It looked like nothing—then scientists found a world 10x the size of Jupiter

Earlier observations of this star, called MP Mus, suggested that it was all alone without any planets in orbit around it, surrounded by a featureless cloud of gas and dust.

However, a second look at MP Mus, using a combination of results from the Atacama Large Millimeter/submillimeter Array (ALMA) and the European Space Agency's Gaia mission, suggest that the star is not alone after all.

The international team of astronomers, led by the University of Cambridge, detected a large gas giant in the star's protoplanetary disc: the pancake-like cloud of gases, dust and ice where the process of planet formation begins. This is the first time that Gaia has detected an exoplanet within a protoplanetary disc. The results, reported in the journal Nature Astronomy, suggest that similar methods could be useful in the hunt for young planets around other stars.

By studying how planets form in the protoplanetary discs around young stars, researchers can learn more about how our own Solar System evolved. Through a process known as core accretion, gravity causes particles in the disc to stick to each other, eventually forming larger solid bodies like asteroids or planets. As young planets form, they start to carve gaps in the disc, like grooves on a vinyl record.

However, observing these young planets is extremely challenging, due to the interference from the gas and dust in the disc. To date, only three robust detections of young planets in a protoplanetary disc have been made.

Dr Álvaro Ribas from Cambridge's Institute of Astronomy, who led the research, specialises in 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," he said.

Using ALMA, Ribas observed the protoplanetary disc around MP Mus (PDS 66) in 2023. The results showed a young star seemingly all alone in the universe. Its surrounding disc showed none of the gaps where planets might be forming, and was completely flat and featureless.

"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."

Now, Ribas and his colleagues from Germany, Chile, and France have given MP Mus another chance. Once again using ALMA, they observed the star at the 3mm range, a longer wavelength than the earlier observations, allowing them to probe deeper into the disc.

The new observations turned up a cavity close to the star and two gaps further out, which were obscured in the earlier observations, suggesting that MP Mus may not be alone after all.

At the same time, Miguel Vioque, a researcher at the European Southern Observatory, was uncovering another piece of the puzzle. Using data from Gaia, he found MP Mus was 'wobbling'.

"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."

Using a combination of the Gaia and ALMA observations, along with some computer modelling, the researchers say the wobbling is likely caused by a gas giant - less than ten times the mass of Jupiter - orbiting the star at a distance between one and three times the distance of the Earth to the Sun.

"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."

This is the first time an exoplanet embedded in a protoplanetary disc has been indirectly discovered in this way - by combining precise star movement data from the Gaia with deep observations of the disc. It also means that many more hidden planets might exist in 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 says that upcoming upgrades to ALMA, as well as future telescopes such as the next generation Very Large Array (ngVLA), may be used to look deeper into more discs and better understand the hidden population of young planets, which could in turn help us learn how our own planet may have formed.

The research was supported in part by the European Union's Horizon Programme, the European Research Council, and the UK Science and Technology Facilities Council (STFC), part of UK Research and Innovation (UKRI).