First UV/X-ray study of star PDS 70 offers insight into planetary formation

A Leicester-led team of astronomers, working as part of an international consortium, have made a breakthrough discovery which changes our understanding of the timescale and process of planetary formation.

PDS 70 is a nearby young star – 370 light years away, about 5 million years old - with an accretion disc at an angle of about 45°–50° when viewed from Earth which makes it one of the few stars where planetary formation can actually be seen happening. The system has two proto-planets, PDS 70 b at an orbital distance similar to Uranus, and PDS 70 c a bit further out than Neptune. There is even a suggestion of a smaller disc around PDS 70 b which could be forming proto-moons.

The Leicester team used the Neil Gehrels Swift satellite to observe PDS 70. Launched in 2004 and originally designed for studying gamma-ray bursts, Swift has both an X-ray telescope – with a camera built at the University of Leicester – and an ultraviolet/optical telescope (plus a gamma ray detector). The team pointed both these telescopes at PDS 70 on five occasions in July and August this year. This is the first time that PDS 70 has been observed using UV wavebands.

Swift also performed the first detailed examination of PDS 70 at X-ray wavelengths, allowing the temperature structure of the hottest region of the star’s outer atmosphere, the corona, to be measured, where the gas reaches several million degrees C

Material from a disc accreting onto a star emits UV radiation as it is heated up to about 10,000 degrees. However, the Leicester team’s research showed surprisingly low UV luminosity which suggests that accretion isn’t happening in the PDS 70 system and the observed UV radiation probably originates in the corona of the star itself. So where is the disc material going? The answer would seem to be that it’s dissipating into space due to high-energy radiation from the star, a process known as photoevaporation.

If photoevaporation rather than accretion is the main process breaking down the disc, then the whole system may be evolving much faster than previously believed and PDS 70 could be a fully functioning solar system in under a million years.

Dr Simon Joyce, a postdoctoral researcher in our School of Physics and Astronomy who led the project, said: “If the accretion phase has already ended and photoevaporation is as rapid as we predict, then PDS 70 could be a fully functioning solar system in less than one million years.

“When you consider that our solar system is 4.6 billion years old, we are seeing PDS 70 during the first years of its life in human terms, yet these observations show that the planet formation process is already nearly finished.

“These observations help to tell us how long planets take to form and are a fascinating demonstration of how our own solar system came to be the way it is. The University of Leicester’s continuing dedication to building and operating space instruments is making discoveries about the origin of life possible, even though they were originally designed for a very different purpose.”

The project also involved Dr John Pye, Dr Jonathan Nichols, Dr Kim Page and Professor Richard Alexander from Leicester as well as colleagues from the University of Vienna. It was conducted within the remit of the Exoplanets-A consortium, a collaboration between Leicester, Vienna, UCL and universities in France, Spain, Germany and the Netherlands. The consortium is EU-funded through the Horizon 2020 research and innovation programme. The research was published last month in Monthly Notices of the Royal Astronomical Society: Letters.

• A Swift view of X-ray and UV radiation in the planet-forming T Tauri system PDS 70. doi:10.1093/mnrasl/slz169