Massive hidden planet found orbiting young star

Astronomers thought the young star MP Mus had no planet in sight. But a closer look revealed a giant planet hidden in the surrounding gas and dust – one they had completely missed.

A team of scientists from the University of Cambridge, along with colleagues from Germany, Chile, and France, revisited MP Mus using the Atacama Large Millimeter/submillimeter Array (ALMA) and data from the European Space Agency’s Gaia mission.

The findings point to a gas giant planet – anywhere from three to ten times the size of Jupiter – lurking inside the disc. This is the first time Gaia has helped identify a planet still embedded in its protoplanetary disc.

Planets hidden inside their discs

Studying young planets like this gives us important clues about how our own Solar System might have formed. When stars are young, they’re surrounded by discs of gas, dust, and ice.

Over time, gravity pulls these particles together to form larger bodies like asteroids and planets. As planets take shape, they start to clear paths in the disc – like grooves in a vinyl record.

But spotting those young planets is tough. Gas and dust interfere with most observations. So far, scientists have only made three solid detections of young planets still sitting inside their discs. This discovery adds a fourth.

Signs of a hidden planet

The study was led by Dr Álvaro Ribas from Cambridge’s Institute of Astronomy. He has spent years studying protoplanetary discs.

“We first observed this star at the time when we learned that most discs have rings and gaps, and I was hoping to find features around MP Mus that could hint at the presence of a planet or planets,” said Dr. Ribas.

In 2023, his team used ALMA to observe MP Mus, also known as PDS 66. What he saw was underwhelming. The star looked alone. Its disc showed no gaps, no ridges – just a flat, lifeless expanse.

“Our earlier observations showed a boring, flat disc,” said Dr. Ribas. “But this seemed odd to us, since the disc is between seven and ten million years old. In a disc of that age, we would expect to see some evidence of planet formation.”

So the team tried again. They used ALMA to look at the star at a longer wavelength – 3mm – which allowed them to see deeper into the disc. This time, the picture changed.

The experts spotted a cavity near the star, plus two distant gaps. These features had been invisible in the first set of images. It was the first hint that a planet might be shaping the disc from the inside.

A star that wobbles

At the same time, another researcher was noticing something strange. Miguel Vioque, a scientist at the European Southern Observatory, was analyzing Gaia data when he noticed MP Mus wasn’t sitting still.

“My first reaction was that I must have made a mistake in my calculations, because MP Mus was known to have a featureless disc,” said Vioque.

“I was revising my calculations when I saw Álvaro give a talk presenting preliminary results of a newly-discovered inner cavity in the disc, which meant the wobbling I was detecting was real and had a good chance of being caused by a forming planet.”

A wobbling star is a classic clue. When a planet orbits a star, its gravity pulls slightly on the star, causing a small but measurable motion. Combine that motion with the new disc structures seen by ALMA, and things start to add up.

Indirect observations of new planets

The team used computer models to pull the clues together. What they found fits the pattern of a gas giant orbiting between one and three times the distance between Earth and the Sun.

“Our modeling work showed that if you put a giant planet inside the new-found cavity, you can also explain the Gaia signal,” said Dr. Ribas. “And using the longer ALMA wavelengths allowed us to see structures we couldn’t see before.”

This marks the first time a planet has been indirectly discovered inside a protoplanetary disc by combining data from both ALMA and Gaia. It opens the door to new possibilities. If other discs hide their planets just as well, these tools together might help reveal them.

Revealing more hidden planets

“We think this might be one of the reasons why it’s hard to detect young planets in protoplanetary discs, because in this case, we needed the ALMA and Gaia data together,” said Dr. Ribas.

“The longer ALMA wavelength is incredibly useful, but to observe at this wavelength requires more time on the telescope.”

Dr. Ribas noted that upgrades to ALMA and the upcoming next-generation Very Large Array (ngVLA) will allow scientists to peer deeper into these dusty discs and possibly uncover many more hidden planets.

The full study was published in the journal Nature Astronomy.

Image Credit: ALMA(ESO/NAOJ/NRAO)/A. Ribas et al.

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