An artist's impression of a gas giant orbiting a white dwarf star
In this artist's rendering, a gas giant orbits a white dwarf. The blast of high-energy photons it receives from the white dwarf evaporates its atmosphere, which is composed mainly of hydrogen, oxygen, and sulfur. Whereas much of the hydrogen is forced away from the white dwarf by its ultraviolet photons in a comet-like tail, oxygen and sulfur fall toward the white dwarf, forming the disk researchers detected. Credit: University of Warwick/Mark Garlick

The solar system’s giant planets may stain the dying Sun. New research suggests that gas giants like Jupiter may have their atmospheres evaporated by hot stars at the end of their lifetime. The trace elements could provide a new way for scientists to investigate some of the most difficult exoplanets to identify.

Roughly 4 billion years from now, the Sun will swell into a red giant before collapsing into a white dwarf, its fusion process finally spent. At first, the new hot white dwarf will emit radiation at temperatures of around 100,000 K. Over a few hundreds of millions of years, it will gradually cool down, placing it in the cool white dwarf category.

Both hot and cool white dwarfs have hydrogen-rich atmospheres, but scientists have long noticed other elements mixed on the surface. Researchers have identified the elements on cool white dwarfs as bits and pieces of rocky planets that collided with their dying star. But the source of the stains on hot white dwarf atmospheres has remained hidden.

“This idea…opens the potential to investigate the composition of extrasolar planetary atmospheres by looking at white dwarfs.”

“There has always been this mystery about why hot white dwarfs accrete different material” from their cooler counterparts, said Matthias Schreiber, a white dwarf researcher at the University of Valparaíso in Chile and the first author of a new paper proposing a solution: In its hottest stages, the white dwarf evaporates the atmospheres of giant planets, and the material is gravitationally pulled into a disk around the star.

The new idea does far more than reveal what the future Sun may look like to alien astronomers. It provides a way to identify how frequently gas giants orbit their stars at a distance, a metric difficult to study with current astronomical techniques. It also provides a tantalizing glimpse at the hard-to-study atmospheres of distant gas giants around other stars.

“This idea…opens the potential to investigate the composition of extrasolar planetary atmospheres by looking at white dwarfs,” Schreiber said.

Dying Stars, Distant Atmospheres

For decades, astronomers have puzzled over the presence of elements like carbon, sulfur, and phosphorus in hot white dwarfs. Although the lightweight hydrogen and helium atmospheres of white dwarfs should cause heavier material to sink to their cores, radiative pressure from the interior could struggle against gravity, occasionally tossing material left over from the star’s lifetime up into the atmosphere.

“But the abundances predicted by this mechanism never matched what was seen,” Schreiber said. “It has been a huge problem.”

Now Schreiber and his team propose that hot white dwarfs are consuming their larger, more gaseous worlds by siphoning off their atmospheres.

The new theory provides a way to probe the atmospheres of gas giants around other stars. The new method could be a “way to study the atmosphere of the giant planets that’s different or better than the ways people have now to study the atmospheres of planets,” said Ben Zuckerman, professor emeritus of astronomy at the University of California, Los Angeles. Zuckerman, who studies cool white dwarfs, was not part of the new research.

“The potential is there that we can use white dwarfs to understand or measure the composition of the atmospheres of gas giant planets,” Schreiber said.

“Difficult to Detect”

Evidence indicates that distant gas giants are more common than the largest exoplanet searches suggest and the solar system isn’t as unique as it seems.

Although thousands of exoplanets have been discovered, very few collections resemble the solar system. The first wave of exoplanets identified was gas giants that orbited their stars in days or hours. NASA’s Kepler telescope found a wealth of worlds, but because the mission lasted only 9 years, it couldn’t confirm exoplanets with longer orbits. Jupiter, by comparison, takes a dozen years to go around the Sun.

Microlensing, a process that involves measuring the increase in brightness of a “lensing” star as it passes in front of a source star, provides the closest glimpse of solar system–like gas giants. A 2016 paper suggested that roughly 60% of stars have gas giants orbiting at distances similar to those found in the solar system. But microlensing relies on the chance lineup of a planet-bearing star with a background object, and observations are rarely repeatable.

The new study notes that roughly 60% of hot white dwarfs are stained by heavy elements. “That combines nicely with the statistics from microlensing,” Schreiber said. Together the two lines of evidence indicate that distant gas giants are more common than the largest exoplanet searches suggest and the solar system isn’t as unique as it currently appears.

The study was published in Astrophysical Journal Letters.

—Nola Taylor Redd (@NolaTRedd), Science Writer


Tillman, N. T. (2020), Hot white dwarfs may reveal cold gas giants, Eos, 101, Published on 02 March 2020.

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