From Eos: “Hot White Dwarfs May Reveal Cold Gas Giants”

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From Eos

2 March 2020
Nola Taylor Redd

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.

“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 [The Astrophysical Journal Letters] 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.

“Difficult to Detect”

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.

Gravitational microlensing, S. Liebes, Physical Review B, 133 (1964): 835

A 2016 paper [The Astrophysical Journal] 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.

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Eos is the leading source for trustworthy news and perspectives about the Earth and space sciences and their impact. Its namesake is Eos, the Greek goddess of the dawn, who represents the light shed on understanding our planet and its environment in space by the Earth and space sciences.