Hubble observes dead star devouring Pluto-like object

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With help from NASA’s Hubble Space Telescope in low-Earth orbit, scientists recently observed a Pluto-like object being “eaten” by its host star. Hubble’s unique sensitivity to ultraviolet light allowed it to study the planetary system, which lies around 260 light-years away from Earth.

The hungry star is a white dwarf — the dense, stellar core left behind after a low-mass star dies and blows away its outer layers. The white dwarf observed by Hubble is about the size of Earth but much heavier, at around half the mass of our Sun. The star’s immense density gives it a strong gravitational field that likely pulled the Pluto-like object from a region within the planetary system similar to our Kuiper Belt.

Much like Pluto and other Kuiper Belt objects within our solar system, the Pluto-like object was composed of ice, carbon, and other volatiles, suggesting the presence of water on the object. The team of scientists, led by Snehalata Sahu of the University of Warwick in the U.K., detected these volatiles being ejected from the object and falling onto the white dwarf — allowing them to measure the chemical composition of the object.

“We were surprised. We did not expect to find water or other icy content. This is because the comets and Kuiper Belt-like objects are thrown out of their planetary systems early, as their stars evolve into white dwarfs,” said Sahu. “But here, we are detecting this very volatile-rich material. This is surprising for astronomers studying white dwarfs as well as exoplanets, planets outside our solar system.”

To observe the white dwarf and the object, the team utilized the Cosmic Origins Spectrograph (COS) on Hubble — an instrument designed for ultraviolet spectroscopy that was installed on the telescope in May 2009. COS was able to determine that the fragments ejected from the object were composed of approximately 64% water ice. Such a high percentage of ice suggests that the object was once part of a much more massive object, likely a Pluto-sized planet, that formed in the system’s Kuiper Belt analog.

COS also detected a high percentage of nitrogen in the fragments. In fact, the amount of nitrogen detected is the highest amount ever detected in a white dwarf debris system.

“We know that Pluto’s surface is covered with nitrogen ices. We think that the white dwarf accreted fragments of the crust and mantle of a dwarf planet,” Sahu explained.

COS before being launched and installed on Hubble in 2009. (Credit: NASA)

Using COS was necessary for observing the fragments of the object falling onto the white dwarf, due to its sensitivity to ultraviolet wavelengths. As the fragments fall onto the white dwarf, the star accretes them, emitting primarily ultraviolet light and very little visible light. If Hubble’s other visible light instruments observed the white dwarf, the star would look no different than any other white dwarf.

The white dwarf was likely once a Sun-like star that hosted a suite of planets and circumstellar disks similar to the Kuiper Belt. This means that by observing this system, scientists can look into the future of our solar system billions of years from now, when the Sun burns away all its layers and becomes a white dwarf. The Sun’s remnant core will likely pull in objects from the Kuiper Belt and accrete them much like the white dwarf studied by Hubble.

“These planetesimals will then be disrupted and accreted. If an alien observer looks into our solar system in the far future, they might see the same kind of remains we see today around this white dwarf,” said Sahu.

Sahu et al. are planning to continue their observations of hungry white dwarfs in the infrared with the James Webb Space Telescope. With a much larger mirror and sensitivity to different wavelengths, the team will be able to detect molecular features of the volatiles, such as water vapor, carbonates, and other compounds.

The team is also planning to study the chemical composition of the recently discovered interstellar comet 3I/ATLAS and determine the amount of water it contains.

“These types of studies will help us learn more about planet formation. They can also help us understand how water is delivered to rocky planets,” Sahu said.

“We observed over 500 white dwarfs with Hubble. We’ve already learned so much about the building blocks and fragments of planets, but I’ve been absolutely thrilled that we have now identified a system that resembles the objects in the frigid outer edges of our solar system. Measuring the composition of an exo-Pluto is an important contribution toward our understanding of the formation and evolution of these bodies,” said Boris Gänsicke of the University of Warwick. Gänsicke served as the principal investigator of the Hubble program used to observe the hungry white dwarf.

Sahu et al.’s study was published in the Monthly Notices of the Royal Astronomical Society on Sept. 18.

(Lead image: artist’s impression of a Pluto-like object being torn apart by a white dwarf. Credit: NASA/Tim Pyle (NASA/JPL-Caltech))

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