Young star with four gas giant planets baffles astronomers

A young star has a planetary system befitting one much older, creating questions about how planets form. Ben Lewis reports.

An artist's impression of the four large planets orbiting CI Tau.

An artist's impression of the four large planets orbiting CI Tau.

Amanda Smith, Institute of Astronomy

A two-million-year-old star has been found to have four gas giant planets in orbit around it, the first time that so many massive planets have been detected in such a young system. The discovery has raised new questions about how such a solar system forms and evolves.

The star, called CI Tau and located about 500 light-years away from Earth, was already interesting to astronomers because it contains the first so-called “hot Jupiter” – a massive planet orbiting very close to its parent – to have been discovered around a young star.

Now, using the Atacama Large Millimetre/submillimetre Array (ALMA) in Chile, a group of researchers led by Cathie Clarke from the University of Cambridge’s Institute of Astronomy in the UK has found evidence of three more gas giants. The research is published in The Astrophysical Journal Letters.

Images from ALMA revealed three distinct gaps in the protoplanetary disc of debris surrounding the star which, the researchers say, were most likely caused by the gravitational effects of planets as they orbit the young star.

Those orbits differ greatly: the closest is closer to CI Tau than Mercury is to the sun, while the outermost is more than 1000 times further out, creating a record for the largest range of orbits ever observed.

The two outer planets are about the mass of Saturn, while the two inner ones are respectively around one and 10 times the mass of Jupiter.

While around 1% of stars host hot Jupiters, most of them are hundreds of times older than CI Tau. It is not known if these other planetary systems share a similar extreme architecture, because their protoplanetary discs have long disappeared.

The researchers are also unsure whether the outer planets pushed the inner one into its ultra-close orbit, and whether this would explain hot Jupiters in general.

The formation of the Saturn-sized outer planets around such a young star is also a curious discovery, say Clarke and colleagues, because formation of gas planets of that size is usually a very slow process.

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