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Scientists Get First Glimpse of Small Planet Orbiting White Dwarf

artist's rendition of planetesimal orbiting white dwarf
Artist's conception of a planetary fragment orbiting a white dwarf star, leaving a trail of gas in its wake. | University of Warwick/Mark Garlick

The first small, intact planetary body orbiting a white dwarf has been discovered, scientists report. The new findings, published in the April 4 issue of Science, shed light on the twilight years of planetary systems and could help other researchers identify similar objects around other white dwarfs, which are the leftover remnants of a long-dead star.

To date, scientists have identified about 3,000 stars that host one or more planets. Most of these stars are main sequence stars similar to the Sun, meaning they have a lifespan ranging from tens of millions of years to over 100 billion years, depending on their mass.

When main sequence stars run out of fuel, they undergo an event known as a planetary nebula and transition into smaller white dwarfs, which are the remnants of the star's core. Following this transition period, the white dwarf greatly expands and then sheds material, shattering any planets that are close by, according to a related Perspective by Luca Fossati, a researcher at the Austrian Academy of Sciences in Graz, Austria.

Some astronomers suspect that white dwarfs could still harbor planets or planetary remnants that survived the expansion period. For example, a previous study indicated that some white dwarfs show signatures of debris around them, probably from planets that were destroyed after the transition period.

However, gathering evidence of intact planets around these stellar remnants has proved more challenging. Traditional methods to detect planets, which examine disturbances in the star's orbit or light signatures caused by an orbiting body, are difficult to apply to white dwarfs because they are much dimmer than main sequence stars, Fossati notes.

Taking a different approach, Christopher Manser, a postdoctoral researcher at the University of Warwick in the United Kingdom and colleagues turned to spectroscopy — a scientific technique that examines the interaction between matter and light — to study the properties of gas in a disc of debris surrounding a white dwarf named SDSS 122859.93+104032.9.

The scientists first used the massive, 10.4-meter, or 34-foot, Gran Telescopio Canarias in the Canary Islands to observe light coming from the debris disc of the white dwarf in the spring of 2017 and 2018. This light changed in color as it orbited the white dwarf, similar to how the sound of a car changes in pitch as it passes an individual, Manser explained.

By studying the light signatures they gathered every few minutes, the researchers could then determine how much of each color they detected was being emitted by the gas, allowing them to search for any peculiarities or variations in the debris disc.

The analysis revealed that there was an extra stream of gas orbiting the white dwarf approximately every two hours, according to Manser. After considering several possibilities, such as a lower-mass companion star, the scientists concluded the periodic variation must be caused by a small planetary body — called a planetesimal — orbiting within the disc.

They also calculated that the planetesimal, which orbits very close to the white dwarf, must be unusually dense and no larger than 600 kilometers, or 372 miles, in diameter to avoid being ripped apart by the star's strong gravitational forces.

One intriguing question the authors considered was where this planetesimal originated. Based on the object's high density, they speculate that the planetesimal may be the remnant core of a planet whose outer layers were stripped away by the star's tidal forces.

The scientists were also surprised by the two-hour orbital period of the planetesimal, Manser said. The group had previously assumed that any body that entered a certain radius surrounding a white dwarf star would be torn apart. However, the planetesimal orbits well within this radius, indicating that large fragments of planets can survive in these harsh astronomical conditions.

"Our discovery is the first spectroscopic detection of a planetesimal around a white dwarf," said Manser. "This body is orbiting closer than was previously thought possible, and opens new and exciting questions for how these bodies are delivered from the planetary system to the white dwarf."

Manser and colleagues theorize that some properties of the gas disc could be fueled by the planetesimal, and plan on further examining whether these signals are present in other white dwarfs known to host gaseous debris discs.

The research team also said that their study's spectroscopy methods could help search for other planetesimals orbiting white dwarfs that cannot be detected with traditional planet-hunting techniques. Fossati believes that the extremely large telescope used by the scientists will likely be the ideal platform for identifying similar planetesimals in the near future.

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Joseph Cariz