Institute of Astronomy

The death of a planet nursery?

Published on 22/09/2016 

Planetary disc around the star known as TW Hydrae. Source: S. Andrews (Harvard-Smithsonian CfA); B. Saxton (NRAO/AUI/NSF); ALMA (ESO/NAOJ/NRAO)

The dusty disc surrounding the star TW Hydrae exhibits circular features that may signal the formation of protoplanets. Cambridge astronomer Giovanni Rosotti is one of a team of astronomers led by LMU astrophysicist Barbara Ercolano who argue that the innermost feature actually points to the impending dispersal of the disc.

When the maps appeared at the end of March, experts were electrified. The images revealed an orange-red disk pitted with circular gaps that looked like the grooves in an old-fashioned long-playing record. But this was no throwback to the psychedelic Sixties. It was a detailed portrait of a so-called protoplanetary disc, made up of gas and dust grains, associated with a young star – the kind of structure out of which planets could be expected to form. Not only that, the maps showed that the disc around the star known as TW Hydrae exhibits several clearly defined gaps. Astronomers speculated that these gaps might indicate the presence of protoplanets, which had pushed away the material along their orbital paths. And to make the story even more seductive, one prominent gap is located at approximately the same distance from TW Hydrae as Earth is from the Sun – raising the possibility that this putative exoplanet could be an Earth-like one.

The international team  has compared the new observations with theoretical models of planet formation. The study indicates that the prominent gap in the TW Hydrae system is unlikely to be due to the action of an actively accreting protoplanet. Instead, the team attributes the feature to a process known as photoevaporation. Photoevaporation occurs when the intense radiation emitted by the parent star heats the gas, allowing it to fly away from the disc.  Dr. Rosotti is a member of a leading world group studying this process at the Institute of Astronomy and took care of the photo-evaporation model used in this study. But although hopes of a new exo-Earth orbiting in the inner gap of TW Hydrae may themselves have evaporated, the system nevertheless provides the opportunity to observe the dissipation of a circumstellar disc in unprecedented detail.

The dusty disc that girdles TW Hydrae has long been a favored object of observation. The star lies only 175 light-years from Earth, and is it relatively young (around a million years old). Moreover, the disc is oriented almost perpendicular to our line of sight, affording an ideal view of its structure. The spectacular images released in March were made with the Atacama Large Millimeter/submillimeter Array (ALMA), an array of detectors in the desert of Northern Chile. Together, they form a radiotelescope with unparalleled resolving power that can detect the radiation from dust grains in the millimeter size range.

Photoevaporation is one of the major forces that shape the fate of circumstellar discs. Not only can it destroy such discs --which typically have a life expectancy of around 10 million years -- it can also stop young planets being drawn by gravity and by the interaction with the surrounding disc gas into their parent star. The gaps caused by the action of photoevaporation on the disc, park the planets at their location by removing the gas, allowing the small dusty clumps to grow into fully fledged planets and steering them into stable orbits. However, in the case of the TW Hydrae system, the astronomers believe that the inner gap revealed by the ALMA maps is not caused by a planet, but represents an early stage in the dissipation of the disc. This view is based on the fact that many characteristic features of the disc around TW Hydrae, such as the distance between the gap and the star, the overall mass accretion rate, and the size and density distributions of the particles, are in very good agreement with the predictions of her photoevaporation model.

This work has been partially supported by a grant from the European Research Council.
published paper in MNRAS 2016

Local contact: Dr Giovanni Rosotti, Institute of Astronomy, Cambridge University

LMU press release

Page last updated: 22 September 2016 at 17:05