"It was one of those serendipitous discoveries," says researcher Boris Gänsicke of the University of Warwick (UK), who led the study published today in Nature. The team had studied about 7000 white dwarfs observed by the Sloan Digital Sky Survey and discovered that one was different from the others. By analyzing subtle variations in the star's light, they found traces of chemical elements in amounts that scientists had never before observed in a white dwarf. "We knew there had to be something exceptional about this system, and we speculated that it might be related to some kind of planetary remnant."
To learn more about the properties of this unusual star, called WDJ0914+1914, the team analyzed it with the X-shooter instrument on ESO's Very Large Telescope (VLT) in Chile's Atacama Desert. These follow-up observations confirmed the presence of hydrogen, oxygen and sulfur associated with the white dwarf. By studying the fine details in the spectra taken by ESO's X-shooter, the team discovered that these elements were in a disk of gas swirling toward the white dwarf and did not come from the star itself.
"It took several weeks of work to come to the conclusion that the only way to make such a disk is the evaporation of a giant planet," said Matthias Schreiber of the University of Valparaiso in Chile, who computed the past and future evolution of this system. The detected amounts of hydrogen, oxygen and sulfur are similar to those found in the deep atmospheric layers of icy giant planets such as Neptune and Uranus. If such a planet were orbiting near a hot white dwarf, the star's extreme ultraviolet radiation would strip it of its outer layers and some of this stripped gas would swirl into a disk, accreting onto the white dwarf. This is what scientists believe they are seeing around WDJ0914+1914: the first evaporating planet orbiting a white dwarf.
By combining observational data with theoretical models, the team of astronomers from the UK, Chile and Germany was able to define a clearer picture of this unique system. The white dwarf is small and extremely hot: 28,000 degrees Celsius (five times the temperature of the Sun). In contrast, the planet is icy and large, at least twice as big as the star. Since it orbits the hot white dwarf at close range, making its full orbit in just 10 days, the star's high-energy photons are gradually stripping away its atmosphere. Most of the gas escapes, but some is trapped in a disk that swirls around the star at a speed of 3,000 tons per second. It is this disk that makes the Neptune-like planet visible, which would otherwise remain hidden.
"This is the first time we can measure the amounts of gases such as oxygen and sulfur in the disk, which provides clues about the composition of exoplanet atmospheres," says Odette Toloza of the University of Warwick, who developed a model for the gas disk surrounding the white dwarf.
"The discovery also opens a new window to learn more about the ultimate fate of planetary systems," adds Gänsicke.
Stars like our Sun burn hydrogen in their cores for most of their lives. Once they run out of this fuel, they swell up, becoming red giant stars, hundreds of times larger and engulfing nearby planets. In the case of the Solar System, this will include Mercury, Venus and even Earth, which will be consumed by the red giant Sun in about 5 billion years. In the end, Sun-like stars lose their outer layers, leaving behind only a dying core, a white dwarf. These stellar remnants can still harbor planets and it is believed that there are many such star systems in our galaxy. However, until now, scientists have never found evidence of a surviving giant planet around a white dwarf. The detection of an exoplanet orbiting WDJ0914+1914, located about 1,500 light-years away in the constellation Cancer, may be the first of many to orbit such stars.
According to the research team, the exoplanet discovered with the help of ESO's X-shooter instrument orbits the white dwarf at a distance of only 10 million kilometers, or 15 times the solar radius, indicating that in the past it must have been submerged in the depths of the red giant star. The unusual position of the planet implies that, sometime after the host star became a white dwarf, the planet moved closer to it. Astronomers believe that this new orbit could be the result of gravitational interactions with other planets in the system, meaning that more than one planet may have survived the violent transition from its host star.
"Until recently, very few astronomers stopped to think about the fate of planets orbiting dying stars. This discovery of a planet orbiting near a finished stellar core firmly demonstrates that the Universe is challenging our minds again and again, pushing us to go beyond our established ideas," concludes Gänsicke.
Additional information
This research work has been presented in a scientific paper published in the journal Nature. The team consists of: Boris Gänsicke (Department of Physics & Centre for the Study of Exoplanets and Habitability, University of Warwick, UK); Matthias Schreiber (Institute of Physics and Astronomy, Millennium Nucleus for the Study of Planetary Formation, University of Valparaiso, Chile); Odette Toloza (Department of Physics, University of Warwick, United Kingdom); Nicola Gentile Fusillo (Department of Physics, University of Warwick, United Kingdom); Detlev Koester (Institute for Theoretical Physics and Astrophysics, University of Kiel, Germany); and Christopher Manser (Department of Physics, University of Warwick, United Kingdom).
ESO is the leading intergovernmental astronomical organization in Europe and the most productive astronomical observatory in the world. It has sixteen member countries: Austria, Belgium, Czech Republic, Denmark, Finland, France, Germany, Ireland, Italy, the Netherlands, Poland, Portugal, Spain, Sweden, Switzerland and the United Kingdom, together with Chile, the host country, and Australia as a strategic ally. ESO has an ambitious program focused on the design, construction and operation of powerful ground-based observing facilities that enable astronomers to make important scientific discoveries. ESO also plays an important role in promoting and organizing cooperation in astronomical research. ESO operates three unique observing facilities in Chile: La Silla, Paranal and Chajnantor. At Paranal, ESO operates the Very Large Telescope together with its VLTI (Very Large Telescope Interferometer), the most advanced interferometer in the world, as well as two survey telescopes: VISTA (Optical and Infrared Survey Telescope for Astronomy), which works in the infrared, and the VST (VLT Survey Telescope), which surveys in visible light. ESO is also a partner in two facilities at Chajnantor, APEX and ALMA, currently the largest operating astronomical project in the world. Finally, at Cerro Armazones, near Paranal, ESO is building the 39-meter ELT (Extremely Large Telescope), which will become "the world's biggest eye on the sky".
Translations of ESO press releases are carried out by members of the ESO Science Outreach Network (ESON), which includes outreach experts and science communicators from all ESO member countries and other nations.
The Spanish node of the ESON network is represented by J. Miguel Mas Hesse and Natalia Ruiz Zelmanovitch.
Source: European Southem Observatory
Retrieved from https://www.eso.org/public/chile/news/eso1919/
Links
Scientific article: https://www.eso.org/public/archives/releases/sciencepapers/eso1919/eso1919a.pdf
VLT photos: https://www.eso.org/public/images/archive/category/paranal/
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