Astronomers have identified a colossal exoplanet, measuring between three and ten times the size of Jupiter, concealed behind the swirling disc of gas and dust encircling a nascent star.

Previous investigations of the star MP Mus indicated that it existed in isolation, devoid of any orbiting planets, encircled by a uniform cloud of gas and dust.

However, a further analysis of MP Mus using a combination of data from the European Space Agency’s Gaia mission and the Atacama Large Millimeter/submillimeter Array (ALMA) suggests that the star is not alone after all.

A multinational team of astronomers, spearheaded by the University of Cambridge, identified a substantial gas giant within the star’s protoplanetary disc, a flattened cloud of gases, dust, and ice where planetary formation commences. Gaia has found an exoplanet within a protoplanetary disc for the first time. The findings, published in the journal Nature Astronomy, indicate that analogous techniques may be beneficial in the search for nascent planets orbiting other stars.

Investigating the formation of planets within protoplanetary discs surrounding nascent stars enables researchers to gain insights into the evolution of our Solar System. Via a mechanism termed core accretion, gravitational forces induce particles inside the disc to adhere to one another, ultimately resulting in the formation of bigger solid entities such as asteroids or planets. As nascent planets develop, they begin to create indentations in the disc, akin to grooves on a vinyl record.

Nonetheless, viewing these nascent planets is highly difficult due to the obstruction caused by the gas and dust in the disc. As of yet, there have been only three substantial detections of nascent planets within a protoplanetary disc.

Dr. Álvaro Ribas, a researcher at Cambridge’s Institute of Astronomy, specializes in the study of 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,” he said.

In 2023, Ribas utilised ALMA to investigate the protoplanetary disc surrounding MP Mus (PDS 66). The findings indicated a solitary young star in the cosmos. The surrounding disc exhibited no gaps indicative of planetary formation and was entirely flat and devoid of features.

“Our earlier observations showed a boring, flat disc,” said 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.”

Combining ALMA and Gaia to Uncover the Unexpected

Ribas and his colleagues from Germany, Chile, and France have provided MP Mus with another opportunity. Utilising ALMA once more, scientists examined the star at the 3mm wavelength, which is longer than previous observations, enabling a more profound investigation of the disc.

The recent observations revealed a cavity near the star and two more gaps at a greater distance, previously concealed in earlier analyses, indicating that MP Mus may not be solitary after all.

Simultaneously, Miguel Vioque, a researcher at the European Southern Observatory, was revealing another component of the enigma. Utilising data from Gaia, he discovered that MP Mus was exhibiting instability.

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

By integrating Gaia and ALMA measurements with computational modelling, the researchers propose that the observed wobble is likely induced by a gas giant, possessing less than ten times Jupiter’s mass, orbiting the star at a distance ranging from one to three astronomical units.

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

A New Method for Detecting Hidden Planets

This is the first indirect finding of an exoplanet in a protoplanetary disc, made by combining detailed Gaia stellar motion data with widespread disc observations. This suggests that there might be many hidden planets on other discs just waiting to be found.

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

Ribas asserts that forthcoming enhancements to ALMA, along with future observatories like the next generation Very Large Array (ngVLA), may provide deeper investigations into more discs and enhance our comprehension of the concealed population of nascent planets, thereby aiding in our understanding of the formation of our own planet.

The research received partial funding from the European Union’s Horizon Programme, the European Research Council, and the UK Science and Technology Facilities Council (STFC), which is part of UK Research and Innovation (UKRI).

Original Publication
Authors: Álvaro Ribas, Miguel Vioque, Francesco Zagaria, Cristiano Longarini, Enrique Macías, Cathie J. Clarke, Sebastián Pérez, John Carpenter, Nicolás Cuello and Itziar de Gregorio-Monsalvo.
Journal: Nature Astronomy
DOI: 10.1038/s41550-025-02576-w
Article Title: A young gas giant and hidden substructures in a protoplanetary disc
Article Publication Date: 14-Jul-2025