Artist’s impression of the WDJ0914+1914 system
Location of WDJ0914+1914 in the constellation of Cancer
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ESOcast 212 Light: First Giant Planet around White Dwarf Found
Artist’s animation of the WDJ0914+1914 system
Artist’s animation of the Sun becoming a red giant
ESO observations indicate the Neptune-like exoplanet is evaporating
Researchers using ESO’s Very Large
Telescope have, for the first time, found evidence of a giant planet
associated with a white dwarf star. The planet orbits the hot white
dwarf, the remnant of a Sun-like star, at close range, causing its
atmosphere to be stripped away and form a disc of gas around the star.
This unique system hints at what our own Solar System might look like in
the distant future.
“It was one of those chance discoveries,” says researcher Boris Gänsicke, from the University of Warwick in the UK, who led the study, published today in Nature. The team had inspected around 7000 white dwarfs observed by the Sloan Digital Sky Survey
and found one to be unlike any other. By analysing subtle variations in
the light from the star, they found traces of chemical elements in
amounts that scientists had never before observed at a white dwarf. “We
knew that there had to be something exceptional going on in this
system, and speculated that it may be related to some type of planetary
remnant.”
To get a better idea of the properties of this unusual star, named WDJ0914+1914, the team analysed it with the X-shooter instrument on ESO’s Very Large Telescope
in the Chilean Atacama Desert. These follow-up observations confirmed
the presence of hydrogen, oxygen and sulphur 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 disc of gas
swirling into the white dwarf, and not coming from the star itself.
“It took a few weeks of very hard thinking to figure
out that the only way to make such a disc is the evaporation of a giant
planet,” says Matthias Schreiber from the University of Valparaiso
in Chile, who computed the past and future evolution of this system.
The detected amounts of hydrogen, oxygen and sulphur are
similar to those found in the deep atmospheric layers of icy, giant
planets like Neptune and Uranus. If such a planet were orbiting close to
a hot white dwarf, the extreme ultraviolet radiation from the star
would strip away its outer layers and some of this stripped gas would
swirl into a disc, itself accreting onto the white dwarf. This is what
scientists think they are seeing around WDJ0914+1914: the first
evaporating planet orbiting a white dwarf.
Combining observational data with theoretical models, the
team of astronomers from the UK, Chile and Germany were able to paint a
clearer image of this unique system. The white dwarf is small and, at a
blistering 28 000 degrees Celsius (five times the Sun's temperature),
extremely hot. By contrast, the planet is icy and large—at least twice
as large as the star. Since it orbits the hot white dwarf at close
range, making its way around it in just 10 days, the high-energy photons
from the star are gradually blowing away the planet's atmosphere. Most
of the gas escapes, but some is pulled into a disc swirling into the
star at a rate of 3000 tonnes per second. It is this disc that makes the
otherwise hidden Neptune-like planet visible.
“This is the first time we can measure the amounts of
gases like oxygen and sulphur in the disc, which provides clues to the
composition of exoplanet atmospheres,” says Odette Toloza from the University of Warwick, who developed a model for the disc of gas surrounding the white dwarf.
“The discovery also opens up a new window into the final 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 puff up into red
giants, becoming 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 all be consumed by the red-giant Sun in about 5
billion years. Eventually, Sun-like stars lose their outer layers,
leaving behind only a burnt-out core, a white dwarf. Such stellar
remnants can still host planets, and many of these star systems are
thought to exist in our galaxy. However, until now, scientists had never
found evidence of a surviving giant planet around a white dwarf. The
detection of an exoplanet in orbit around WDJ0914+1914, located about
1500 light years away in the constellation of Cancer, may be the first
of many orbiting such stars.
According to the researchers, the exoplanet now found with
the help of ESO’s X-shooter orbits the white dwarf at a distance of only
10 million kilometres, or 15 times the solar radius, which would have
been deep inside the red giant. The unusual position of the planet
implies that at some point after the host star became a white dwarf, the
planet moved closer to it. The 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 its host
star’s violent transition.
“Until recently, very few astronomers paused to ponder
the fate of planets orbiting dying stars. This discovery of a planet
orbiting closely around a burnt-out stellar core forcefully demonstrates
that the Universe is time and again challenging our minds to step
beyond our established ideas,” concludes Gänsicke.
More Information
This research was presented in a paper to appear in Nature.
The team is composed of Boris Gänsicke (Department of
Physics & Centre for Exoplanets and Habitability, University of
Warwick, UK), Matthias Schreiber (Institute of Physics and Astronomy,
Millennium Nucleus for Planet Formation, Valparaiso University, Chile),
Odette Toloza (Department of Physics, University of Warwick, UK), Nicola
Gentile Fusillo (Department of Physics, University of Warwick, UK),
Detlev Koester (Institute for Theoretical Physics and Astrophysics,
University of Kiel, Germany), and Christopher Manser (Department of
Physics, University of Warwick, UK).
ESO is the foremost intergovernmental astronomy organisation in
Europe and the world’s most productive ground-based astronomical
observatory by far. It has 16 Member States: Austria, Belgium, the Czech
Republic, Denmark, France, Finland, Germany, Ireland, Italy, the
Netherlands, Poland, Portugal, Spain, Sweden, Switzerland and the United
Kingdom, along with the host state of Chile and with Australia as a
Strategic Partner. ESO carries out an ambitious programme focused on the
design, construction and operation of powerful ground-based observing
facilities enabling astronomers to make important scientific
discoveries. ESO also plays a leading role in promoting and organising
cooperation in astronomical research. ESO operates three unique
world-class observing sites in Chile: La Silla, Paranal and Chajnantor.
At Paranal, ESO operates the Very Large Telescope and its world-leading
Very Large Telescope Interferometer as well as two survey telescopes,
VISTA working in the infrared and the visible-light VLT Survey
Telescope. Also at Paranal ESO will host and operate the Cherenkov
Telescope Array South, the world’s largest and most sensitive gamma-ray
observatory.
ESO is also a major partner in two facilities on
Chajnantor, APEX and ALMA, the largest astronomical project in
existence. And on Cerro Armazones, close to Paranal, ESO is building the
39-metre Extremely Large Telescope, the ELT, which will become “the
world’s biggest eye on the sky”.
Links
Contacts
Boris Gänsicke
University of Warwick
UK
Tel: +44 247 657 4741
Matthias Schreiber
Valparaiso University
Chile
Tel: +56 32 299 5518
Email: matthias.schreiber@uv.cl
Odette Toloza
University of Warwick
UK
Email: odette.toloza@warwick.ac.uk
Nicola Gentile Fusillo (study co-author)
European Southern Observatory and University of Warwick
Germany
Tel: +49 8932 0067 50
Cell: +44 7476 9595 49
Email: ngentile@eso.org
Christopher Manser (study co-author)
University of Warwick
UK
Tel: +44 7516 8167 53
Email: c.manser@warwick.ac.uk
Bárbara Ferreira
ESO Public Information Officer
Garching bei München, Germany
Tel: +49 89 3200 6670
Email: pio@eso.org
Source: ESO/News