Evaporating Neptune-like giant exoplanet found orbiting dying star
Astronomers have discovered a massive icy planet that is evaporating as it orbits the super-hot remains of a Sun-like star. According to the researchers, this is the first time that evidence of a huge planet orbiting such a star has been observed. The discovery provides a rare window into the fate of our own solar system, billions of years into the future.
Yellow dwarf stars like our Sun create massive amounts of energy by combining hydrogen atoms to form helium atoms at its core, through a process called thermonuclear fusion. In about 5 billion years our Sun will have exhausted its supply of hydrogen, and begin fusing helium atoms instead.
At this point our yellow dwarf will transform into a majestic red giant. During this phase of its existence the star will shine 2,000 times brighter than it does today, and will grow so massive that it could consume Mercury, Venus and Earth. Eventually, our once-nurturing star will shed its outer layers, leaving behind a small, slowly dying core, known as a white dwarf.
Now, astronomers have discovered what they believe to be the first indirect evidence of a massive planet that has survived its star’s chaotic red giant phase, and is currently orbiting a white dwarf.
The international team behind the study examined 7,000 white dwarfs that were observed as part of the Sloan Digital Sky Survey. The light signature of one star – called WDJ0914+1914 – was found to contain trace elements of chemicals that had never before been associated with a white dwarf.
In an attempt to unravel the mysteries of WDJ0914+1914, the astronomers turned to the European Southern Observatory’s Very Large Telescope (VLT) located in the Atacama Desert. The VLT turned its gaze on the strange white dwarf, and with the help of a powerful spectrograph instrument called X-shooter created a detailed spectrum, or breakdown, of the star’s light. By analyzing this spectrum, and searching for black bars called "absorption lines," scientists are able to determine what chemicals are present in or around a star.
The followup observations confirmed the presence of hydrogen, oxygen and sulfur embedded in WDJ0914+1914’s starlight. According to the new paper, the unexpected material was not a part of the star itself, but rather was located in a disk of gas that was spiralling into the white dwarf.
The amounts of the gasses detected in the spectra were discovered to be similar to those found in the atmospheres of massive icy planets such as Neptune and Uranus. If such a planet were to orbit the white dwarf, its atmosphere would be slowly evaporated by the intense ultraviolet radiation pouring forth from the white dwarf. This would explain the presence of the gaseous disk.
The researchers then combined their data with theoretical models in order to figure out just what is transpiring around WDJ0914+1914. According to their experiments, the icy planet is likely to be at least twice as large as the tiny star. This icy giant travels at a distance of just 10 million km (6 million miles) from the super-hot surface of the white dwarf, and completes a full orbit in just 10 days. For reference, Earth travels around the Sun at an average distance of 150 million km (93 million miles).
It is estimated that roughly 3,000 tonnes (3,307 US tons) of disrupted planetary material falls into the white dwarf each second.
The very presence of such a planet orbiting so close to its star raises questions of its own. The gas giant could not have been created in that orbit and simply remained there, as it would have been destroyed when its parent star bloated outward during its red giant phase.
Instead, the authors of the new study believe, the planet could have formed much farther out, and once WDJ0914+1914 had shrunk to a white dwarf, been pushed inward by another body’s gravitational influence. This theory suggests that at least one other planet must have survived the violent transformation of its star, and is lurking in the outer reaches of the solar system undetectable to modern telescopes.
The observations made of WDJ0914+1914 provide a window into the fate of our own solar system at a time when our Sun has made the tumultuous transition from yellow dwarf, to red giant and finally, into a dying white dwarf. Perhaps then Neptune will keep vigil, slowly evaporating as our exhausted star inexorably cools over the next billion years.
The study has been published in the journal Nature.