heat from its formation. Despite being about 13 million years old, astronomically speaking, it is a newborn.

The planet’s extreme orbit is what makes it especially intriguing. Although it sits just six times closer to its binary stars than any previously imaged planet in a similar system, the distance is still vast enough that it requires three centuries to complete a single loop.

The host stars themselves whirl around each other rapidly once every 18 Earth days. How such a massive planet settled into this peculiar configuration remains an open question.

“Exactly how it works is still uncertain,” Wang said. “Because we have only detected a few dozen planets like this, we don’t have enough data yet to put the picture together.”

For Wang, the value of the discovery lies in its potential to reveal the complex mechanics of three bodies moving through space together. “Of the 6,000 exoplanets that we know of, only a very small fraction of them orbit binaries,” he said. “Imaging both the planet and the binary is interesting because it’s the only type of planetary system where we can trace both the orbit of the binary star and the planet in the sky at the same time.”

His team hopes to continue tracking the system as GPI undergoes a major upgrade before being installed at Gemini North in Hawaii as GPI 2.0. Team member Nathalie Jones has already requested more telescope time to monitor how the orbits evolve. “We want to track the planet and monitor its orbit, as well as the orbit of the binary stars,” Jones said.

More planets hiding in old data?

Jones and Wang are also diving further into archival observations, searching for additional missed planets. “There are a couple of suspicious objects, but what they are, exactly, remains to be seen,” Jones added.

The team’s findings were published on December 11 in The Astrophysical Journal Letters, marking a significant step forward in understanding how worlds might form and survive in systems with dual suns just like the one imagined in a galaxy far, far away.