Diligent sleuthing by astronomers has broken open a cold case in the data archive of NASA’s Hubble Space Telescope. In a study published Monday in Nature Astronomy, researchers report uncovering a surprising chemical clue that indicates the white dwarf star HS 0209+0832 may host a second-generation planet.
A white dwarf is the remnant core of a low-mass star that has burned through all its nuclear fuel and lost its outer envelope of gas and dust to space. A second-generation planet is a world that forms around the stellar remnant from its cast-off material.
“Rather than the white dwarf stage being a kind of epilogue to the story of a star and its planets, this research points to the systems we are familiar with only being the first chapter of a potentially much longer tale, with some new characters showing up. That’s a really exciting prospect to pursue,” said Jamie Williams, astronomer and lead author, a doctoral candidate at the University of Warwick in the United Kingdom.
Earth and the other planets in our solar system are first-generation planets, which form from material left over from a star’s birth.
This artist’s concept, not to scale, imagines the evolution of a Sun-like star (1) into an aging red giant (2) and then a small, bright white dwarf surrounded by a disk of its expelled outer layers (3), from which a second-generation planet forms (4).
Illustration: NASA, ESA, Leah Hustak (STScI)
“What Hubble is showing us in this white dwarf system is something we haven’t seen before: a high abundance of the element niobium, the signature of which I was unfamiliar with when I first found it in the archival data,” Williams said.
When Hubble first observed the star in 1999, the data contained roughly 100 chemical features that could not be identified. Williams went back to those records armed with an updated chemical database and found that niobium matched many of the mystery features.
Williams explained that, while niobium is found in our solar system and has multiple uses on Earth, including in jewelry and medical imaging devices, the amount Hubble found in the HS 0209+0832 system points to a planet forming not from a star’s birth, but from the material ejected as it dies.
“Niobium and other elements heavier than iron are astronomically special because, unlike many common elements, they are not formed in the cores of stars by thermonuclear fusion,” said Nicholas Stone, a theoretical astrophysicist at the University of Wisconsin – Madison and member of the research team. “Instead, these heavy elements can only be synthesized in the exotic conditions that briefly emerge inside dying stars. The presence of niobium is a signpost of these ‘death’ throes, and the expulsion of the dying star’s innards into space.”
Once the star ejected this chemically enriched material, the team theorizes that some of it coalesced into a gas giant planet. The remainder of the ejecta dispersed long ago, but the planet remains.
“When Jamie asked me about niobium in relation to this study I was truly gobsmacked, as that element had not been reported in any other white dwarf analyzed to date. Once we realized it was there, everything fell into place,” said astronomer and study co-author Boris Gaensicke, also at the University of Warwick.
NASA’s Hubble Space Telescope spectrum shows the unusual abundances of certain elements it found in the HS 0209+0832 system. Dips indicate where niobium, nickel, and calcium are absorbing light, so less light in that part of the spectrum reached Hubble.
Illustration: NASA, ESA, Leah Hustak (STScI)
The research team confirmed the Hubble observations with data from NASA’s retired FUSE (Far Ultraviolet Spectroscopic Explorer) mission, which also showed strong signatures of niobium in the HS 0209+0832 system.
NASA’s TESS (Transiting Exoplanet Survey Satellite) also observed the white dwarf for four months, allowing it to detect periodic brightness variations that indicate that a planet orbits at a distance of about 3.7 million miles (6 million kilometers), much closer than Mercury orbits the Sun.
The research team estimates the candidate planet is a gas giant about the size of Jupiter that is rapidly losing atmosphere. Because the white dwarf star is relatively new, it is still very hot and likely blasting this planet with energy that is stripping its outer material. This could result in the planet having a comet-like tail of material that would form a disk around the white dwarf star and fall back on to its surface, leading to Hubble detecting the niobium when studying the star. Despite this mass loss, Williams said that the planet is likely not a temporary blip on the cosmic radar.
“If the second-generation planet is there, I think it is likely to survive. Eventually the white dwarf will cool and then maintain a consistent temperature, with the planet in its stable habitable zone for millions of years,” Williams said.
Williams added that there is still a lot of work to do to understand these types of systems — how second-generation planets form, how common or rare they are, and how they evolve in orbit around a “dead” star. He’ll use Hubble to explore these questions for the next several years, hoping to build up substantial data and statistics about these new types of celestial bodies.
“I think this research is an important example of the fact that scientific discovery is not a straight path,” Gaensicke said. “It often needs that magical moment when people discuss big questions on their minds and realize that together they can find unexpected answers.”
The Hubble Space Telescope has been operating for more than three decades and continues to make ground-breaking discoveries that shape our fundamental understanding of the universe. Hubble is a project of international cooperation between NASA and ESA (European Space Agency). NASA’s Goddard Space Flight Center in Greenbelt, Maryland, manages the telescope and mission operations. Lockheed Martin Space also supports mission operations at Goddard. The Space Telescope Science Institute in Baltimore, which is operated by the Association of Universities for Research in Astronomy, conducts Hubble science operations for NASA.
